Structural span › Loads
Beam load calculator: how much weight is on a beam, header, joist or post
Enter the joist spans that frame into the member, its length, and what sits on top, and get the tributary area, the design load in psf from IRC 2021 Table R301.5 or Table R301.6, the total pounds, the load per foot and the reaction at each end. Free, no signup, printable.
The load on a beam is its tributary area times the design load. Tributary width is half the joist span on each side of the beam, and tributary area is that width times the beam length. A beam between two 14 ft joist spans and 12 ft long carries 14 × 12 = 168 sq ft; at the 40 psf floor live load of Table R301.5 plus 10 psf dead load it carries 168 × 50 = 8,400 lb, which is 700 lb per foot of beam and 4,200 lb on each of its two supports.
That number is a load, not a size. The calculator stops at pounds on purpose: the IRC prescriptive span tables already carry these load cases, so the shortest route from pounds to lumber is the beam span calculator, the header size calculator or the deck tables, and beyond those tables an engineer. Jump to the R301.5 live load table or the roof load table.
How to use the beam load calculator
Pick the member first, because the tributary geometry changes with it: a beam with joists on both sides, a beam with joists on one side only, a deck beam, a header over an opening, a single joist, or a post under a beam. Then enter the spans in feet and inches, choose what is on top from the Table R301.5 list, and set the dead load. Every result prints the arithmetic it used, line by line, so it can be checked by hand.
- Joist span on each side: the span of the joists that land on this member, support to support, not the width of the room. Enter 0 on the far side if nothing frames into it.
- Length of the member: for a beam, its span between posts or walls; for a header, the width of the opening; for a joist, its own span.
- What is on top of it: the occupancy row of Table R301.5. Roof switches the live load to Table R301.6 or your ground snow load, whichever is greater, and asks for the roof slope. Enter my own live load takes any psf.
- Dead load: the weight of the framing, sheathing, ceiling and finishes the member holds up. R301.4 asks for actual weights; the span tables are built for 10 or 20 psf and this tool warns when you go past 20.
The post option returns the pounds on the post only, with no per-foot or reaction rows, because a post is a point, not a span. Sizing a deck post from that load is Table R507.4 on the deck post size calculator; a post inside the house is sized by an engineer.
How the beam load calculation works: tributary width, area, plf and reactions
A joist is a small beam. It rests on two supports, and a uniformly loaded, simply supported joist delivers half its load to each end. So a beam that catches one end of every joist in a 14 ft span collects half of that 14 ft, a 7 ft strip. If joists of the same span land on its other side too, it collects another 7 ft strip, and the beam's tributary width is 14 ft. Not 28 ft, which is the doubling mistake the deck forums repeat every summer, and not 7 ft, which forgets the second side.
Where the beam sits between the two outer supports does not change its share. Move a center beam 2 ft off center and one strip grows while the other shrinks; the total tributary width stays half the distance between the outer supports. The outer walls take the other half, 7 ft each in the example.
| Member | Tributary width | Length carried | Tributary area |
|---|---|---|---|
| Beam with joists on both sides | span A ÷ 2 + span B ÷ 2 | beam span | width × beam span |
| Beam with joists on one side | span A ÷ 2 | beam span | width × beam span |
| Header over an opening | half of each span landing on the wall | opening width | width × opening width |
| Single joist or rafter | its spacing, 16 in = 1.33 ft | its span | spacing × span |
| Post under a beam | half the joist span (one side) + any overhang | half the beam span each side | width × length (DCA 6 Eq. B-1) |
Tributary widths for the five member types the calculator covers. The post row follows AWC DCA 6-2015 Appendix B; an end post gets half of one beam span plus any beam cantilever, and a corner post half of each.
From area to pounds. Multiply the tributary area by the design load in pounds per square foot. The design load is the live load from Table R301.5 plus the dead load, so a floor at 40 + 10 = 50 psf on 168 sq ft is 8,400 lb. Multiply the tributary width alone by the same psf and you get the load per running foot: 14 ft × 50 psf = 700 plf, which is the number a span table or an engineer's beam chart is keyed to. Check: 700 plf × 12 ft = 8,400 lb.
