Floor Joist Calculator

About this calculator

Floor joists are spaced at a fixed on-center distance (commonly 12, 16, or 24 inches) to distribute the floor's load evenly across the framing, so the number of joists needed for a given span is found by dividing the span length by that spacing and adding one for the starting joist at the edge. This calculator applies that standard framing formula to your entered span and spacing to estimate the joist count for a floor section.

It's a routine planning step for carpenters, framers, and DIY builders ordering lumber for a deck, subfloor, or addition before construction begins, giving a quick material estimate that can then be checked against local building code span tables, which also factor in joist size and load requirements.