Floor Joist Count & Span Calculator
Count the floor joists for a room and check whether your joist size spans the distance at your chosen on-center spacing.
How it works
joists = ceil(run×12 ÷ spacing) + 1 · lumber = joists × span · compare span to the max allowable for the joist size and spacing
The joist count follows on-center spacing along the run: divide the run in inches by the spacing and add one for the joist at the far end. The span — how far each joist reaches unsupported — must stay under the allowable maximum for its size, which drops as spacing widens and grows with a deeper joist. The built-in limits are for SPF #2 at 40 psf live load and an L/360 deflection limit, typical for a residential floor.
Worked example
A 16 ft run of 2×10 joists at 16 in on-center spanning 12 ft: ceil(192 ÷ 16) + 1 = 13 joists, using 13 × 12 = 156 linear ft. A 2×10 at 16 in o.c. spans about 16 ft, so a 12 ft span passes comfortably.
Sources: AWC Span Calculator
Frequently asked questions
Is the run or the span what limits joist size?
The span does. The run just sets how many joists you buy; the span is the unsupported distance each joist bridges, and it is what deflection and strength tables limit. Two rooms with the same run can need different joists if one spans farther.
Why is 16 inches on-center so common?
It divides evenly into the 48-inch width of plywood and OSB, so sheet edges land on joists with no cutting, and it balances lumber cost against stiffness. Twelve-inch spacing stiffens a floor for tile or stone; 24 inches saves lumber where a deeper joist allows it.
What if my span is too long?
Step up to a deeper joist, tighten the spacing, switch to engineered I-joists or LVL, or add a mid-span beam and posts to cut the span in half. Any of these is a code-driven decision — confirm it against the current span tables for your species and load.
Related calculators
Estimates only — add for waste and confirm every load-bearing decision against local building codes and a design professional.