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Deck Beam Calculator

Source: IRC Table R507.5(1) · NDS design values

Find the maximum allowable span between posts for a built-up or solid deck beam, or work backwards from the span you have, and it names the smallest beam that covers it. Values are read directly from the prescriptive span table in the International Residential Code, published in full below so you can check the result yourself.

Beam span calculator

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Table values: IRC Table R507.5(1). Deck live load 40 psf, dead load 10 psf, L/360 deflection, No. 2 grade, wet service.

ledgerhousejoiststhe half the beam carries(tributary width)beampostpostpostjoist span, ledger to beambeam spanbeam spanThe span table asks for the full joist span, not the shaded half.
How beam span and joist span are measured

Worked example

A 20 ft × 12 ft deck, ledger-attached to the house, with a single beam at the outer edge. Joists run 12 ft from the ledger to the beam. The beam is a built-up 3-ply 2×10 in Douglas fir-larch.

  1. Tributary width. Joists span 12 ft from ledger to beam. The beam carries half of that: 6 ft. But R507.5 is written against the full joist span, not the tributary half. Enter 12 ft.
  2. Read the table. Douglas fir-larch group, 3-2×10, joist span 12 ft → 8 ft 6 in maximum span between posts.
  3. Post count. 20 ft ÷ 8.5 ft = 2.35 → round up to 3 bays, giving 4 posts at 6 ft 8 in on center.
  4. Check the cantilever. R507.5.1 allows the beam to cantilever past the post up to one quarter of the actual backspan.

Switching the same beam to southern pine gives 9 ft 2 in instead of 8 ft 6 in, still 3 bays. Dropping to a 2-ply 2×10 in Douglas fir-larch gives 5 ft 11 in, which forces 4 bays and 5 posts.

The formula

IRC Table R507.5(1) is prescriptive: you look the answer up rather than deriving it. The values in it were generated from ordinary NDS bending and deflection checks, and if your condition falls outside the table (a species not listed, a load above 40 psf, an engineered beam) this is what you or your engineer works from:

fb = M / S ≤ Fb′   where   M = w·L² / 8   and   S = b·d² / 6
fb
actual bending stress, psi
M
maximum moment, in·lb (simple span, uniformly loaded)
w
uniform load on the beam, lb/in = (40 + 10 psf) × tributary width
L
clear span between posts, in
S
section modulus of the beam, in³
b, d
actual beam width and depth, in (a 3-ply 2×10 is 4.5 × 9.25)
Fb′
adjusted allowable bending stress, psi (NDS reference value × CD, CM, Ct, CL, CF, Cr)

Deflection governs as often as bending on deck beams. The parallel check is Δ = 5wL⁴ / 384EI ≤ L/360, with E the modulus of elasticity for the species and I = bd³/12.

Where these numbers come from

The table below is IRC Table R507.5(1), “Deck Beam Spans.” It appears in the 2015, 2018 and 2021 editions of the International Residential Code, in Section R507.5 (“Deck beams”). The 2018 and 2021 values are the same.

The table assumes a 40 psf live load, a 10 psf dead load, No. 2 grade lumber, wet service conditions, and deflection limited to L/360 at the main span and L/180 at a cantilever with a 220 lb point load. Southern pine values include the incising factor; the northern species group does not.

Note that spans are measured between the centers of the posts, and the joist-span column is the full span of the joists the beam carries, not the tributary half. This is the single most common misreading of the table.

