Floor Joist Span Calculator
How far floor joists span: enter the species, spacing and span, and see the smallest 2x joist that works, with the longest span of every size at 12, 16 and 24 in apart, for bedrooms, living areas or your own load.
2x10 joists 16 in apart work for a 14′ span, using 80% of their capacity (limited by bending).
Longest span, Douglas Fir-Larch No.2
| Joist | 12 in apart | 16 in apart | 24 in apart |
|---|---|---|---|
| 2x6 | 10′ 8″ | 9′ 8″ | 8′ 2″ |
| 2x8 | 14′ 1″ | 12′ 9″ | 10′ 5″ |
| 2x10 | 18′ | 15′ 7″ | 12′ 8″ |
| 2x12 | 20′ 10″ | 18′ 1″ | 14′ 9″ |
Spans in bold reach 14′.
At a 14′ span, 16 in apart
| 2x6Fails live load deflection (span/360) | 297% |
| 2x8Fails live load deflection (span/360) | 130% |
| 2x10Works | 80% |
| 2x12Works | 60% |
Every check for the 2x10
| Bending | 80% |
| Shear | 28% |
| Live load deflection (span/360) | 62% |
| Total load deflection (span/240) | 52% |
Check the 2x10 in the beam designer
10 psf dead and 40 psf live load on a floor of Douglas Fir-Larch No.2, dry indoors. The beam designer link checks one joist without the repetitive member factor, so it allows a little less.
Floor joist span tables
The longest span for No.2 floor joists of each species under 40 psf live and 10 psf dead load. Worked out by the calculator, not copied from the code tables.
Douglas Fir-Larch No.2
| Joist | 12 in apart | 16 in apart | 24 in apart |
|---|---|---|---|
| 2x6 | 10′ 8″ | 9′ 8″ | 8′ 2″ |
| 2x8 | 14′ 1″ | 12′ 9″ | 10′ 5″ |
| 2x10 | 18′ | 15′ 7″ | 12′ 8″ |
| 2x12 | 20′ 10″ | 18′ 1″ | 14′ 9″ |
Southern Pine No.2
| Joist | 12 in apart | 16 in apart | 24 in apart |
|---|---|---|---|
| 2x6 | 10′ 3″ | 9′ 3″ | 7′ 7″ |
| 2x8 | 13′ 6″ | 11′ 9″ | 9′ 7″ |
| 2x10 | 16′ 2″ | 14′ | 11′ 5″ |
| 2x12 | 19′ | 16′ 6″ | 13′ 5″ |
Hem-Fir No.2
| Joist | 12 in apart | 16 in apart | 24 in apart |
|---|---|---|---|
| 2x6 | 10′ | 9′ 1″ | 7′ 11″ |
| 2x8 | 13′ 2″ | 11′ 11″ | 10′ 1″ |
| 2x10 | 16′ 9″ | 15′ 1″ | 12′ 4″ |
| 2x12 | 20′ 3″ | 17′ 7″ | 14′ 4″ |
Spruce-Pine-Fir No.1/No.2
| Joist | 12 in apart | 16 in apart | 24 in apart |
|---|---|---|---|
| 2x6 | 10′ 3″ | 9′ 3″ | 8′ 1″ |
| 2x8 | 13′ 6″ | 12′ 3″ | 10′ 3″ |
| 2x10 | 17′ 2″ | 15′ 4″ | 12′ 6″ |
| 2x12 | 20′ 7″ | 17′ 10″ | 14′ 6″ |
Spans are between supports. For bedrooms (30 psf live load), enter 30 above.
How the calculator works out a joist span
Each joist is a simple span checked to the 2018 National Design Specification (NDS) for wood, with allowable stress design (ASD). Joists 24 in apart or closer share load, so they take the repetitive member factor of 1.15. The longest span is the longest one that passes every check:
- Bending and shear under the dead and live load.
- Deflection: live load sag up to the span over 360, and total load sag up to the span over 240, the limits in the International Residential Code (IRC).
The IRC's floor joist tables (R502.3) come from the same kind of check, so its spans are close to these. A building department may ask for the code table; for a floor outside it, a calculation like this one.
Floor joist questions
How far can a 2x10 floor joist span?
Douglas Fir-Larch No.2 2x10 joists 16 in apart span 15′ 7″ under 40 psf live and 10 psf dead load; 12 in apart, 18′.
How far can a 2x8 floor joist span?
12′ 9″ at 16 in apart and 14′ 1″ at 12 in apart for Douglas Fir-Larch No.2.
How far can a 2x12 floor joist span?
18′ 1″ at 16 in apart and 14′ 9″ at 24 in apart for Douglas Fir-Larch No.2.
Can bedroom floor joists span farther?
Yes: the IRC uses 30 psf live load for sleeping rooms. Douglas Fir-Larch No.2 2x10 joists 16 in apart then span 17′ 5″.
Will a floor that passes feel bouncy?
A joist at its longest span sags the span over 360 under the full live load, which the code allows but some people feel. For a stiffer floor, or under tile or stone, use a deeper joist or set them closer.
What size beam carries the joists?
Use the wood beam span calculator, or the beam designer for steel, glulam and LVL.