Deck Joist Span Calculator (IRC Table R507.6 Estimate)

Pick the lumber species group, joist size and spacing to see the maximum deck joist span in the IRC’s prescriptive table. Enter the span you need and the calculator lists every size and spacing in the table that reaches it.

Planning estimate from a code table, not an engineering approval. The table assumes a 40 psf live load and 10 psf dead load, No. 2 grade lumber and wet service. It does not cover hot tubs, snow loads over 40 psf, other grades or engineered lumber. Confirm your design with your local building department or an engineer.

Deck joist span calculator

How it works

The spans come straight from IRC Table R507.6, which gives the allowable joist span (face of support to face of support) for each species group, size and spacing. The same values appear in the 2018 IRC and in the 40 psf live-load column of the 2021 IRC.

Southern pine 2×8: 13 ft 1 in at 12 in, 11 ft 10 in at 16 in, 9 ft 8 in at 24 in
Douglas fir-larch, hem-fir, SPF 2×10: 15 ft 8 in, 13 ft 7 in, 11 ft 1 in
Redwood, western cedars, ponderosa and red pine 2×12: 17 ft 5 in, 15 ft 1 in, 12 ft 4 in

The calculator compares your span to the table value. It does not interpolate or extrapolate.

Worked example

Southern pine 2×8 joists at 16 in on center, for a 12 ft span:

Species group: Southern pine
Joist size: 2×8
Joist spacing: 16 in on center
Span you need (optional): 12 ft

The calculator gives:

Maximum joist span: 11′ 10" (table value 11-10)
Your span of 12′ 0": TOO LONG for this joist and spacing
Table options that reach it: 2×8 at 12 in, 2×10 at 12 in, 2×10 at 16 in, 2×12 at 12 in, 2×12 at 16 in, 2×12 at 24 in

The table allows 11 ft 10 in, so 12 ft is too long. The table options that reach 12 ft are 2×8 at 12 in, or 2×10 and 2×12 at 12 or 16 in, or 2×12 at 24 in.

Notes and limits

Joist cantilevers have their own limits in the same table, and depend on the back span; they are not covered here.
Species groups matter: southern pine spans further than the other groups at most sizes.
Composite and PVC decking often needs 12 or 16 in joist spacing regardless of span. Check the decking maker’s instructions.
Your jurisdiction may use a different edition of the IRC or local amendments.

Frequently asked questions

How far can a 2×8 deck joist span?

In southern pine at 16 in on center, 11 ft 10 in. In Douglas fir-larch, hem-fir or SPF, 11 ft 1 in. In redwood or cedar, 10 ft 7 in (IRC Table R507.6).

How far can a 2×10 deck joist span?

At 16 in on center: 14 ft 0 in in southern pine, 13 ft 7 in in the fir group, and 13 ft 0 in in redwood and cedar.

Is this the same as a floor joist span?

No. Deck joists use the deck table (R507.6), which assumes wet service. Floor joists inside a house use the floor tables in R502.

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.