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Joist Span Calculator

The actual IRC span table, as a pass/fail check.

Every "can I span 14 feet with 2×10s?" question is a table lookup — but the table is a wall of numbers. This calculator turns IRC Table R502.3.1(2) into a direct answer: pick size and spacing, enter your span, get PASS or FAILS with the maximum shown. Values are Douglas Fir-Larch #2 at 40 psf live / 10 psf dead with L/360 deflection — the standard living-area floor case.

Why people use this

Deck builders, basement finishers, and anyone staring at an undersized floor use this: the IRC table answer to "will 2×10s at 16 inches cover 14 feet" without flipping through code tables. The pass/fail verdict is the feature — max span alone makes people do interpolation math in their heads.

How it works

1. Pick the joist size (2×6 through 2×12) and spacing (12", 16", or 19.2" on center).
2. Enter the clear span you need to cover.
3. Read the maximum allowable span and the pass/fail verdict.
4. If it fails, tighten spacing or size up — or consider engineered joists for long spans.

Common questions

How far can a 2×10 floor joist span?

Douglas Fir-Larch #2 at 16" on center, 40 psf live load: 15'-5" maximum per IRC Table R502.3.1(2). At 12" spacing it stretches to 17'-9"; at 19.2" it drops to 14'-1". Southern Pine runs longer, SPF shorter — check the stamp on your lumber.

Do these spans work for decks?

No — deck joists use different tables (IRC R507) because exterior decks carry different loads and get no deflection credit from a ceiling below. Tile floors, hot tubs, and bearing walls from above also change the residential floor math. When the answer matters, your local code edition and building department control.

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