A 2x10 Douglas fir-larch No. 2 floor joist at 16 inches on center, carrying the 40 psf live load of a living area and 10 psf of floor weight, can span 15 ft 7 in. Stiffness alone would allow 16 ft 5 in, but this grade's bending strength runs out first. A 2x12 at the same spacing reaches 18 ft 1 in. The calculator above gives the maximum span for four common species, three grades, five sizes and four spacings, checks your span and counts the joists.
How the calculator works
Choose the lumber, the spacing and the loads, then enter the span between supports. The result is the maximum span and what limits it, the smallest size that works for your span, a table of all sizes and spacings, and the joists to buy.
The method
Spans follow the American Wood Council's Span Tables for Joists and Rafters, the basis of the floor joist tables in the building codes. A joist is a single span with a uniform load. Stiffness limits the span so that the sag under the live load, 5·w·L⁴ ÷ (384·E·I), stays under L/360, the usual limit for floors; strength limits it to √(8·Fb·S ÷ w) under the full dead plus live load. The calculator checks both and reports the shorter. It reproduces tabulated AWC spans to the inch: a 2x8 with E of 1.6 million psi at 16 in spans 12 ft 10 in and needs an Fb of 1,255 psi, exactly as in AWC Table F-1.
Design values
Fb and E come from AWC's Design Values for Joists and Rafters (Table W-1), with the size factor and the repetitive member factor already included. Floors use the normal (ten-year) load duration. Here is how species compare for a 2x10 No. 2 at 16 in, 40 + 10 psf, L/360:
| Species | Fb, psi | E, psi | Maximum span |
|---|---|---|---|
| Douglas fir-larch | 1,140 | 1,600,000 | 15' 7" |
| Spruce-pine-fir (No. 1/No. 2) | 1,105 | 1,400,000 | 15' 4" |
| Hem-fir | 1,075 | 1,300,000 | 15' 2" |
| Southern pine | 920 | 1,400,000 | 14' 0" |
Loads
The IRC sets 40 psf as the live load for living areas and 30 psf for sleeping rooms (Table R301.5). Dead load is the weight of the floor itself: 10 psf for a typical wood floor with subfloor, finish and a drywall ceiling below, 20 psf for tile on a mortar bed or other heavy finishes. Decks, garages and storage areas carry more; use the load your code or engineer gives.
Count and lumber
Joists across a floor are the floor length divided by the spacing, plus one: a 24 ft floor at 16 in takes 19 joists. Each joist is cut from the next even lumber length that covers the span; with bearing at both ends a 14 ft span needs 14 ft or 16 ft boards. Doubled joists under walls, rim joists and blocking come on top.
Common mistakes
- Reading the E column only. A stiff joist of a weak grade can fail in bending before it sags too much. The calculator checks both.
- Counting from the outside of the floor. Spans are between supports: from the face of one wall or beam to the next.
- Heavy finishes on a 10 psf design. Stone tile on a mortar bed needs the 20 psf dead load and a stiffer floor.
- Bouncy floors at the limit. L/360 is a code minimum; for tile or a stiffer feel use a deeper joist or closer spacing.
For rafters, use the Rafter Span Calculator.