From pounds to reactions. A uniform load on a simply supported member splits evenly between its two supports, so each end reaction is half the total: 4,200 lb. That is the load each post, jack stud or wall must take down to the footing, and it is the number the deck footing size calculator needs. A post under the middle of a beam picks up half of the beam span on each side of it, which is why the post option halves in two directions. A single 16 in. joist across 14 ft carries 18.7 sq ft, 933 lb, 67 plf and 467 lb at each end.
IRC 2021 Table R301.5: the live loads the beam load calculator uses
R301.5 says the minimum uniformly distributed live load "shall be as provided in Table R301.5." These are the psf figures behind every floor, deck and attic option in the tool, printed as the IRC prints them, with the concentrated loads the table lists beside them.
| Use | Uniform load (psf) | Concentrated load (lb) |
|---|---|---|
| Uninhabitable attics without storage | 10 | — |
| Uninhabitable attics with limited storage | 20 | — |
| Habitable attics and attics served with fixed stairs | 30 | — |
| Balconies (exterior) and decks | 40 | — |
| Fire escapes | 40 | — |
| Guards | — | 200 |
| Guard in-fill components | — | 50 |
| Handrail | — | 200 |
| Passenger vehicle garages | 50 | 2,000 |
| Areas other than sleeping areas | 40 | — |
| Sleeping areas | 30 | — |
| Stairs | 40 | 300 |
IRC 2021 Table R301.5, page 3-30 of the Seattle SDCI reprint, original IRC values. Footnote a: an elevated garage floor takes the 50 psf or a 2,000 lb load on a 4 1/2 by 4 1/2 in. area, whichever is worse. Footnote c: a stair tread takes 40 psf or 300 lb on 2 by 2 in. Footnote e sends attached decks to R507.1. Footnotes b and g define the attic rows by a 42 in. clear height between joists and rafters, and the limited-storage load applies only where the access opening is at least 20 by 30 in., the joist slope is 2:12 or flatter and the insulation sits below the top of the joists. The guard and handrail loads are concentrated only, so they never enter a uniform beam calculation.
Decks: 40 psf in the IRC, 60 psf in some states. The IRC prints 40 psf for balconies and decks, and the deck joist, beam and post tables in R507 are built on that 40 psf. Washington amends the row to 60 psf, and its reprint shows the 40 struck through next to it; check whether your state does the same. This calculator uses the IRC figure and flags the deck option so you can check your local amendment. If your code says 60, enter it under the custom live load; the tool will take it, but the deck beam span calculator and deck joist span calculator will not, because their tables were never printed for it.
Dead load is yours to enter. R301.4 requires "the actual weights of materials and construction" including fixed equipment. As a rule of thumb, not a code figure, ordinary wood floor framing with subfloor and drywall below lands near 10 psf; and R301.2.2.2 caps the average floor dead load at 10 psf and roof plus ceiling at 15 psf for the seismic provisions, with a 25 psf roof exception where bracing is increased. The span tables print 10 and 20 psf columns and nothing above, so tile over mortar, stone, a plaster ceiling, a hot tub or a green roof takes the member out of the tables even when the pounds here are correct. The difference between the two loads and why the tables treat them differently is covered in the live load vs dead load post.
Roof loads: Table R301.6 changes with the tributary area, or ground snow governs
Roofs are the one case where the live load depends on the tributary area the tool just worked out. R301.6 designs the roof "for the live load indicated in Table R301.6 or the ground snow load indicated in Table R301.2, whichever is greater," and Table R301.6 drops the live load as the loaded area grows, because a large area is less likely to be fully loaded at once. That is why the calculator asks for the roof slope and your ground snow load, and reports which of the two won.
| Roof slope | 0 to 200 sq ft | 201 to 600 sq ft | Over 600 sq ft |
|---|---|---|---|
| Flat or rise less than 4 in. per ft (1:3) | 20 | 16 | 12 |
| Rise 4 in. per ft (1:3) to less than 12 in. per ft (1:1) | 16 | 14 | 12 |
| Rise 12 in. per ft (1:1) and greater | 12 | 12 | 12 |
IRC 2021 Table R301.6, minimum roof live loads in pounds per square foot of horizontal projection, by tributary loaded area for any structural member, page 3-31 of the Seattle SDCI reprint. Ground snow load comes from Figures R301.2(3) and R301.2(4) or the local building department; where the ground snow load exceeds 70 psf, R301.2.3 requires engineered design and the prescriptive roof tables no longer apply.
