Joist span calculator
Maximum allowable floor joist spans per IRC Table R502.3.1(1). Residential floors — 40 psf live load, 10 psf dead load.
| Size | 12" o.c. | 16" o.c. | 19.2" o.c. | 24" o.c. |
|---|---|---|---|---|
| 2x6 | 12'-6" | 11'-4" | 10'-8" | 9'-10" |
| 2x8 | 16'-6" | 15'-0" | 14'-1" | 13'-0" |
| 2x10 | 21'-0" | 19'-1" | 17'-11" | 16'-6" |
| 2x12 | 25'-7" | 23'-3" | 21'-10" | 20'-1" |
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The IRC (International Residential Code) provides maximum allowable spans for floor joists based on species, grade, size, and spacing. These tables assume a residential floor loading of 40 psf live load plus 10 psf dead load.
The span is measured as the clear distance between supports — wall to beam, beam to beam, or wall to wall. It does not include the bearing length on each end.
Common Configurations
- 2x8 @ 16" o.c. — Typical for spans up to 13-14 feet
- 2x10 @ 16" o.c. — Standard for most residential floors, spans to 17-19 feet
- 2x12 @ 16" o.c. — For longer spans up to 22+ feet
- 12" o.c. spacing — Increases span but uses more lumber
- 24" o.c. spacing — