Beam & Joist Span Calculator
A quick reference for common residential floor-joist spans. Pick a member size and spacing to see the maximum allowable span — then verify with an engineer.
Enter a target span to highlight which member sizes can reach it at the chosen spacing.
Preliminary reference only — based on common IRC residential floor-joist spans for SPF #2 lumber at 40 psf live / 10 psf dead, L/360 deflection. Not a substitute for engineered design. Confirm species, grade, loads, and spans with a structural engineer and your local building official.
Understanding Floor-Joist Spans
A joist's maximum span depends on its depth, the spacing between joists, the wood species and grade, and the loads it carries. Deeper members and tighter spacing both let you span farther. The values here reflect common IRC residential floor joists in SPF #2 lumber under a typical 40 psf live load and 10 psf dead load, limited to an L/360 deflection so floors feel stiff underfoot.
Use this table to get in the right ballpark while planning, but treat it as a starting point rather than a final answer. Real projects involve point loads, cantilevers, openings, bearing details, and local amendments that a span table can't capture. Always confirm your final joist or beam sizing with a structural engineer and your building official before you build.
Frequently Asked Questions
How far can a 2x8 floor joist span?
For SPF #2 lumber at 40 psf live / 10 psf dead and L/360 deflection, a 2x8 floor joist spans about 14 feet 2 inches at 12 inches on center, 12 feet 10 inches at 16 inches on center, and 10 feet 9 inches at 24 inches on center. Verify locally.
What size joist do I need for a 16-foot span?
For a clear span around 16 feet under typical residential floor loads, a 2x10 at 12 inches on center (about 18 ft) or 16 inches on center (about 16 ft) works in SPF #2, and a 2x12 gives extra margin. Confirm with an engineer for your exact loads.
Are these spans code-approved?
No — these are reference values only, drawn from common IRC residential floor-joist span tables for one species, grade, and load case. They are not a code approval or an engineered design. Always verify the correct span for your conditions with your local building official and a structural engineer.
Turn Your Measurements Into a Real Floor Plan
Once you know your dimensions, generate a labeled, to-scale floor plan in seconds — then export it to DXF or build it in 3D. No CAD experience needed.
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