Floor joist span is read from the IRC table for your lumber, size and spacing: a No. 2 Douglas fir-larch 2x10 at 16 in on center spans 15 ft 7 in under a 40 psf living-area floor. Enter your span above and the calculator tells you whether the joist passes, by how much, and which size to move to if it does not.
How the floor joist span is found
The 2021 International Residential Code prints two floor joist span tables in Section R502.3. Table R502.3.1(1) is for sleeping rooms and attics reached by a fixed stair (30 psf live load), and Table R502.3.1(2) is for every other room (40 psf live load). Both assume a dead load of 10 or 20 psf and limit sag to span divided by 360. Each row gives the longest joist span for one species, grade and spacing, in sizes from 2x6 to 2x12.
Sag limit: L ÷ 360. Longest span from stiffness alone: L = cube root of (384 × E × I ÷ (5 × w × 360)), where w is the load in pounds per inch of joist.
Take the default: a Douglas fir-larch No. 2 2x10 (E = 1,600,000 psi, I = 98.9 in⁴) at 16 in on center under 40 psf of live load carries 4.44 lb per inch. Stiffness alone allows about 16 ft 5 in. The table prints 15 ft 7 in because it also checks bending and shear at 50 psf total load, and the lower number wins. At 15 ft 7 in the joist may sag 187 in ÷ 360 = 0.52 in.
Worked example: a 13 ft floor
A 13 ft 0 in joist span, No. 2 Douglas fir-larch at 16 in on center, living-area floor, 10 psf dead load:
- 2x8 spans 12 ft 9 in, so it is 3 in short and fails.
- 2x10 spans 15 ft 7 in, so it passes with 2 ft 7 in to spare.
- 2x12 spans 18 ft 1 in, which is more depth than you need.
Floor joist span chart: Douglas fir-larch No. 2, living areas
From IRC 2021 Table R502.3.1(2), 40 psf live load, 10 psf dead load, L/360. The calculator shows the same chart for every species, grade and load.
| Joist spacing, o.c. | 2x6 span | 2x8 span | 2x10 span | 2x12 span |
|---|---|---|---|---|
| 12 in | 10 ft 9 in | 14 ft 2 in | 18 ft 0 in | 20 ft 11 in |
| 16 in | 9 ft 9 in | 12 ft 9 in | 15 ft 7 in | 18 ft 1 in |
| 19.2 in | 9 ft 2 in | 11 ft 8 in | 14 ft 3 in | 16 ft 6 in |
| 24 in | 8 ft 3 in | 10 ft 5 in | 12 ft 9 in | 14 ft 9 in |
Floor joist spacing
Closer joists carry more span. A 2x10 goes from 12 ft 9 in at 24 in on center to 15 ft 7 in at 16 in and 18 ft 0 in at 12 in. Sixteen inches is the usual choice because it matches 4 ft sheet goods and leaves a floor that does not bounce. Moving from 16 in to 12 in adds 2 ft 5 in of span for a 2x10 and takes a third more joists.
Bedrooms, living areas and heavy floors
Sleeping rooms use 30 psf of live load, so the same 2x10 at 16 in spans 17 ft 5 in in a bedroom against 15 ft 7 in in a living room. A tile floor on a mortar bed or gypcrete raises the dead load from 10 to 20 psf and shortens that 2x10 to 14 ft 3 in. Use the 20 psf choice for any heavy finish.
Species and grade
At 16 in on center, a No. 2 2x10 spans 15 ft 7 in in Douglas fir-larch, 15 ft 5 in in spruce-pine-fir, 15 ft 2 in in hem-fir and 14 ft 0 in in Southern pine. Grade matters more: a 2x8 of Douglas fir-larch at 16 in spans 13 ft 7 in in Select Structural, 13 ft 1 in in No. 1, 12 ft 9 in in No. 2 and only 9 ft 9 in in No. 3, which is not a floor grade you want to rely on.
Floor truss span
Floor trusses and I-joists are not in this table. A floor truss span comes from the manufacturer's layout for your loads, and trusses are usually sold with the layout drawn for the house. Use the span chart here for solid sawn lumber only.
Floor beam span
The joists need something to bear on. For a girder or a header under the floor, use the beam span calculator for sawn lumber or the LVL beam calculator for an engineered floor beam, and check its sag with the beam deflection calculator.
Canada, the UK, Australia and New Zealand
These tables are the US IRC. Span tables for Canada (CSA O86 and the National Building Code), UK C24 timber and Australian and New Zealand joists (AS 1684, NZS 3604) are separate documents with different loads and grades, and none of them is included here. Use your local tables for those.
Next questions
- Decking is different: the deck joist span calculator uses wet-service deck tables, where a 2x10 of Douglas fir-larch at 16 in spans 13 ft 7 in.
- The joists above an attic or a ceiling use lighter loads: see the ceiling joist span calculator.
- An opening in the floor or wall needs a header size.
- Sheet the floor with the plywood calculator and price the finish with the flooring cost calculator.
- All the span and beam tools are in the framing and lumber calculators.
The IRC tables are for ordinary uniform floor loads and uncut joists. Have an engineer or your building department verify anything with notches, holes, cantilevers or heavy point loads.