Deckwright
Framing · IRC R507.5

Deck Beam & Post Spacing Calculator

See every beam size side by side: how far it can span between posts for your joist span, and how many posts and footings it needs across your deck.

1 Inputs
ft
Ledger to beam
in
ft
Table column
8′
effective joist span 7.8 ft (× 0.66)
2-ply pick
(2) 2x8
3 posts @ 7′
Fewest posts
3
(2) 2x8
Load case
40psf
live load
The 2021 IRC sizes beams by an effective joist span: your span × a factor from Table R507.5(5) that depends on the cantilever (0.66 with no cantilever, up to 1.00 at a quarter-span cantilever). We use the next higher factor row and table column rather than interpolating, which is on the safe side.
BeamMax post spacingPostsActual spacingEnd overhang
(2) 2x65′-11″44′ 8″1′
(2) 2x8pick7′-7″37′1′
(2) 2x109′-0″37′1′
(2) 2x1210′-7″37′1′
(3) 2x67′-5″37′1′
(3) 2x89′-6″37′1′
(3) 2x1011′-2″37′1′
(3) 2x1213′-3″37′1′
2021 IRC Tables R507.5(1)–(5) (Southern pine): beams carrying one span of joists, No. 2 grade, wet service, 10 psf dead load. Posts sit up to 12″ in from each beam end (never more than ¼ of the beam span). Fasten plies with two rows of 10d nails at 16″ o.c., and land splices over posts. Not a permit approval.

How beam size, post spacing and joist span trade off

A deck beam carries half of every joist that lands on it, plus any cantilever. The longer the joists, the more deck area each foot of beam supports. That is why Table R507.5 is organized by joist span: the same southern pine (2) 2x10 that can reach 10′-4″ at a 6 ft effective joist span is limited to 6′-0″ at 18 ft.

The 2021 IRC uses an effective joist span: the actual span times a factor from Table R507.5(5), from 0.66 with no cantilever up to 1.00 when the joists cantilever a quarter of their span. The tables assume the worst case, and the factor gives back capacity when your cantilever is shorter. The 2018 table has no factor, so in 2018 mode we read the column for the span plus twice the cantilever, which is conservative.

Picking a beam is really picking how many holes you want to dig. Deeper or triple-ply beams mean fewer posts and footings, but each footing gets bigger, because it carries more area. Run the footing calculator with the post spacing you choose here.

Snow loads

The 2021 IRC adds beam tables for 50, 60 and 70 psf ground snow loads (Tables R507.5(2)–(4)). Select your snow load and the calculator switches tables. The 2018 edition only covers 40 psf.

Building the beam

  • Fasten 2-ply and 3-ply beams with two rows of 10d (3″ × 0.128″) nails at 16″ on center, or with approved screws.
  • Make splices only over a post, and stagger the plies.
  • Bear the beam fully on a notched 6x6 post with two ½″ through-bolts, or use a manufactured post cap. Bolting a beam to the side of a post without bearing is prohibited.

Questions people ask

How far can a double 2x10 deck beam span?
Under the 2021 IRC, with joists that don’t cantilever, a southern pine (2) 2x10 beam can span 10′-4″ between posts when it carries 8 ft joists, 9′-0″ for 12 ft joists and 7′-4″ for 18 ft joists (Table R507.5(1), using the effective joist span = actual span × 0.66). In Douglas fir, hem-fir or SPF: 10′-0″, 8′-7″ and 7′-0″. Cantilevered joists raise the effective span and shorten these.
How far apart should deck posts be?
That depends on the beam size and how far the joists span to the beam. Longer joists put more load on the beam, so the posts must be closer. Under the 2021 IRC, a southern pine (2) 2x12 beam supporting 12 ft joists with no cantilever allows posts up to 10′-7″ apart; a (2) 2x8 up to 7′-7″.
Can a deck beam overhang the post?
Yes. The IRC allows a beam to cantilever past the end post by up to one quarter of the allowable beam span. This calculator keeps the overhang to 12″ or less.
Do I need a 3-ply beam?
Only to reduce the number of posts and footings. A 3-ply beam allows wider post spacing but is heavier, needs a wider post or bracket, and costs more lumber. Compare both in the table above.

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