Span lookup
Look up an allowable span
Pick a member and load case, species, grade, size, and spacing to read the maximum span straight from the published table - with the source cited and a diagram of the geometry.
Not sure which grade or load case to pick? How to read a span table covers each of the five choices this tool asks for, and how far a 2x8 joist spans shows how much the answer moves when you change just one of them. If the species and grade choices are the sticking point, lumber species and grades explained covers what the groups and stamps mean and why the ranking between them shifts with grade.
Maximum allowable span
12′-9″
A 2x8 Douglas Fir-Larch no. 2 floor joist at 16″ o.c. may span up to 12′-9″ under this table's load case (40 psf live, 10 psf dead, L/360).
Source: International Residential Code (IRC), 2018 - Table R502.3.1(2) - Floor Joist Spans, 40 psf Live Load. Published by International Code Council.
Edition: These values are transcribed from the IRC 2018 tables. The ICC has since published IRC 2021 and IRC 2024, and the edition in force varies by jurisdiction - confirm the table and edition adopted by your local building department before relying on a span.