By Zencalcy Editorial TeamLast reviewed About us
Floor joists are counted from the span or length they cross and the spacing. The count is straightforward; the harder question is whether the chosen size can span the distance, which this calculator checks against bending and deflection for a nominal load.
Joist count, spacing and a span check for a floor.
Count = ⌈width ÷ spacing⌉ + 1 Span check: L = √(8 Fb S ÷ w)
A 40 psf live plus 10 psf dead load is typical for residential floors; the tributary load per joist is (live + dead) × spacing in feet. Use Fb ≈ 1,000 psi and E ≈ 1,600,000 psi for visually graded lumber.
| Symbol | Meaning |
|---|---|
| L | Joist span. |
| s | Spacing on centre. |
| N | Number of joists. |
20 × 12 ÷ 16 = 15 spaces, so 16 joists. At 16 inches on centre, each carries 66.7 lb/ft of load.
Continue with the Beam Deflection Calculator, Beam Load Calculator, Bending Stress Calculator, or the Wood Beam Span Calculator — or browse more structural & workshop and all construction calculators.
16 inches for tile or stone, 24 for carpet and vinyl if the span table allows it.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026