Joist Span Calculator
Bending, deflection, and shear checked against your live load, dead load, spacing, and lumber grade, with the span each joist depth will carry and the spacing that span needs.
A 2x10 at 16 in on-center with 40 psf live and 10 psf dead spans about 15 ft on this check, and 16 ft 10 in in the IRC span table for Douglas Fir-Larch No.1. The code table is the permit answer; this model is the engineering cross-check, and it runs conservative.
How the math works
- Load: each joist carries a tributary width equal to its spacing, so plf = spacing ÷ 12 × (live + dead). 16 in on-center with 40 psf live and 10 psf dead is 66.7 plf.
- Bending: span = √(8 × Fb′ × S ÷ w), with S = 7.567 in³ for 2x6, 12.237 for 2x8, 18.437 for 2x10, 26.517 for 2x12. Fb′ is the tabular bending value times the NDS size factor (1.00 / 1.05 / 1.10 / 1.15 by depth), a 1.15 load-duration factor for the 7-day live load, and the 1.10 flat-use factor for 2x8 through 2x12 at 24 in on-center or tighter.
- Deflection: with Δ = L/N, span = ∛(384 × E′ × I ÷ (5 × w × N)). Live load drives L/360 for floors, total load drives L/240, and L/480 is the plaster- ceiling case. I = 29.5 / 52.4 / 85.0 / 131.9 in⁴.
- Shear: horizontal shear = 1.5 × V ÷ (b × d) against 215 psi Douglas Fir-Larch, 165 Hem-Fir, and 135 to 115 Spruce-Pine-Fir. Shear only bites on short spans with heavy loads, and a notch at a bearing is where it fails.
- Grade values: 24F-V4 (Douglas Fir-Larch No.1 and No.2 machine-evaluated) Fb 875 psi E 1,200,000; 21F-V3 Hem-Fir Fb 825 E 1,200,000; 15F-V3 Spruce-Pine-Fir Fb 675 E 875,000; 12F Spruce-Pine-Fir Fb 525 E 875,000.
What span will a 2x10 floor joist carry at 16 in on-center?
About 15 ft 0 in on this check with 40 psf live and 10 psf dead, and 16 ft 10 in in IRC Table R502.3.1(1) for Douglas Fir-Larch No.1. The difference is the flat diaphragm and size factors the table assumes, so a permit drawing follows the table and this model is the conservative cross-check.
Is L/360 or L/240 the right deflection limit?
Floors are designed to L/360 under live load only, and to L/240 under the total load when the finish is brittle. A plaster ceiling calls for L/480, which is why tile and stone floors want the deeper joist at the same span.
Can I space 2x12 joists at 24 in on-center?
On this check a 2x12 at 24 in on-center carries about 17 ft with 40 psf live and 10 psf dead, and the deflection limit is what governs at that spacing. Bounce is a serviceability complaint, not a code violation, and a glued-and-screwed subfloor with construction adhesive cures most of it.