Stairs
Stair calculators for rise and run, stringers, deck stairs and baluster spacing, checked against IRC 2021
Four calculators on three pages take a stair from the first measurement to the last baluster: whether the flight fits, how the stringer is cut, how a deck stair comes out of its tread boards, and how the railing is spaced. Lengths come out as tape fractions, the formulas are written out on this page, and every limit a tool warns about is tied to its section of the 2021 IRC or labelled as the deck guide it comes from. Free, no signup.
Every stair calculator on this site starts from the same two lines: risers = total rise ÷ target riser height, rounded up, and riser height = total rise ÷ risers. A 3 ft 6 in rise at a 7½ in target is 6 risers of 7 in and 5 treads; on a 10 in tread run that is 4 ft 2 in of total run at 35.0°. The 2021 IRC caps the riser at 7¾ in and sets the tread at 10 in or more (R311.7.5.1, R311.7.5.2). What changes between the four tools is where the tread run comes from, what gets cut, and which code figure each one checks.
Pick the calculator by the stair in front of you, not by the name on the page. If you are not sure, start with the which stair calculator do I need table; for the numbers behind the warnings, the IRC 2021 stair and guard limits are listed with their sections.
The four stair calculators, and what each one runs, returns and checks against the code
Each tool covers one job and hands the rest to its neighbours. The first two share a page: the stair calculator on top is the stairs calculator most people mean, a free online stair rise and run calculator that runs the basic calculations for stairs, answers as a stair riser, stair tread and stair angle calculator, and says whether the staircase fits the room, and the stair stringer calculator below it turns the same rise into a cut layout. The deck stair calculator is the outdoor version, an exterior stair calculator that works as a deck stairs, deck step and deck stringer calculator in one, and the only one that takes its tread run from boards. The baluster spacing calculator is the railing tool, a spindle spacing calculator for the balustrade of a level deck railing or a sloping stair rail, whether you call the uprights balusters, spindles or pickets. The result on each card is what the tool shows when it loads, or with the one setting the card names switched on, so the page can be checked against the tool.
Stair calculator: rise, run and whether the staircase fits
Runs: risers = total rise ÷ target riser height, rounded up; riser height = total rise ÷ risers; treads = risers − 1; total run = treads × tread run; slope length = √(rise² + total run²). With the headroom check on, it measures the clearance under the floor above from the sloped line of the nosings.
Returns: riser count and riser height, tread count, total run, slope length, stair angle, stringer count for the width, the run to spare in the space you have, and the floor opening length that gives 6 ft 8 in of headroom.
Code: R311.7.5.1 riser 7¾ in at most; R311.7.5.2 tread 10 in at least; R311.7.2 headroom 6 ft 8 in; R311.7.3 height of one flight, warned at the deck guide's 12 ft with the IRC's 12 ft 7 in stated beside it.
Default 9 ft 1 in rise on a 10 in run: 15 risers of 7 1/4 in, 14 treads, total run 11 ft 8 in, 36.0°, inside the 12 ft available. With the headroom check on and its 10 ft floor opening: headroom 6 ft 2 11/16 in, and 10 ft 7 5/16 in of opening needed
Stair stringer calculator: cut layout, throat and the board to buy
Runs: the same riser arithmetic, then the stringer length nose to nose, √(rise² + total run²), a board length that adds the end cuts, a bottom cut shortened by the tread thickness, and the throat left under the notch: board depth less riser × run ÷ √(riser² + run²).
Returns: riser and tread counts, total run, stair angle, throat, bottom cut, stringer length, board length and the stock length to buy, stringer count, and a cut diagram you can print and use as a stair stringer template.
Code: R311.7.5.1 and R311.7.5.2 for the riser and the tread. The 2x12 minimum, the 5 in throat and the 6 ft span of a cut stringer are AWC DCA 6 deck figures and are labelled that way, because the IRC sets no stringer size.
Default 3 ft 6 in rise, 2x12, 1 in treads: 6 risers of 7 in, 5 treads, total run 4 ft 2 in, 35.0°, throat 5 1/2 in, stringer 5 ft 5 5/16 in, board 7 ft 5 1/4 in, buy 8 ft
Deck stair calculator: deck height to grade, tread boards, stringers and footings
Runs: total rise = deck height less any landing pad, then the riser arithmetic above; tread run = the depth the boards cover less the overhang, so two 2x6 with a ¼ in gap cover 11 1/4 in and a 1 in overhang leaves a 10 1/4 in run; stringers at no more than 18 in on center across the width; one post under cut stringers wherever the horizontal run passes 6 ft.
