Joist Span Calculator

This tool is a lookup, not a design. It reads the maximum allowable floor-joist span straight from the published span tables in the International Residential Code — Table `R502.3.1(1)` for sleeping rooms at a `30` psf live load and Table `R502.3.1(2)` for living areas at `40` psf, both at a `10` psf dead load and an `L/360` deflection limit. Those tables are derived from the American Wood Council's Span Tables for Joists and Rafters. Pick the species group stamped on your lumber, the grade, the nominal size, and the on-center spacing, and the tool returns the exact span the code table prints — nothing is calculated from bending or deflection formulas.

Four things move the number. Deeper joists span farther, so a `2x12` reaches well past a `2x6`. Tighter spacing spans farther, which is why `12` inches on center beats `24`. Higher grades and stiffer species span farther — Select Structural tops `No. 2`, and Douglas Fir-Larch generally edges out Spruce-Pine-Fir. And a `30` psf sleeping-room floor is allowed a longer span than the same joist under a `40` psf living-room load. The span shown is the clear distance the joist carries between supports; it does not include bearing.

Read the grade stamp on the actual board — species and grade are the most common source of error. This helper covers uniform-load cases only; heavy point loads such as a stone-tile floor, a tub parallel to the joists, or a knee wall fall outside the table and need a separate check. Structural work is safety-critical: confirm every span with your local building department or a licensed engineer before you frame.