Stair stringer calculator: rise, run, fit check and cut layout

From the bottom. When treads sit on top of the stringer, every tread lifts its step by the tread thickness, but nothing lifts the floor the stringer stands on, so the first step would come out one tread thickness too tall. Shortening the bottom riser cut by that thickness fixes it: with a 7 in riser and a 1 in deck board the bottom cut is 6 in. Nothing is cut off the top: the stringer simply hangs with its top tread cut one riser plus one tread thickness below the upper surface. The stringer calculator prints the shortened cut as its own row when the box for treads on top of the stringer is ticked.

Usually because the stringer was hung too high or the rise was divided for the wrong number of steps. The top tread cut on the stringer must sit one full riser plus the tread thickness below the finished upper surface; hung higher than that, the last step shrinks by the difference. Store-bought stringers cause the same fault, since they are cut for one fixed riser and your total rise is rarely a multiple of it. Measure the finished rise, divide it equally, and check that the top riser equals the others before fastening anything. The IRC allows 3/8 in between the tallest and shortest riser.

Normally there is one tread fewer than risers. The risers are the vertical steps between the lower surface and the upper one, and the upper floor or deck acts as the final tread, so a stair with 6 risers has 5 treads on the stringer. The two counts are equal only when a tread is built level with the upper floor, which the tools handle with the top-tread box. The count matters because total run is treads times tread run: mixing the two numbers up moves the bottom of the stair by a full tread, 10 in or more, and changes the stringer and board length with it.

Longer than the stringer length. The nose-to-nose length is the diagonal of total rise and total run, but the level cut at the bottom and the plumb cut at the top both extend past that diagonal, and on a 2x12 the two together add about two feet. For a 3 ft 6 in rise the diagonal is 5 ft 5 5/16 in and the board length is 7 ft 5 1/4 in, an 8 ft board. For an 8 ft rise the diagonal is 12 ft 9 11/16 in and the board length 14 ft 9 3/16 in, so a 14 ft board that looks long enough is not; buy 16 ft. The stringer calculator prints both lengths and the stock size.

Not for a cut stringer on a deck stair, and it is a poor choice anywhere steps are notched into it. AWC DCA 6-2015 requires 2x12 minimum for all deck stair stringers and shows a 5 in minimum throat. A 2x10 is 9¼ in deep; a 7 in riser with a 10 in run notches 5.73 in out of it and leaves a 3 1/2 in throat, the depth of a 2x4. A 2x12 with the same steps keeps 5 1/2 in. The IRC itself names no stringer size for interior stairs, so local practice and your inspector decide there. The stringer tool lets you select 2x10 or 2x8 and shows the throat and the warning.

Three for a 36 in stair and four for a 48 in stair, on the deck guide's rule of at least three cut stringers at no more than 18 in on center. At 36 in that puts one at each side and one in the middle, 18 in apart; at 48 in four stringers sit 16 in apart. A 60 in stair takes five. Treads that span two outer stringers with nothing in the middle are the usual reason a stair feels springy. Composite treads often need closer spacing than 18 in; use the decking maker's stair figure. Both tools print the stringer count for the width you enter.

Under the 2021 IRC a riser may be no more than 7¾ in high and a tread no less than 10 in deep, from R311.7.5.1 and R311.7.5.2. The riser is measured vertically between the leading edges of adjacent treads and the tread horizontally between the same edges, so nosing does not count toward the 10 in. Treads under 11 in deep also need a nosing of ¾ in to 1¼ in. These are limits, not targets: a riser near 7 in on a 10 to 11 in tread is the common residential choice. Some states and cities amend the figures, so check the locally adopted code.

By 3/8 in at most. R311.7.5.1 says the greatest riser height within any flight shall not exceed the smallest by more than 3/8 in, measured across the whole flight and not only between neighbouring steps. The allowance exists for construction error; it is not room to leave the first or last step taller to use up a remainder. An uneven step is a trip hazard because people set their stride on the first two or three risers and stop looking. Divide the total rise by the riser count so every riser is identical on paper, then build as close to that as the materials allow.

No. Rise plus run near 17 to 18 in is a carpenter's comfort rule, and so is twice the rise plus the run near 24 to 25 in. Neither appears in the IRC, which sets only a maximum riser of 7¾ in and a minimum tread of 10 in. The rules are useful for choosing between legal layouts, since a stair that satisfies them matches a normal stride. They fail as a test on their own: a 9 in riser with a 9 in run adds up to 18 and breaks both code limits. A 7 in riser on an 11 in run passes the rules and the code together.

Between about 30° and 38° for a house stair built to the IRC limits. The angle is arctan(riser ÷ run): a 7 in riser on an 11 in run is 32.5°, 7 on 10 is 35.0°, and the steepest legal pair, 7¾ on 10, is 37.8°. The code does not state an angle; it follows from the riser and tread limits. The same figure is the plumb-cut angle on the stringer and the bevel on a rail post or baluster top, so both tools print it to a tenth of a degree for whatever riser and run you enter.

For a deck stair, AWC DCA 6-2015 Figure 28 limits a cut 2x12 stringer to 6 ft and a solid stringer to 13 ft 3 in, measured horizontally. Past that the guide calls for a 4x4 post under the stringer, on its own footing, or an intermediate landing. At a 10 in run, 6 ft is a little over seven treads, so a deck more than about 5 ft high usually needs the post. The IRC gives no stringer span for interior stairs, which are normally supported by a wall or framing along their length. The stringer calculator warns when the total run passes 6 ft.

A landing or floor is needed at the top and bottom of a stairway, and a flight may not rise more than 12 ft 7 in between floors or landings under IRC R311.7.3. The deck guide, AWC DCA 6-2015, is stricter and requires an intermediate landing once a deck stairway is over 12 ft high. A landing must be at least as wide as the stair and at least 36 in deep in the direction of travel. A landing is also the fix when a straight flight will not fit the floor space, because it lets the stair turn. Run the calculator once for each flight with that flight's rise.

Long enough that the nosing line has dropped 6 ft 8 in plus the floor thickness by the time it passes under the edge of the opening. As a formula, opening length = (80 in + floor thickness) ÷ riser height × tread run. For 15 risers of 7 1/4 in on a 10 in run under a 12½ in floor that is 10 ft 7 5/16 in; under a 10½ in floor it is 10 ft 4 9/16 in. A gentler stair needs a longer opening. The stair calculator's headroom check prints the headroom for the opening you have and the length needed. It checks geometry only; the opening's headers and trimmers are framing design.

A handrail, 34 to 38 in above the nosings, is required on at least one side of a flight with four or more risers (R311.7.8). A guard is required where the walking surface is more than 30 in above the floor or grade below (R312.1). Baluster layout is on the baluster spacing calculator.

The same way: measure the rise from the decking to the landing spot, divide it into equal risers and multiply the treads by the run. Deck stairs add a landing pad, stringer posts and guards, which the deck stair calculator handles; common questions are in deck stair questions.

Measure the total rise, divide by about 7.5 in and round up for the riser count, divide the rise by that count for the riser height, then multiply the treads by the tread run for the total run. The full walk-through with worked examples is in how to calculate stairs.