Returns: riser height, risers and treads, the run your boards give, total run, stair angle, stringer count, stringer length and board to buy, tread boards, the open riser gap, the landing, stair footings, and whether a guard and a handrail are required.
Code: R311.7.5.1 and R311.7.5.2, the open riser rule in R311.7.5.1, a guard over 30 in (R312.1.1) and a handrail at four risers (R311.7.8). Stringer count, span and tread sizes are from AWC DCA 6-2015 and say so.
Default 3 ft 6 in deck, two 2x6 treads, 3 ft wide: 6 risers of 7 in, 5 treads, run 10 1/4 in, total run 4 ft 3 1/4 in, 34.3°, 3 stringers from 8 ft boards, 10 tread boards, guard yes, handrail yes
Baluster spacing calculator: spindle count, exact gap and layout marks
Runs: balusters = (opening − largest gap allowed) ÷ (baluster width + largest gap allowed), rounded up; gap = (opening − balusters × width) ÷ (balusters + 1). A gap that lands on the limit adds one baluster. On a stair the opening along the rail is turned into a level opening with the stair angle first.
Returns: the baluster or spindle count, the exact equal gap, on-center spacing, a mark for every baluster measured from one post, post spacing for a whole run of deck railing, and on a stair rail the angle, the level gap and the 6 in triangle check.
Code: R312.1.3: a 4 in sphere on a level guard, 4⅜ in on the open side of a stair, 6 in at the triangle of riser, tread and bottom rail. Guard height 36 in, or 34 in on a stair (R312.1.2). The 6 ft post spacing is AWC DCA 6 Figure 24.
Default 60 in opening, 2x2 balusters, level guard: 11 balusters at a 3 5/8 in gap, 5 1/8 in on center. As a stair guard on a 7 1/2 in riser and 10 in run: 8 balusters at a 4 in level gap, marked every 6 7/8 in along the rail
Which stair calculator do I need? Pick by the stair you are building
The table routes by the job. Read the second column as a check on the tool: if the arithmetic there is not what your stair needs, the row is wrong for you, whatever the page is called. Three of the linked calculators are planned and not yet live; they are marked.
| What you are building | Arithmetic it runs | Returns | Code figure | Calculator |
|---|---|---|---|---|
| Interior stairway, floor to floor: how many steps, and does it fit the room | rise ÷ risers; treads × run against the space you have; headroom on the nosing line | risers, riser height, total run, angle, run to spare, floor opening length | R311.7.5.1; R311.7.5.2; R311.7.2 | Stair calculator |
| Cutting stringers for any straight stair | stringer length, end cuts, bottom cut less the tread, throat under the notch | cut diagram, throat, board length and stock length, stringer count | R311.7.5.1; R311.7.5.2; DCA 6 for 2x12, throat and span | Stair stringer calculator |
| Steps from a deck to the ground or to a landing pad | deck height less the pad; run from the tread boards less the overhang | risers, run, stringers, tread boards, landing, footings, guard and handrail | R311.7.5.1; R311.7.5.2; R312.1.1; R311.7.8; DCA 6 | Deck stair calculator |
| Deck stair on a tall deck | horizontal run against the 6 ft span of a cut stringer; posts and footings added | posts under the stringers, board to buy, a flag where a landing is needed | DCA 6 Figure 28; R311.7.3 | Deck stair calculator |
| Balusters or spindles on a level deck, porch or balcony railing | count from the clear opening, equal gaps, a gap on the limit adds one | count, exact gap, on-center spacing, a mark per baluster | R312.1.3, 4 in sphere | Baluster spacing calculator |
| Balusters on a stair railing or banister | rail length turned level with the stair angle, then the same count | level gap, centers along the rail, stair angle, triangle sphere | R312.1.3 exceptions 1 and 2 | Baluster spacing calculator, stair guard setting |
| A whole side of deck railing with posts in it | posts taken out of the run, equal sections, one section spaced | opening per section, post spacing, balusters for the run | R312.1.3; DCA 6 Figure 24 | Baluster spacing calculator, whole run setting |
| Stair with a landing between two flights | each flight run on its own rise; the landing is not laid out | one result per flight | R311.7.3; R311.7.6 | Stair calculator, once per flight |