Worked through the tool: a ridge beam with 14 ft of horizontal rafter run on each side and 20 ft long has a tributary area of 14 × 20 = 280 sq ft, which sits in the 201 to 600 column. On a 4:12 to 12:12 roof the table gives 14 psf. With a 30 psf ground snow load, snow governs, and at a 15 psf roof dead load the design load is 30 + 15 = 45 psf: 280 × 45 = 12,600 lb on the ridge, 630 plf, 6,300 lb on each end post. Snow replaces the roof live load; it is never added to it. Rafters themselves are sized by span, not by pounds, on the rafter span calculator, and the difference between a rafter roof and a trussed roof for the walls below is in rafters vs trusses.
What this beam load calculator does not do
It gives the uniform gravity load on one member from one level, and the reactions that load produces. Everything below changes the answer and is left out on purpose, so the page says so plainly.
- Point loads. A post from the floor above, a bearing wall that lands mid-span, a header reaction, a hot tub or a stair stringer puts a concentrated load on the member. The reactions then depend on where along the span it lands, and a 200 lb person standing at mid-span is a different problem from 200 lb spread over the floor. That is statics, not a table lookup, and it belongs to an engineer.
- Continuous joists. Joists that run over the beam in one piece, rather than stopping at it, deliver more than half their load to that beam. The usual continuity factor is above 1.0 and the tool does not apply one. The deck beam table excludes this case by footnote and so does this page.
- More than one level. The tool loads one floor or one roof. A center beam under a two-storey house with a bearing wall above carries its own floor, plus the ceiling that wall holds, plus the roof. Run each level separately and add them: 14 ft tributary width, 16 ft beam, floor at 50 psf = 11,200 lb; ceiling over an attic without storage at 10 psf live + 10 psf dead over the same area = 4,480 lb; a 4:12 to 12:12 roof on 224 sq ft at 14 + 15 psf = 6,496 lb; total 22,176 lb, plus the weight of the wall itself, which R301.2.2.2 caps at 10 psf of wall area for an interior wood wall.
- The member's own weight. A triple 2x12 or a steel beam adds a real load of its own that is not in the psf you entered.
- Load factors and combinations. The figures are service loads, added the way the IRC span tables add them. No ASD or LRFD combination is applied.
- Bending, shear, deflection and size. The tool never says whether a member can carry the load. That is the job of the span tables where they apply and of an engineer where they do not, and the beam deflection explained post covers why a beam that is strong enough can still be too bouncy.
Live-load reduction, the ASCE 7 formula that lowers the psf on very large tributary areas, is a commercial-code provision. The IRC residential values are minimums with no reduction, so the tool applies none.
From load to size: which IRC span table to use next
Every IRC span table already has the live and dead loads in its heading, so you do not carry the pounds into it; you carry the spans. The pounds matter for the things the tables do not cover, for checking a footing, for talking to an engineer, and for understanding why a table says no. Once you know what the member is, this is where it gets sized.
| You are loading | Size it with | IRC 2021 table it reads |
|---|---|---|
| Floor beam or girder | beam span calculator | R602.7(1) and (2) |
| Header over a door or window | header size calculator | R602.7(1) to (3) |
| Floor joist | floor joist span calculator | R502.3.1(1) and (2) |
| Ceiling joist | ceiling joist span calculator | R802.5.1(1) and (2) |
| Rafter | rafter span calculator | R802.4.1(1) to (8) |
| Deck joist | deck joist span calculator | R507.6 |
| Deck beam | deck beam span calculator | R507.5(1) to (5) |
| Deck post | deck post size calculator | R507.4 |
| Deck footing under that post | deck footing size calculator | R507.3, DCA 6 Table 4 |
Where a table runs out, the tool has already warned you: more than 400 sq ft on one member, a dead load over 20 psf, ground snow over 70 psf, or a member the tables do not print, such as LVL, steel or a built-up girder outside R602.7. Those go to an engineer with the pounds from this page in hand, and the beam span and size questions post explains what the engineer will do with them. How the tables are read, and why their headings carry the loads, is in how to read a span table. All ten structural span tools are listed on the structural span hub.
