Joist Span Calculator
Calculate the maximum allowable span for floor joists by lumber size, species and spacing - to IRC code.
The joist span calculator returns the maximum allowable span for floor and deck joists per IRC R502.3.1. Span is the unsupported distance between bearing points - a joist resting on a sill plate at one end and a girder at the other has a span equal to that distance, not the total joist length. Standard residential floor loading is 40 psf live + 10 psf dead. Deck loading is the same but with a wet-service adjustment that reduces allowable span by about 10%. Sleeping rooms have lower live loads (30 psf) which allows longer spans. Always check your local code - some jurisdictions adopt the IRC with amendments, and your inspector has the final say.
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Frequently asked questions
Doug fir #2 2x10 at 16 in OC: 16 ft 5 in maximum span (40 psf live + 10 psf dead). SPF #2 2x10 at 16 in OC: 15 ft 2 in. For a deck (wet service), reduce by 10% - so Doug fir 2x10 at 16 in OC spans 14 ft 9 in on a deck. Sleeping room load (30 psf) allows 18 ft 1 in for Doug fir 2x10 at 16 in OC.
Joist size is the lumber dimension (2x6, 2x8, 2x10, 2x12). Span is the unsupported distance the joist covers between bearing points - the maximum length the joist can be without sagging or breaking. A 16 ft long 2x10 with a center girder has two 8 ft spans, not one 16 ft span - the center girder is a bearing point that divides the span.
Wood loses strength when wet repeatedly. The IRC wet service factor reduces allowable design values by about 10% for joists exposed to weather - which means deck joists. Doug fir 2x10 at 16 in OC spans 16 ft 5 in as a floor joist but only about 14 ft 9 in as a deck joist. Always use pressure-treated lumber for deck joists, and oversize when possible to allow for long-term moisture cycling.
Doug Fir-Larch and Southern Yellow Pine have the longest spans among common dimensional lumber - both are ~10% stiffer than SPF. SPF (Spruce-Pine-Fir) is the most commonly available in northern markets and Canada; Doug Fir is common in the western US; Southern Pine is common in the southeast. For pressure-treated outdoor use, Southern Pine is the dominant species. Always design to your locally available species.
Yes for some spans - Doug fir 2x10 at 24 in OC handles 14 ft 2 in for floor loads. But 16 in OC is far more common because it allows shorter subfloor spans (3/4 in OSB or plywood spans 24 in OC with minor deflection issues; 16 in OC is rock solid). 24 in OC saves about 30% on joist count and lumber cost but the floor will feel more bouncy. Engineered I-joists make 24 in OC feasible without bounce.