| Poured concrete steps | a solid stepped block in cubic feet and cubic yards | cubic yards, ready-mix order, bags | none checked for the stair shape | Concrete calculator, steps shape |
| Concrete for the landing pad and the stair post footings | slab volume; hole volume less the post | cubic yards and bags | R507.3.2 for the depth of a deck footing | Concrete slab · post hole concrete |
| The deck frame the stair hangs from | IRC R507 span tables for joists, beams and posts | member sizes and spans | R507.4; R507.5; R507.6 | Deck joist span · deck beam span · deck post size (planned) |
| Tread and deck boards as a board count | area or run divided by board coverage | boards and lengths | none | Deck board calculator (planned) |
| A ramp instead of steps | rise × slope ratio for the run; landings | ramp length and landings | ADA slope, not the stair section | ADA ramp slope calculator (planned) |
Indoor and outdoor stairs are measured to different things. An interior stair has a fixed rise, finished floor to finished floor, and a tread run you choose; the questions are whether 14 treads fit the room and whether the head clears the floor above. A deck stair has a rise that depends on where it lands, on sloping ground or on a pad that may not be poured yet, and a tread run that falls out of the boards. That is why the deck tool asks for the pad and the boards and the interior tool asks for the space and the floor opening.
One stair is usually three jobs. The layout fixes the riser and the run, the stringer tool cuts to them, and the railing is spaced last, once per opening between posts. A deck stair adds concrete: a landing at the bottom at least 36 in deep in the direction of travel, and footings under the stair, both figured on the concrete pages in the table. The concrete calculators and the structural span calculators have their own routers for the pour and the frame.
A guard and a handrail are different parts with different triggers. The guard stops a fall and is required where the walking surface is more than 30 in above the grade or floor below; the handrail is the grip and is required at four or more risers. A deck 24 in high has a stair of four risers: handrail yes, guard no. The deck stair calculator prints both answers, and the baluster spacing calculator spaces the in-fill of whichever railing is built.
One 3 ft 6 in stair read by the stair calculators: where the numbers agree and where they differ
The stringer tool and the deck stair tool both load with a 3 ft 6 in rise, which makes it the easiest stair to follow across the site. Both divide 42 in by a 7½ in target, get 5.6, round up to 6 risers and land on a 7 in riser with 5 treads, because the upper floor or the deck is the top step. From there the two tools part company on one input, the tread run.
| Read by, and what sets the tread run | Riser | Tread run | Total run | Angle | Stringer | Board |
|---|---|---|---|---|---|---|
| Stair stringer calculator: the run you enter, 10 in | 7 in | 10 in | 4 ft 2 in | 35.0° | 5 ft 5 5/16 in | 8 ft |
| Deck stair calculator: two 2x6 treads, 1 in overhang | 7 in | 10 1/4 in | 4 ft 3 1/4 in | 34.3° | 5 ft 6 1/4 in | 8 ft |
| Deck stair calculator: same boards, 1 1/4 in overhang | 7 in | 10 in | 4 ft 2 in | 35.0° | 5 ft 5 5/16 in | 8 ft |
| Deck stair calculator: one 2x10 tread, 1 in overhang | 7 in | 8 1/4 in (fails) | 3 ft 5 1/4 in | 40.3° | 4 ft 10 7/8 in | 8 ft |
| Deck stair calculator: two 2x6 treads on a 4 in landing pad, rise 3 ft 2 in | 6 5/16 in | 10 1/4 in | 4 ft 3 1/4 in | 31.7° | 5 ft 3 13/16 in | 8 ft |
With the overhang at 1¼ in the deck tool's run is 10 in and its total run, angle and stringer length are the stringer tool's to the sixteenth: 4 ft 2 in, 35.0° and 5 ft 5 5/16 in. The two calculators run one formula, and the page that looks different is only starting from boards. Stringer is the length nose to nose and Board is the stock length to buy. The fourth row is the same stair failing: a single 2x10 leaves an 8 1/4 in run, and the tool flags it as under the IRC R311.7.5.2 minimum of 10 in. The fifth is the stair after a 4 in pad is poured: the rise is 3 ft 2 in, the riser 6 5/16 in, and stringers already cut for 7 in no longer fit.
