Floor Joist Span Calculator
The maximum floor joist span depends on lumber size, spacing, species, and live load. This calculator uses simplified AWC span table values for Douglas Fir-Larch #2 — the most common framing lumber. Always verify with your local building department.
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How to Use This Calculator
- 1Enter your measurements in the fields above. Use the units shown next to each field (ft, in, etc.).
- 2Click Calculate to get your material estimate instantly.
- 3Review the highlighted result — it shows the key quantity you need to order.
- 4Use Copy Results to save your estimate, or bookmark the URL — your inputs are encoded in the link.
Pro tip: Always add 10% to your material order to account for waste, cuts, and defects.
Frequently Asked Questions
What is the maximum span for a 2×10 floor joist at 16" OC?
For Douglas Fir-Larch #2 at 16" OC with a 40 PSF live load, the maximum span for a 2×10 is approximately 19.3 feet. At 12" OC, that extends to 21.2 feet.
How do I know if my floor joists are adequate?
Check the span against AWC span tables for your lumber species and size. The joist must span from one bearing point to the next without exceeding the tabulated value. Signs of undersized joists: floor bounce, squeaking, or visible sag.