🏠 Floor Joist Calculator
Determine the right joist size based on span, spacing, wood species, and loads
📊 Result Chart
❓ FAQ
How to calculate floor joist size?
Joist size is determined by span, spacing, wood species, and loads. The core formula: Section modulus S = (w×L²)/(8×Fb). Select the smallest joist with S ≥ required S, while meeting L/360 and L/240 deflection limits.
What's the difference between SPF, Douglas Fir, SYP, and LVL?
The main difference is bending strength: SPF #2 ≈ 875 psi, Douglas Fir #2 ≈ 900 psi, SYP #2 ≈ 950 psi, LVL ≈ 2900 psi. LVL is strongest for long spans; SPF is most economical for short residential spans.
What do L/360 and L/240 deflection limits mean?
L/360 means maximum live load deflection must not exceed span/360; L/240 means total load deflection must not exceed span/240. For a 12 ft span, live load max deflection = 0.4 inches, total load max = 0.6 inches.
Should I use 12-inch or 16-inch joist spacing?
12-inch spacing provides higher capacity for heavy loads or long spans. 16-inch is the most common residential standard with good economy. Closer spacing means each joist carries less load, allowing smaller sections.