The stair calculator at the top of the stringer page loads with a taller stair, 9 ft 1 in floor to floor, and runs the same two lines: 15 risers of 7 1/4 in, 14 treads, 11 ft 8 in of run. The baluster tool picks the stair up at the rail. On the deck stair above, a 7 in riser on a 10 1/4 in run, a 60 in opening along the rail is 49 9/16 in measured level and takes 8 2x2 balusters at a 4 3/16 in level gap, set every 6 7/8 in along the rail; the same 60 in on the level deck takes 11. The stair angle the baluster tool prints for that rail, 34.3°, is the angle the deck stair tool printed for the stair.
The conventions are the same on every stair page. The riser count is rounded up, never down, so the riser comes out at or under the target. Treads are one fewer than risers unless you add a tread level with the upper floor. Lengths print to the nearest sixteenth as a tape reads. A board is the stock length at or above what the cut needs, never the length below it. A count of stringers, posts, balusters or boards is rounded up once. And a code limit is a warning on your input, never a change to it.
IRC 2021 stair and guard limits behind the calculator warnings, with the section for each
Every warning a stair calculator on this site prints traces to a line below. The values are the 2021 International Residential Code, Section R311.7 for stairways and R312.1 for guards, as carried in the Seattle SDCI 2021 Chapter 3 reprint the site reads from, pages 3-51 to 3-54, with Table R301.5 on page 3-30. The tools check a figure and tell you when an input is past it; they never change the input to meet it.
| Limit | Value | IRC 2021 section | Checked or stated by |
|---|---|---|---|
| Stair width | 36 in minimum clear width | R311.7.1 | stated on the deck stair page |
| Headroom | 6 ft 8 in minimum, measured plumb from the sloped line of the nosings | R311.7.2 | stair calculator, headroom check |
| Height of one flight | 12 ft 7 in at most between floors or landings | R311.7.3 | stair, stringer and deck stair tools; warned at 12 ft, see below |
| Riser height | 7¾ in maximum; 3/8 in at most between the tallest and shortest in a flight | R311.7.5.1 | stair, stringer and deck stair tools |
| Tread depth | 10 in minimum, nosing to nosing; 3/8 in variation at most in a flight | R311.7.5.2 | stair, stringer and deck stair tools |
| Nosing | ¾ in to 1¼ in projection; not required where the tread is 11 in or more | R311.7.5.3 | overhang input on the deck stair tool |
| Open risers | an opening more than 30 in above the floor or grade must not pass a 4 in sphere | R311.7.5.1 | deck stair calculator, open riser gap row |
| Landing | at least as wide as the stair and 36 in in the direction of travel | R311.7.6 | deck stair calculator, landing row |
| Slope of treads and landings | no steeper than 1 unit in 48, which is 2 percent | R311.7.7 | stated on the deck stair page |
| Handrail | at least one side of a flight with four or more risers; 34 to 38 in above the line of the nosings | R311.7.8; R311.7.8.1 | deck stair calculator, handrail row |
| Guard required | open sides more than 30 in above the floor or grade below, measured within 36 in of the edge | R312.1.1 | deck stair and baluster spacing tools |
| Guard height | 36 in minimum; 34 in on the open side of a stair, measured from the nosing line | R312.1.2 | baluster spacing calculator, height warning |
| Openings in a guard | must not pass a 4 in sphere; 4⅜ in on the open side of a stair; 6 in at the riser, tread and bottom rail triangle | R312.1.3 | baluster spacing calculator |
| Load on a guard and its in-fill | 200 lb on the top of the guard; 50 lb on one square foot of balusters or panel | Table R301.5, note f | stated, not checked |
The guard opening rule is short enough to read in full. This is Section R312.1.3 as printed on page 3-54 of that reprint, where it carries no amendment mark:
R312.1.3 Opening limitations. Required guards shall not have openings from the walking surface to the required guard height that allow passage of a sphere 4 inches (102 mm) in diameter.
