Floor joist span calculator
How far a 2x6, 2x8, 2x10 or 2x12 floor joist may span under the 2021 IRC's own tables, for four common species in three grades at 12", 16", 19.2" or 24" on centre — and, when yours falls short, the deeper joist, closer spacing or better grade that would reach.
Bedrooms, and attics reached by a fixed stair, use the 30 psf table. Every other room uses 40.
From the grade stamp on the board.
The weight of the floor itself. Ask what your plans or the building department assume if you are not sure — a heavy finish like tile on a mortar bed is the usual reason for 20.
= 174"
Maximum clear span, IRC Table R502.3.1(2)
15 ft 7 in
Room to spare
1 ft 1 in
Your span
14 ft 6 in
Loads
40 live + 10 dead psf
- Clear span within Table R502.3.1(2) — passes2x10 at 16" on centre, Douglas fir-larch No. 2, 10 psf dead load: the table allows 15 ft 7 in, and 14 ft 6 in is 1 ft 1 in inside it.
Douglas fir-larch No. 2, 40 live + 10 dead psf
| Spacing | 2x6 | 2x8 | 2x10 | 2x12 |
|---|---|---|---|---|
| 12" | 10' 9" | 14' 2" | 18' 0" | 20' 11" |
| 16" | 9' 9" | 12' 9" | 15' 7" | 18' 1" |
| 19.2" | 9' 2" | 11' 8" | 14' 3" | 16' 6" |
| 24" | 8' 3" | 10' 5" | 12' 9" | 14' 9" |
Your joist is highlighted. Figures in full ink reach the span you entered; faint ones fall short.
Defaults, not law
These are the 2021 IRC's prescriptive spans, Tables R502.3.1(1) and R502.3.1(2), for dimension lumber at L/360 deflection. They are what the model code says, not necessarily what your jurisdiction adopted — ring the building department before you order the lumber, not after.
They hold only for a simple span carrying an even floor load: no bearing wall or post landing on the joists, no cantilever, no heavy load concentrated in one spot, no notches or holes beyond what the code allows, and three or more joists. Anything outside that is a design question for somebody who will stamp it. Deck joists have their own table in the code and are not these.
About floor joist spans
- How far can a 2x10 floor joist span?
- At 16" on centre in No. 2 Douglas fir-larch — the everyday floor joist — the 2021 IRC allows 15 ft 7 in clear under a living-area floor with a 10 psf dead load (Table R502.3.1(2)). The species and grade move it more than people expect: No. 2 Southern pine in the same size and spacing is allowed 14 ft, and Select structural Douglas fir-larch 17 ft 4 in. Read the grade stamp on the board rather than assuming it.
- Why does a bedroom floor get a longer span than a living room?
- Because the code designs it for less weight. The IRC takes a sleeping room at 30 psf of live load and every other room at 40, and gives each its own table — R502.3.1(1) and R502.3.1(2). The same No. 2 Douglas fir-larch 2x10 at 16" spans 17 ft 5 in under a bedroom and 15 ft 7 in under a living room. A room that might become an office or a playroom later is worth framing to the 40 psf table.
- Is a joist span measured centre to centre of the supports?
- No. The spans in the IRC tables are clear spans: from the face of one support to the face of the other, not centre to centre of the bearings and not the length of the board. The joist itself is longer by its bearing at each end. Measure the gap the joist has to jump, and put that into the calculator.
- What do I need to span 16 feet?
- Not a No. 2 Douglas fir-larch 2x10 at 16" — the table stops it at 15 ft 7 in. A 2x12 at the same spacing reaches 18 ft 1 in, or the same 2x10 at 12" on centre reaches 18 ft. The calculator runs the same comparison for any species, grade and span, and never changes the span you asked about to make a joist pass.
- Why are spruce-pine-fir No. 1 and No. 2 the same span?
- Because spruce-pine-fir No. 1 and No. 2 are graded and sold together as one combined grade, "No. 1/No. 2", with one set of design values — so the code table prints the same span for both: 15 ft 5 in for a 2x10 at 16" under a living area. The other three species list the two grades separately because their No. 1 is genuinely stronger.
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