Joist Span Chart

Floor joist spans depend on lumber size, spacing, species, and load. These are typical maximum spans for No. 2 grade softwood under a 40 psf live load at 16-inch spacing — always confirm against your local code.

Joist size Max span at 16 in OC
2×6 about 9 ft 9 in
2×8 about 12 ft 10 in
2×10 about 16 ft 5 in
2×12 about 19 ft 11 in
Spacing matters. Tightening to 12 in OC adds roughly 10% span; opening to 24 in OC drops it 10 to 15%. Species and grade shift these numbers, so verify with official span tables.

These figures are estimates for planning. A structural professional or code official should confirm any load-bearing design.

FAQ

Does joist spacing affect how far a joist can span? Yes. Closer joist spacing, like 12 inches on-center versus 16 or 24 inches, allows a given joist size to span farther or carry more load, since each joist supports a narrower strip of floor or ceiling.

See the Beam Span Chart and the Header Span Chart.

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The calculators and tools on Formula Factory are provided for general guidance and informational purposes only. Results are estimates based on standard formulas and the values you enter — they do not constitute professional engineering, electrical, or architectural advice. Always verify calculations with a qualified professional before making decisions for any safety-critical, code-compliance, or commercial application. Formula Factory makes no representations or warranties as to the accuracy or completeness of any result, and accepts no liability for errors, omissions, or any outcomes arising from reliance on this information.