Exceptions: 1. The triangular openings at the open side of stair, formed by the riser, tread and bottom rail of a guard, shall not allow passage of a sphere 6 inches (153 mm) in diameter. 2. Guards on the open side of stairs shall not have openings that allow passage of a sphere 4-3/8 inches (111 mm) in diameter.
What the IRC does not set matters as much on a stair. It gives no stringer size, no stringer spacing, no stringer span and no throat; no on-center spacing for balusters, only the sphere; no post spacing for a guard; and no comfort rule: there is no standard stair rise and run in the code, only the two limits, so rise plus run near 17 or 18 and two risers plus a tread between 24 and 25 are trade rules of thumb and are labelled that way. The stringer and post figures the tools use come from AWC DCA 6-2015, the prescriptive guide for wood decks, and belong to deck stairs:
| Deck guide figure | Value | Where in DCA 6-2015 | Used by |
|---|---|---|---|
| Stringer size | 2x12 minimum | Stair Requirements text | stringer and deck stair tools |
| Cut stringers | three at least where only cut stringers are used; 18 in on center at most | Stair Requirements; Figure 29 | stringer count on both tools |
| Span of a cut stringer | 6 ft, measured horizontally; a 4x4 post where it is exceeded | Figure 28 | deck stair tool, post row |
| Span of a solid stringer | 13 ft 3 in, measured horizontally | Figure 28 and commentary | deck stair tool, solid stringer setting |
| Throat of a cut stringer | 5 in left under the notch | Figure 28 | stringer tool, throat row |
| Treads between solid stringers | sizes by species | Table 6 | deck stair tool, tread warning |
| Intermediate landing | where the stairway height is over 12 ft | Stair Requirements | all stair tools, warning |
| Stair footings | 12 in below undisturbed ground, or below the frost line; the stair is carried on footings, not on a patio slab | Stair Footing Requirements | deck stair tool, footing row |
| Guard posts | 4x4, 6 ft apart at most | Figure 24 | baluster tool, opening warning |
The code and the guide disagree in one place the tools have to pick a side on. R311.7.3 lets a flight rise 12 ft 7 in between landings; the deck guide calls for an intermediate landing above 12 ft. The calculators warn at 12 ft, the stricter figure, and print both, and the guide itself says the code governs where the two differ. Sections R312.1.1 and R312.1.2 carry Washington amendment marks in the Seattle reprint; the numbers used here are the model code's. Your jurisdiction can amend any line, and how the site reads its sources is on the methodology page.
Where a stair layout goes wrong, in numbers
These are the stair mistakes that come up again and again, each with the arithmetic that catches it. The finished floor left out. Total rise is measured finished surface to finished surface. Leave ¾ in of flooring out at the top and the top riser is ¾ in off, twice the 3/8 in the code allows between the tallest and shortest riser in a flight. The pad poured afterwards. Stringers cut for a 3 ft 6 in rise have 7 in risers; set them on a 4 in pad and the bottom riser is 3 in. Measured to the pad first, the same stair is 6 risers of 6 5/16 in. Board depth taken for tread run. Two 2x6 cover 11 1/4 in, but with a 1 in overhang the run is 10 1/4 in; over five treads that is 4 ft 3 1/4 in of total run, not 4 ft 8 1/4 in, and the landing moves 5 in.
Stringer length taken for board length. The default stringer is 5 ft 5 5/16 in nose to nose and the board it is cut from is 7 ft 5 1/4 in, because the board has to carry the cuts at both ends. A 6 ft board is short; the tool says buy 8 ft. Headroom measured from the tread. The code measures from the sloped line of the nosings. A 9 ft 1 in stair under a 10 ft floor opening with a 12½ in floor has 6 ft 2 11/16 in of headroom, 5 5/16 in short of 6 ft 8 in, and needs an opening of 10 ft 7 5/16 in. A tall deck on unsupported stringers. An 8 ft deck takes 13 risers of 7 3/8 in and a horizontal run of 10 ft 3 in, well past the 6 ft a cut stringer spans in the deck guide; the deck stair calculator adds one post and a footing and buys 16 ft boards.
