Floor Joist Calculator
Free Floor joist Calculator for structural engineering projects. Enter dimensions to get material lists and cost estimates.
Formula
fb = M/S | delta = 5wL4/(384EI) | M = wL2/8
The bending stress fb is the maximum moment M divided by the section modulus S. The moment for a uniformly loaded simply supported joist is wL-squared over 8, where w is the load per linear foot and L is the span. Deflection uses the standard elastic formula 5wL4/(384EI). Both bending stress and deflection must be within allowable limits per the NDS.
Worked Examples
Example 1: Typical Residential Floor
Problem: 2x10 No. 2 SPF joists at 16 inches OC spanning 14 feet, with 40 psf live load and 10 psf dead load.
Solution: Tributary width = 16/12 = 1.333 ft\nw = (40+10) * 1.333 = 66.7 plf\nM = 66.7 * 14^2 / 8 = 1,633 ft-lb\nfb = 1633*12 / 21.39 = 916 psi\nFb_adj = 1000 * 1.15 = 1,150 psi -> PASS\nDelta = 0.285 in, L/360 = 0.467 in -> PASS
Result: Both bending and deflection checks pass
Example 2: Long Span with 2x12
Problem: 2x12 No. 1 joists at 12 inches OC spanning 18 feet, 40 psf live, 12 psf dead.
Solution: Tributary width = 1.0 ft\nw = 52 plf\nM = 52 * 18^2 / 8 = 2,106 ft-lb\nfb = 2106*12 / 31.64 = 799 psi\nFb_adj = 1200 * 1.15 = 1,380 psi -> PASS\nDelta = 0.263 in, L/360 = 0.600 in -> PASS
Result: Both checks pass with good margin
Frequently Asked Questions
How do I determine the correct floor joist size for my span?
Floor joist size depends on the span length, spacing, load, and lumber grade. As a general rule, the joist depth in inches should be roughly the span in feet. A 2x10 can typically span 14-16 feet at 16-inch spacing with No. 2 lumber under standard residential loads (40 psf live, 10 psf dead). A 2x12 can reach 18-20 feet. Always verify by checking both bending stress and deflection, as deflection often controls for longer spans.
Why does deflection often control floor joist design instead of bending?
Deflection limits are serviceability criteria (L/360 for live load) that prevent bouncy, uncomfortable floors and damage to finishes like tile or drywall. For longer spans, the required moment of inertia to meet the deflection limit is more demanding than the section modulus required for bending stress. This is because deflection depends on L to the fourth power, while bending depends on L squared. As a result, you may need a larger joist to satisfy deflection even though a smaller one would be strong enough.
How accurate are the results from Floor Joist Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.
How do I get the most accurate result?
Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.
What formula does Floor Joist Calculator use?
The formula used is described in the Formula section on this page. It is based on widely accepted standards in the relevant field. If you need a specific reference or citation, the References section provides links to authoritative sources.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.