Deck Beam Span Calculator (IRC Table R507.5 Estimate)

Pick the beam and species and enter the joist span the beam carries. The calculator reads the maximum beam span between posts from the IRC’s deck beam table, and the cantilever allowed past the end posts.

Planning estimate from a code table, not an engineering approval. The table assumes 40 psf live load, 10 psf dead load, No. 2 grade lumber and wet service. Snow loads over 40 psf, hot tubs and elevated or multi-level decks need other tables or an engineer. Confirm with your local building department.

Deck beam span calculator

How it works

Beam span depends on how much deck the beam holds up, which is set by the joist span. The table lists maximum beam spans for joist spans of 6, 8, 10, 12, 14, 16 and 18 ft. If your joist span falls between two columns, the calculator uses the next larger column, which is always on the safe side. The IRC notes allow interpolation, which gives a slightly longer span.

Southern pine 2-ply 2x10: 10 ft 4 in (6 ft joists) down to 6 ft 0 in (18 ft joists)
Southern pine 2-ply 2x12: 8 ft 0 in for 14 ft joists
A beam may cantilever past a post by up to one-quarter of its span (IRC R507.5)

Values are from the 2018 IRC Table R507.5. We checked them against the Pennsylvania Housing Research Center's worked example for the 2021 IRC (Table R507.5(1), 40 psf live load), which gives the same 8 ft 0 in for 2-ply southern pine 2x12 at a 14 ft joist span.

Worked example

A 2-ply southern pine 2x12 beam carrying 14 ft joists:

Species: Southern pine
Beam: 2-ply 2x12
Joist span the beam carries: 14 ft

The calculator gives:

Maximum beam span between posts: 8' 0" (table value 8-0)
Joist span column used: 14 ft
Beam may cantilever past a post: up to 2' 0" (one-quarter of the span)

The table allows 8 ft 0 in between posts, and the beam may overhang each end post by up to 2 ft.

Notes and limits

Single-ply beams are left out on purpose. They are unusual on decks, and one value in the published copy of that row appears to be a misprint, so we did not use it.
Built-up beams need their plies fastened together; the IRC calls for two rows of 10d nails at 16 in on center along each edge.
Solid 4x beams are listed only for the non-southern-pine species group, as in the table.
The joist span that governs is the longer of the joists on either side for a beam in the middle of a deck. With a joist cantilever, the 2021 IRC uses an "effective" joist span (Table R507.5(5)); ask your building department how they apply it.

Frequently asked questions

How far can a double 2x10 beam span on a deck?

In southern pine, 8 ft 0 in when it carries 10 ft joists and 7 ft 4 in with 12 ft joists. In the fir, cedar and redwood group, 6 ft 6 in and 5 ft 11 in.

How far can a double 2x12 beam span?

In southern pine, 9 ft 5 in for 10 ft joists and 8 ft 7 in for 12 ft joists.

How far can a deck beam cantilever?

Up to one-quarter of the actual beam span at each end (IRC R507.5).

Sources

Last reviewed: 25 September 2026. The calculation is covered by automated tests with known answers taken from the sources above. Results are planning estimates, not an engineering approval: confirm with your local building department or an engineer. Found an error? Please let us know.