A 4 in gap between balusters. The rule is a sphere that must not pass, so a gap of exactly 4 in fails. A 48 in opening with eight 2x2 balusters leaves exactly 4 in; the baluster spacing calculator returns 9 at 3 7/16 in. Balusters set on centers. Space 2x2 balusters at 5½ in on center across a 60 in opening, starting one 4 in space from the first post, and ten fit, every gap is 4 in, and the space left at the far post is 5 in. Equal gaps, counted from the opening, give 11 balusters at 3 5/8 in. The comfort rule taken for code. Rise plus run equal to 18 is a rule of thumb. The code sets the two limits separately, and a 7¾ in riser on a 10 in tread passes at 17¾.
What these stair calculators do not do
They do not put a dollar figure on anything. They do not lay out winder treads, a spiral stair, or the turn of an L-shaped or U-shaped staircase; each flight is run as a straight stair and the landing between two flights is not framed. They do not size a stringer, a guard post or a connection for load: stringer rules are the deck guide's and are labelled, and the 200 lb and 50 lb guard loads of Table R301.5 are stated, not checked. They do not check a handrail's grip size, its returns or its continuity, or the lighting of a stairway; those parts of R311.7 are set out in the IRC stair code guide.
They do not figure concrete: poured steps are the steps shape in the concrete calculator, the landing pad is a slab and the stair footings are post holes. They do not design a ramp; that is the planned ADA ramp slope calculator, which works to accessibility slopes, not to the stair section. They do not cover commercial or public stairs, where the IBC, OSHA and the ADA set different numbers. They do not rate composite treads, whose span between stringers is the maker's figure, space cable railing, whose spacing and tension come from the cable maker, or count carpet, runners or finish. And they do not know your local amendments or which edition your jurisdiction has adopted.
Stair guides and question posts under this hub
The posts below take the stair questions the calculators answer in two lines and give them the long form. Each one links back to the tool it supports.
- IRC stair code: riser, tread, headroom and width in R311.7: the whole stair section in one place, winders and handrail grip included.
- How to calculate stairs: rise, run, risers and stringers: the hand method, step by step, for the questions the tools answer in one line.
- Deck stairs: width and how to calculate the steps: how wide, how many steps, and where the stair should land.
- Deck post and railing spacing, and the baluster count: post layout, corners and stair posts before the balusters go in.
- Deck footing depth, joist size and framing questions: the frame and footings the stair is hung from.
Stair calculators FAQ
Which stair calculator do I use for an indoor stair and which for a deck stair?
Use the stair stringer page for a staircase between two floors and the deck stair calculator for steps that land on the ground. Indoors the total rise is fixed, finished floor to finished floor, the tread run is a number you choose, and the open questions are whether the flight fits the room and clears the floor above; the two tools on the stringer page answer those and draw the cut layout. Outdoors the rise depends on where the stair lands and on any pad, and the run comes from the tread boards, so the deck tool starts from the deck height and the boards and adds the stringer span, footing, guard and handrail checks.
Why do two stair calculators give different numbers for the same total rise?
Because they differ on the tread run, the top step or the rounding, almost never on the riser. For a 3 ft 6 in rise every correct tool gives 6 risers of 7 in. On a 10 in run the total run is 4 ft 2 in and the stringer 5 ft 5 5/16 in nose to nose; with two 2x6 treads and a 1 in overhang the run is 10 1/4 in, the total run 4 ft 3 1/4 in and the stringer 5 ft 6 1/4 in. A tool that counts a tread level with the upper floor adds one more run. And a stringer length is not a board length: that 5 ft 5 5/16 in stringer is cut from a board 7 ft 5 1/4 in long.
Are the IRC stair limits different for deck stairs and interior stairs?
No, the stair section of the 2021 IRC makes no separate set of dimensions for a deck stair. Section R311.7 gives one set: riser no more than 7¾ in, tread at least 10 in, 3/8 in of variation in a flight, 36 in of width, a handrail at four or more risers, with R312.1 for guards. What a deck stair adds comes from AWC DCA 6-2015, the prescriptive deck guide: 2x12 stringers, three cut stringers at no more than 18 in on center, a 6 ft span for a cut stringer, and footings under the stair. The guide is stricter in one place, a landing above 12 ft of rise where the code allows 12 ft 7 in, and it says the code governs where the two differ.
Does the IRC say what size stair stringers must be or how far apart?
