Floor Joist Calculator
Size floor joists fast using span loads spacing and material checks. View bending shear and deflection results plus simple recommendations. Export a shareable CSV or PDF summary instantly.
Developed by: Nohman Habib
Example Data
| Span | Spacing | Live | Dead | Material | Joist | Deflection (LL/Total) |
|---|---|---|---|---|---|---|
| 12 ft | 16 in | 40 psf | 10 psf | SPF #2 | 2×10 | L/360 and L/240 |
| 14 ft | 12 in | 40 psf | 15 psf | SYP #2 | 2×10 | L/480 and L/240 |
| 4.0 m | 400 mm | 1.9 kPa | 0.5 kPa | DF-L #2 | 2×10 | L/360 and L/240 |
Formula
- Tributary width: t = spacing
- Line load on one joist: w = (DL + LL) × t
- Simple span UDL moment: M = wL² / 8
- Simple span UDL shear: V = wL / 2
- Section modulus: S = b d² / 6
- Second moment: I = b d³ / 12
- Bending stress: fb = M / S
- Shear stress (rectangular): fv = 1.5V / (b d)
- Deflection (simple span UDL): δ = 5wL⁴ / (384EI)
- Bearing stress: fc⊥ = reaction / (b × bearing)
How to Use This Calculator
- Select your units, material, and joist size.
- Enter the clear span and the joist spacing.
- Set live and dead loads using your local code or project requirements.
- Pick deflection limits (common: L/360 live and L/240 total).
- Click Calculate to see pass/fail, governing check, and utilization.
- Use Download CSV or Download PDF to save the results.
FAQs
1) What does joist spacing change in the results?
Spacing changes tributary width. Wider spacing increases the line load carried by each joist, raising moment, shear, and deflection. Reducing spacing often improves both strength and stiffness checks.
2) What is a typical live load for a residential floor?
Many residential rooms use 40 psf live load, while sleeping areas may be lower in some codes. Always follow your local code requirements and any project specifications for the intended occupancy.
3) Why are there two deflection limits?
Designers often check live-load deflection for “feel” and cracking control, and total-load deflection for long-term sag. Common defaults are L/360 live and L/240 total, but requirements vary.
4) Can I use this for cantilevered joists?
Cantilevers change internal forces compared to a simple span. This tool reports a warning if a cantilever is entered and checks the main simple-span segment only. For cantilevers, use a dedicated analysis method.
5) Why are the wood strength values labeled “typical”?
Allowable stresses depend on the grading agency, specific species group, adjustment factors, and code. The presets are convenience starting points. For final design, use stamped values and proper adjustment factors.
6) Does passing mean my floor is code compliant?
No. Floors may also require checks for lateral bracing, bearing details, vibration performance, fire protection, and connections. Treat this as an educational calculator and verify with a qualified professional.