TradesHow this works
Pick the species, joist size, and spacing you have in mind and the clear span you need to cover. The tool looks up the maximum allowable span from the IRC living-areas table and tells you whether your member makes it, with the margin. If it comes up short, it lists the lighter members that do span it, ranked so the least lumber comes first.
Use it to narrow choices fast, not to sign off a floor. The building official enforces a specific code edition, and beams, headers, cantilevers, and engineered lumber are a different calculation. Measure the clear span face of support to face of support.
Common questions
What load does this use?
It uses the IRC living-areas table, R502.3.1(2): 40 pounds per square foot live load and 10 pounds dead, No. 2 grade lumber, at an L/360 deflection limit. That covers general rooms. Bedrooms and attics use a lighter sleeping-area table, which this tool does not cover.
Why do different species span differently?
Stiffness and strength vary by species. At the same size, Southern Pine and Douglas Fir-Larch reach farther than Spruce-Pine-Fir or Hem-Fir. So the same 2x10 might pass in one species and fall short in another, which is why the tool asks for species.
Can I use this to sign off a floor?
No. This is a planning lookup to narrow your options quickly. The building official enforces a specific code edition and may have local amendments, and anything beyond a simple repetitive floor joist (beams, headers, cantilevers, point loads, engineered lumber) needs the full table or a design professional.
Sources: IRC Table R502.3.1(2) floor joist spans, living areas 40 psf live / 10 psf dead, No. 2 grade, L/360. Four common species. Never a structural certification. (IRC). Engine version 1.0.0. Risk tier 2.