No. The 2021 IRC sets the shape of the stair, its riser, tread, width and headroom, but it gives no stringer size, spacing, span or minimum throat. The figures in common use are from AWC DCA 6-2015 and are written for deck stairs: 2x12 minimum, three cut stringers at no more than 18 in on center, a horizontal span of 6 ft for a cut stringer and 13 ft 3 in for a solid one. The calculators label them as the deck guide wherever they appear. For an interior stair the code leaves stringer sizing to the plans and the building department, and the tools say so instead of inventing a limit.
Can I use the stair calculators for a stair with a landing or a turn?
Yes for a straight flight on each side of a landing, no for winders or a spiral. Run the calculator once per flight, with the rise from the floor to the landing and then from the landing to the next floor. A landing has to be at least as wide as the stair and 36 in in the direction of travel (R311.7.6), and one flight may rise no more than 12 ft 7 in between floors or landings (R311.7.3); the deck guide draws that line at 12 ft. Each flight is its own stair for the riser arithmetic, and the 3/8 in limit in R311.7.5.1 is written for the risers within one flight. The tools do not lay out winder treads or frame the landing.
In what order do I run the calculators for a deck stair with a railing?
Deck stair first, stringer second, balusters last, with the landing pad in place before any of it is final. The deck stair calculator fixes the riser, the run from your boards, the stringer count and whether a guard and a handrail are required. The stair stringer calculator then gives the cut layout and the throat for that riser and run. The baluster spacing calculator is run once per opening between posts, on the stair guard setting for the sloping rail and the level setting for the deck above. The rise is measured to the finished landing, so a pad poured after the stringers are cut changes every riser: 4 in of pad turns a 7 in riser into 6 5/16 in.
What is the difference between a guard and a handrail on a stair?
A guard stops a fall off an open side; a handrail is what a hand holds. The 2021 IRC requires a guard where the walking surface is more than 30 in above the floor or grade below (R312.1.1), at least 36 in high, or 34 in measured from the line of the nosings on the open side of a stair (R312.1.2). It requires a handrail on at least one side of a flight with four or more risers, 34 to 38 in above the nosing line (R311.7.8). On a stair one top rail can do both jobs if it sits between 34 and 38 in. A low stair can need a handrail and no guard: a deck 24 in high has four risers.
How were the figures on these stair pages checked?
By script against the calculators. Each stair page is built by a script that mirrors the tool's arithmetic, recomputes every riser, run, length and baluster count printed in the copy and tables, and refuses to write the page if a number disagrees with what the tool displays at the same inputs. Code limits are tied to their 2021 IRC section and were read against the Chapter 3 reprint named on the methodology page; deck stair figures are tied to AWC DCA 6-2015 and labelled as the guide. There are no ratings or user counts on the site. If a number on a page disagrees with the tool, that is a fault to report through the contact page.
Are the stair code limits on these pages the whole code?
No, they are the limits a layout calculator can check. Section R311.7 also covers winder and spiral treads, handrail grip size and continuity, and stairway lighting, none of which these tools compute, and R312 says more about guards than the size of their openings. A state or city can amend any figure: the Chapter 3 reprint the site reads from marks Washington and Seattle amendments, and the tools use the model code figures, not the local ones. A jurisdiction may also be on an earlier or later edition than 2021. Take the pages as the model code with its section numbers, and confirm the edition and the amendments with your building department.
What are the IRC riser and tread limits for stairs?
Riser no more than 7¾ in, tread at least 10 in measured nosing to nosing, and no more than 3/8 in between the largest and smallest in a flight (R311.7.5.1, R311.7.5.2). The stair stringer calculator lists every limit with its section and the IRC stair code guide has the long form.
What is the maximum gap between balusters on a deck or stair railing?
A required guard may not pass a 4 in sphere, or a 4⅜ in sphere on the open side of a stair (R312.1.3), so the gap has to be under those figures, not on them. The baluster spacing calculator gives the count and the exact gap for any opening.
How many steps does a deck 4 ft off the ground need?
7 risers of 6 7/8 in and 6 treads at the default 7½ in target. The deck stair calculator has the chart for every deck height, and the deck stair questions post covers width and step counts.
Is there a calculator for concrete steps or for a ramp?
Poured steps are a volume job: the concrete calculator has a steps shape that figures a solid stepped block in cubic yards and bags. A ramp is a slope job, handled by the planned ADA ramp slope calculator.