Joist & Rafter Span Calculator

The tool reads a simplified IRC-style allowable span for your lumber size, spacing, and species, then adjusts it for loads above or below the 50 psf baseline using a square-root load factor. The result is the clear distance between supports.

No. It is a planning estimate only. Allowable spans depend on grade, deflection limits, snow and wind zones, and connection details. Always verify the final span against your local building code and a licensed structural engineer or building official.

Residential floors typically use 40 psf live plus 10 psf dead, so 50 psf total is a common default. Decks, roofs, and snow regions use different values.

Both are sized the same way for bending, but rafters carry sloped roof and snow loads while joists carry flat floor loads. Use the appropriate load and confirm rafter sizing against a roof framing table.

Stiffer, stronger species and higher grades allow longer spans, so the calculator's species factor nudges the base table value up or down.

At tighter spacing, each joist carries less area and can span farther. Moving from 24 to 16 in on center usually adds about 10 to 20% of span.

No. Laminated veneer lumber (LVL), microllam, and I-joists have their own manufacturer tables. This calculator only applies to sawn dimensional lumber of grade No. 2.