Floor Joist Span Calculator
Find maximum allowable floor joist spans based on IRC residential span tables. Select joist size, spacing, lumber species, and live load to verify your joist layout meets code.
Typical: 10 psf for standard floor assembly
Results
Max Allowable Span
15.0
feet
Span Check
PASS
14 ft vs 15.0 ft max
Deflection (L/360)
PASS
0.291" vs 0.467" max
Max Span Comparison — Douglas Fir-Larch #2 at 40 psf
| Joist Size | 12" OC | 16" OC | 24" OC |
|---|---|---|---|
| 2x6 | 10.0 ft | 9.0 ft | 7.5 ft |
| 2x8 | 13.0 ft | 12.0 ft | 10.0 ft |
| 2x10 | 16.5 ft | 15.0 ft | 12.5 ft |
| 2x12 | 20.5 ft | 18.5 ft | 15.5 ft |
How to Use This Calculator
Select the lumber species and grade, joist size, on-center spacing, and the design live load. Enter your actual planned span to see if it passes. The calculator checks both the IRC span table and L/360 deflection limit for live loads. Use the comparison table to quickly compare all sizes and spacings.
Formula Used
Max span from IRC Table R502.3.1. Deflection check: delta = 5wL^4 / (384EI). Allowable deflection = L/360 for live loads. I = bd^3/12 where b=1.5 in (nominal 2x). E varies by species.
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Disclaimer
This calculator is for estimation purposes only. Results may vary based on local conditions, materials, and building codes. Always consult a licensed professional before making decisions based on these calculations. MyContractorTools is not responsible for any errors or omissions.