Deck Beam Span Calculator

A deck beam span calculator checks how far a beam can go between posts. It reads the maximum beam span from AWC DCA 6 Table 3A for the beam size and the joist span it carries. A 3-ply 2x8 beam under 12 ft joists spans up to 7 ft 6 in., so an 8 ft span needs a 3-ply 2x10.

Last reviewed September 2026Method & sources Report an error

Result

This beam span is too long for this beam size

Breakdown
Result	This beam span is too long for this beam size
Maximum Beam Span	5 ft 11 in
Maximum Beam Overhang (each end)	1 ft 5 in

How to Calculate Deck Beam Span

AWC DCA 6 Table 3A gives the maximum beam span for each built-up beam size, based on how far the joists it carries span. This calculator uses the next joist-span column up if your span falls between two columns. It converts the table value to feet and inches and compares your beam span to it. A beam may extend past its outer post by up to one-quarter of the allowable beam span, which the guide allows as the maximum overhang.

Example: Beam with 8 ft post-to-post span, carrying 12 ft joists

A beam with 8 ft post-to-post span, carrying 12 ft joists, Southern Pine, 3-ply 2x8 beam: 1. Table column: 12 ft joist span is the 12 ft column. 2. Allowable beam span for a 3-ply 2x8: 7 ft 6 in. (90 in.) 3. Compare: 8 ft (96 in.) is longer than 90 in., so the 3-ply 2x8 is too short. 4. Next size up: a 3-ply 2x10 allows 8 ft 9 in. (105 in.), which covers 96 in. Use a 3-ply 2x10 beam for this 8 ft span under 12 ft joists.

Deck Beam Span: Compare Scenarios

Scenario	Result	Maximum Beam Span	Maximum Beam Overhang (each end)
Short beam (8 ft, 2-ply 2x8, 12 ft joists)	This beam span is too long for this beam size	5 ft 11 in	1 ft 5 in
Same span (8 ft, 3-ply 2x8, 12 ft joists)	This beam span is too long for this beam size	7 ft 6 in	1 ft 10 in
Same span (8 ft, 3-ply 2x10, 12 ft joists)	This beam works for this span	8 ft 9 in	2 ft 2 in

What Is Deck Beam Span?

A deck beam span calculator checks how far a beam can reach between its supports, which are the posts. The beam carries the joists, so a beam that is too long for its size and ply count will sag or bend under the deck load. The guide sets the limit by beam size, the number of plies, and the joist span the beam supports.

Deck Beam Span: Pros and Cons

Pros

Uses a primary, free-to-read AWC guide table, with the table cell and assumptions stated on the page
Converts the table value to feet and inches and reports the maximum overhang past the post, so both checks appear together
Shows the joist span column used, so you can see how the result changes when joists are longer

Cons

Covers Southern Pine built-up (2-ply and 3-ply) beams in this version. Other species, single-ply beams, and glulam beams need their own tables and are not included
These values differ from IRC Table R507.5 for some sizes. A jurisdiction on the 2021 IRC may require a different beam size
Assumes 40 psf live load and 10 psf dead load, with no snow load beyond that. A heavy snow region or a hot tub needs an engineer or your building department

Deck Beam Span: What a Pro Checks

Check the joist span first. A longer joist span lowers the allowable beam span, so it can change the beam size even when the post spacing stays the same. (AWC DCA 6 Table 3A)
Keep the beam overhang within one-quarter of the allowable beam span. An overhang longer than that is a separate limit the table does not cover. (AWC DCA 6 Table 3A notes)
Confirm with your building department whether they enforce the AWC DCA 6 table or IRC Table R507.5 before ordering lumber. The two can differ for the same beam size. (AWC DCA 6 Table 3A notes)

Frequently Asked Questions

How far can a 3-ply 2x10 beam span between posts?

For Southern Pine under 12 ft joists, AWC DCA 6 Table 3A allows 8 ft 9 in. Under 8 ft joists, the same beam spans 11 ft 0 in. Longer joists reduce the allowable beam span.

Why does joist span change the beam span?

The beam carries the load of the joists resting on it. Longer joists put more load on the beam, so the table allows a shorter beam span for each step up in joist length.

How far can a deck beam overhang past the post?

The guide allows the beam to extend past the post face by up to one-quarter of the allowable beam span. For a 3-ply 2x10 with 8 ft 9 in. allowed, that is about 2 ft 2 in. per end.

Why does the calculator give different numbers than my building department?

The AWC DCA 6 table in this calculator is the 2015 edition. Some jurisdictions enforce IRC Table R507.5 from the 2021 IRC, which has different values for some beam sizes. Ask your building department which table to use before you buy lumber.

Sources

AWC Prescriptive Residential Wood Deck Construction Guide (DCA 6, 2015 edition), Table 3A: Dimension Lumber Deck Beam Spans, Southern Pine (opens in a new tab)

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