Joist Span Calculator
Check a floor joist against the standard span table: pick the lumber size, species and spacing, enter your span, and see the maximum the table allows.
Spans from the standard table
Floor joist spans come from a table, not a formula — the code's span tables fold bending strength and the bounce limit (deflection no more than the span divided by 360) into one number per size, species, grade and spacing. This calculator carries the simplified table for the common case: residential living-area floors at 40 lb per square foot live load and 10 lb dead load, #2 grade lumber — the same basis as IRC Table R502.3.1(2), the table most local codes adopt or adapt.
Maximum span = the table value for your joist size × species & grade × spacing · Your span passes if it is no longer than the table value
Example: a 2 × 10 Douglas fir-larch #2 joist at 16 in centers may span 15 ft 7 in — so a 14 ft span passes with 19 in to spare, while a 16 ft span is over the table and wants the next size up or tighter spacing.
This is a planning aid, not a design. Spans are measured between the faces of supports, and the table assumes dry, single-span joists carrying floor loads only — not tubs, tile showers, stone counters or the point loads of a kitchen island party. Sleeping areas, decks, ceiling joists and every multi-span or engineered case use different tables. Your local code (which may amend the national table) and, for anything beyond the plain case, an engineer govern the real build. When a span fails, the table also shows the way out: one size deeper buys far more span than a grade upgrade, and tightening spacing from 24 to 16 in is the cheap fix nobody regrets.
Frequently asked questions
How far can a 2 × 8 joist span?
At 16 in centers in Douglas fir-larch #2, the table allows 12 ft 9 in for a living-area floor; Southern pine #2 allows 11 ft 10 in, spruce-pine-fir #2 allows 12 ft 3 in. At 12 in centers each gains roughly a foot and a half. The calculator checks your exact combination against the table.
What does 'on center' spacing change?
Spacing is how the load is shared: at 12 in centers each joist carries a third less floor than at 16 in, and half of what it would at 24 in. That sharing is worth real span — the same 2 × 10 that fails a 16 ft span at 24 in centers passes it easily at 12 in. It is often cheaper to add joists than to deepen them.
Is #2 grade lumber the standard assumption?
Yes — #2 is the grade stamped on most framing lumber at a typical yard, and the table here is built on it. Select structural and #1 span further (the full code table lists them), #3 less. The grade is printed on the stamp on every board; mixed bunks happen, so build to the lowest stamp you actually receive.
Why does my span fail when the old floor 'feels fine'?
The table checks two things: outright strength, and stiffness — the L/360 bounce limit, which usually fails first and is about feel and cracked finishes, not collapse. Older floors were often framed to looser standards and simply live with the bounce. Any new work, permit or no, is judged against the current table.
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