IRC Table R507.5(1): Deck Beam Spans (ft-in), by joist span carried
Beam size6 ft8 ft10 ft12 ft14 ft16 ft18 ft
Southern pine
2-2×66 ft 11 in5 ft 11 in5 ft 4 in4 ft 10 in4 ft 6 in4 ft 3 in4 ft 0 in
2-2×88 ft 9 in7 ft 7 in6 ft 9 in6 ft 2 in5 ft 9 in5 ft 4 in5 ft 0 in
2-2×1010 ft 4 in9 ft 0 in8 ft 0 in7 ft 4 in6 ft 9 in6 ft 4 in6 ft 0 in
2-2×1212 ft 2 in10 ft 7 in9 ft 5 in8 ft 7 in8 ft 0 in7 ft 6 in7 ft 0 in
3-2×68 ft 2 in7 ft 5 in6 ft 8 in6 ft 1 in5 ft 8 in5 ft 3 in5 ft 0 in
3-2×810 ft 10 in9 ft 6 in8 ft 6 in7 ft 9 in7 ft 2 in6 ft 8 in6 ft 4 in
3-2×1013 ft 0 in11 ft 3 in10 ft 0 in9 ft 2 in8 ft 6 in7 ft 11 in7 ft 6 in
3-2×1215 ft 3 in13 ft 3 in11 ft 10 in10 ft 9 in10 ft 0 in9 ft 4 in8 ft 10 in
Douglas fir-larch, hem-fir, SPF, redwood, western cedars, ponderosa pine, red pine
3×6 or 2-2×65 ft 5 in4 ft 8 in4 ft 2 in3 ft 10 in3 ft 6 in3 ft 1 in2 ft 9 in
3×8 or 2-2×86 ft 10 in5 ft 11 in5 ft 4 in4 ft 10 in4 ft 6 in4 ft 1 in3 ft 8 in
3×10 or 2-2×108 ft 4 in7 ft 3 in6 ft 6 in5 ft 11 in5 ft 6 in5 ft 1 in4 ft 8 in
3×12 or 2-2×129 ft 8 in8 ft 5 in7 ft 6 in6 ft 10 in6 ft 4 in5 ft 11 in5 ft 7 in
4×66 ft 5 in5 ft 6 in4 ft 11 in4 ft 6 in4 ft 2 in3 ft 11 in3 ft 8 in
4×88 ft 5 in7 ft 3 in6 ft 6 in5 ft 11 in5 ft 6 in5 ft 2 in4 ft 10 in
4×109 ft 11 in8 ft 7 in7 ft 8 in7 ft 0 in6 ft 6 in6 ft 1 in5 ft 8 in
4×1211 ft 5 in9 ft 11 in8 ft 10 in8 ft 1 in7 ft 6 in7 ft 0 in6 ft 7 in
3-2×67 ft 4 in6 ft 8 in6 ft 0 in5 ft 6 in5 ft 1 in4 ft 9 in4 ft 6 in
3-2×89 ft 8 in8 ft 6 in7 ft 7 in6 ft 11 in6 ft 5 in6 ft 0 in5 ft 8 in
3-2×1012 ft 0 in10 ft 5 in9 ft 4 in8 ft 6 in7 ft 10 in7 ft 4 in6 ft 11 in
3-2×1213 ft 11 in12 ft 1 in10 ft 9 in9 ft 10 in9 ft 1 in8 ft 6 in8 ft 1 in

Reproduced from IRC Table R507.5(1). Spans are center-to-center of posts. Verify against the edition your jurisdiction has adopted.

Download this table as CSV. Same cells as printed above, nothing added.

Wooden beam span calculator: other framing members

R507.5 covers deck beams specifically. If you are sizing a beam inside the building envelope rather than on a deck, the governing table is different: floor girders fall under IRC Table R602.7(1), rafters and ceiling joists under R802.4 and R802.5. A generic wooden beam span calculator that does not name its table is not telling you which of these it used: that distinction changes the answer by feet, not inches.

Frequently asked questions

Is the joist span in the table the full span or the tributary width?
The full span of the joists the beam carries. If joists run 12 ft from the ledger to the beam, you use the 12 ft column, not 6 ft. The table already accounts for the beam picking up half the load.
How far apart can deck posts be with a 2×10 beam?
For a doubled 2×10 in Douglas fir-larch carrying 12 ft joists, 5 ft 11 in. Tripling it to a 3-ply 2×10 raises that to 8 ft 6 in, and switching to southern pine raises it again to 9 ft 2 in. Ply count and species both move the answer substantially.
Can a deck beam cantilever past the end post?
Yes. IRC R507.5.1 permits the beam to overhang the post by up to one quarter of the actual backspan.
Does this work for a beam under a hot tub or a roof?
No. Table R507.5(1) is built on a 40 psf live load and 10 psf dead load. A hot tub, a planter wall, a snow-loaded roof over the deck, or a second story all put you outside the table and into an engineered design.
What about 2×8 or 2×12 beams in the same layout?
Change the beam size in the calculator above, or read across the table. The two are the same data.
Which IRC edition do these values come from?
Table R507.5(1) as published in the 2018 and 2021 editions, which are identical for this table. Adoption is state and county by state and county, so confirm which edition your building department enforces.

Check these numbers yourself

Figures on this page last checked against the source documents on 2026-09-05. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Estimates only. This calculator reproduces a prescriptive code table for planning purposes. It is not a structural design and it is not a substitute for your local building department or a licensed engineer. Building codes are amended locally, and a deck failure injures people. Verify every span against the code edition adopted in your jurisdiction before you buy lumber or build.