Worked examples: wall removal, deck beam and ridge beam
Removing a bearing wall: 9,600 lb on a 16 ft beam
Second-floor joists span 12 ft on each side of the wall, and the new beam will span 16 ft between posts. Tributary width 6 + 6 = 12 ft; area 12 × 16 = 192 sq ft; floor, areas other than sleeping, 40 + 10 = 50 psf. Load 192 × 50 = 9,600 lb, 600 plf, 4,800 lb on each post. If a post is added at mid-span the beam becomes two 8 ft spans, each reaction changes, and the beam over a continuous support is outside the simple halving. Bedrooms above would use 30 psf.
Deck beam: 3,000 lb on a 12 ft beam with 10 ft joists
Joists run 10 ft from the ledger to the beam with no cantilever, and the beam spans 12 ft between posts. Tributary width 10 ÷ 2 = 5 ft; area 5 × 12 = 60 sq ft; deck, 40 + 10 = 50 psf. Load 3,000 lb, 250 plf, 1,500 lb per post. At a 60 psf state amendment the same beam carries 4,200 lb. The ledger takes the other 3,000 lb into the house rim.
Ridge beam: 12,600 lb where snow governs
Rafters with 14 ft of horizontal run each side, ridge 20 ft, 6:12 roof, 30 psf ground snow, 15 psf roof dead load. Area 280 sq ft; Table R301.6 gives 14 psf for 201 to 600 sq ft at that slope; 30 psf snow is greater, so 30 + 15 = 45 psf. Load 12,600 lb, 630 plf, 6,300 lb on each end. The same ridge with no snow and a flat roof would use 16 psf from the table, since 280 sq ft crosses the 200 sq ft line.
Beam load questions from builders and homeowners
How do I calculate the load on a beam from the floor joists?
Add half the joist span on each side of the beam to get the tributary width, multiply by the beam length for the tributary area, then multiply by the live load plus the dead load. Joists of 10 ft on one side and 14 ft on the other give 5 + 7 = 12 ft; on a 10 ft beam that is 120 sq ft, and at 40 + 10 psf the beam carries 6,000 lb, or 600 lb per foot. Each end takes half, 3,000 lb. The calculator prints these steps for any spans you enter.
Is the tributary width half the joist span or the whole span?
Half, on each side. A simply supported joist sends half its load to each end, so a beam catching one end of 14 ft joists collects a 7 ft strip. Joists on both sides give 7 + 7 = 14 ft. Adding the two full spans, 28 ft, doubles the load: 16,800 lb instead of 8,400 lb on a 12 ft beam. The deck guide's note about using twice the joist span applies to picking a table column, not to the width of the strip the beam carries.
Can I just divide the whole floor area by the number of posts?
No. On a 28 by 40 ft house the outside walls carry half of everything; a center beam carries the middle 14 ft strip, and each post under it carries only the beam length halfway to its neighbours. A post between two 10 ft beam spans under 14 ft joists on both sides carries 14 × 10 = 140 sq ft, not 1,120 sq ft divided by the post count. The post option takes one joist span, as on a deck; for a center beam, run the beam option for one post-to-post span and split it between its two posts.
What does tributary area mean?
Tributary area is the patch of floor, deck or roof whose weight flows into one particular member. Every joist, beam, post and footing has one, and the boundaries fall halfway between a member and its neighbours, because that is where the load splits. For a joist it is its spacing times its span; for a beam, half the joist span on each side times the beam span; for a post, half the beam span on each side times half the joist span plus any overhang. Multiply the area by the psf and the load on that member falls out.
How much load does each post or each end of the beam carry?
Half of the total, when the load is uniform and the beam sits on two supports. An 8,400 lb beam puts 4,200 lb on each end, and that 4,200 lb is what the post, the jack studs or the foundation wall must carry down. A beam on three posts is different: the middle post under a beam that runs continuously over it takes more than a simple half of each side; AWC DCA 6 uses a 1.25 factor for that post, and this tool does not model it. The deck footing calculator turns a post load into a footing size.
What live load do I use, 40 psf or 30 psf?
Table R301.5 gives 40 psf for areas other than sleeping areas, which covers living rooms, kitchens, hallways and dining rooms, and 30 psf for sleeping areas. Habitable attics also use 30, attics with limited storage 20, attics without storage 10, garages 50, and stairs 40. If a beam carries a bedroom on one side and a living room on the other, the safe choice is 40 for the whole strip; the floor joist tables make the same split with their two tables. The tool's occupancy list follows the table row for row.
