Floor joist span chart
Pick the wood, grade, size and spacing. You get the maximum span from the American Wood Council's 2024 span tables and design values, whether stiffness or strength limits it, and a chart for every size.
Inputs
Result
| Size | 12 in | 16 in | 19.2 in | 24 in |
|---|
Every grade
Maximum span for each grade of the species, size and spacing above.
| Grade | E, million psi | Fb, psi | Span | Limited by |
|---|
Spans are worked from the American Wood Council's Span Tables for Joists and Rafters (2024) and Design Values for Joists and Rafters (2024), using the equations in the tables' own commentary. We checked those equations against every span in the printed tables and they match. Spans assume three or more members at no more than 24 in on centre, sheathed and nailed on top, single span, and stop at 26 ft as the tables do. Your local code and the authority having jurisdiction govern.
How the span is worked out
A joist has to meet two limits. It must not bend more than the code allows, which depends on its stiffness, the modulus of elasticity E; and it must not be overstressed, which depends on its bending strength Fb. Both come from the grade stamp, through the American Wood Council's design values. The span is whichever limit is shorter.
Douglas Fir-Larch No. 2 in a 2x10 has E = 1,600,000 psi and Fb = 1,140 psi. At 16 in on centre under the 40 psf living-area load, stiffness alone would allow 16 ft 5 in, but strength allows only 15 ft 7 in, so 15 ft 7 in is the span.
Which load to use
- 40 psf live, 10 dead for living areas under the IRC. Use the 20 psf dead load row for tile or stone floors.
- 30 psf live for sleeping rooms and for attics reached by a fixed stair.
- Ceiling joists: 10 psf for attics with no storage, 20 psf for attics with limited storage.
Two-span joists that run continuously over a centre beam can use these single-span figures conservatively. Notching, holes, point loads and anything sitting on the floor beyond the table loads need a design.
Questions
How far can a 2x10 floor joist span?
At 16 in on centre under a 40 psf living-area load, 15 ft 7 in in Douglas Fir-Larch No. 2, 15 ft 2 in in Hem-Fir No. 2, 15 ft 4 in in Spruce-Pine-Fir No. 1/No. 2 and 14 ft in Southern Pine No. 2, by the AWC 2024 values. The calculator works out any species and grade.
How far can a 2x8 floor joist span?
At 16 in on centre, 40 psf live, Douglas Fir-Larch No. 2 spans 12 ft 9 in. At 12 in on centre it reaches 14 ft 2 in.
How far can a 2x12 floor joist span?
At 16 in on centre, 40 psf live, Douglas Fir-Larch No. 2 spans 18 ft 1 in. At 24 in on centre, 14 ft 9 in.
How far can a 2x6 ceiling joist span?
With no attic storage (10 psf), Douglas Fir-Larch No. 2 at 16 in on centre spans 17 ft 8 in. With limited storage (20 psf) it drops to 13 ft.
Why are Southern Pine spans shorter than in older tables?
Southern Pine design values were lowered in 2013 after new testing, so current spans for No. 2 Southern Pine are shorter than in span charts printed before then.
Is the span measured from the centre of the supports?
No. AWC spans are clear spans, from the face of one support to the face of the other.
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