Is deck live load 40 psf or 60 psf?
The IRC says 40 psf for exterior balconies and decks, and the R507 deck joist, beam and post tables are built on 40. Washington State amends the row to 60 psf, and its reprint of the code shows the 40 struck through; check whether your jurisdiction has done the same. The calculator uses the IRC 40 and flags the deck option so you check your local code; enter 60 as a custom live load if your jurisdiction requires it. The deck span tools on this site cannot take 60, because no table was printed for it.
What dead load should I use for the beam load calculation?
Enter the actual weight of what the member holds up, which is what R301.4 requires. As a rule of thumb, wood floor framing with a subfloor and a drywall ceiling is about 10 psf, and R301.2.2.2 limits the average floor dead load to 10 psf and roof plus ceiling to 15 psf for the seismic provisions. Tile over a mortar bed, a plaster ceiling, stone, a green roof or a hot tub push the number past 20 psf, and 20 is the top dead-load column the span tables print, so the tool warns you when you cross it.
What is the difference between live load and dead load?
Dead load is the permanent weight of the structure itself; live load is what uses it: people, furniture, snow. The full answer, with the psf values and why the tables treat them differently, is in live load vs dead load.
How do I figure the roof load on a beam?
Take the tributary area of the roof the beam carries, half the rafter or truss span on each side times the beam length, then use the larger of the Table R301.6 roof live load and your ground snow load, plus the roof dead load. Table R301.6 depends on the slope and on the area itself: 20, 16 or 12 psf for a flat roof as the area passes 200 and 600 sq ft. A 280 sq ft ridge strip at 6:12 gets 14 psf from the table, but a 30 psf snow load governs, so 30 + 15 = 45 psf and 12,600 lb.
Does a wall above the beam add to the beam load?
Yes, if it is a bearing wall. It brings down the ceiling joists over it, at 10 psf live for an attic without storage or 20 psf with limited storage plus their dead load; the roof above that, at Table R301.6 or snow plus the roof dead load; and its own weight, which R301.2.2.2 caps at 10 psf of wall face for an interior wood wall. Run each level through the tool and add the totals: a 16 ft beam under a 14 ft strip of floor, no-storage ceiling and 6:12 roof with no snow comes to 22,176 lb before the wall itself.
Can this calculator handle a point load in the middle of the beam?
No. A point load, such as a post from the floor above, a beam landing on this beam, or a hot tub sitting on two joists, is a concentrated force at one spot, and the reactions depend on exactly where it sits along the span. A 3,000 lb post at mid-span sends 1,500 lb to each end, but the same post at the quarter point sends 2,250 lb to the near end. Modelling that is beam statics, and sizing for it is engineering, so the tool leaves point loads out and says so on every result.
Now that I have the pounds, what size beam do I need?
Take the spans, not the pounds, to the prescriptive table for that member. The IRC beam, header, joist and rafter tables already carry the Table R301.5 loads in their headings, so the beam span calculator sizes a floor girder from the joist spans and the building width, and the deck beam table sizes a deck beam from the joist span. The pounds are for what the tables do not cover: LVL, steel, point loads, dead loads over 20 psf, or a footing check. Those go to an engineer with this page's printout.
Do the IRC span tables already include these loads?
Yes. Every span table names its loads in the title or footnotes: the floor joist tables are 40 or 30 psf live with 10 or 20 psf dead; the deck tables carry 40 psf live and 10 psf dead with 50, 60 and 70 psf snow columns; the rafter tables list roof live load and snow columns with 10 and 20 psf dead. So you never multiply anything before reading them. This calculator exists for the step before and the step after: understanding how much load a member sees, and checking the posts, footings and members the tables leave out.
Source and verification of these loads
Live loads are IRC 2021 Table R301.5 and roof live loads Table R301.6, read from the 2021 Seattle Residential Code, Chapter 3, pages 3-30 and 3-31. Washington amends the deck row, so that document shows the original 40 psf struck through beside the local 60; this site uses the original IRC value. Dead load follows Section R301.4 and the caps in R301.2.2.2; the 70 psf snow limit is R301.2.3. Post tributary geometry follows AWC DCA 6-2015, Appendix B, Equations B-1 and B-2. The tool's arithmetic is a formula, not a table lookup, and the plugin test suite pins every example on this page to it. How the site reads and checks code tables is on the methodology page.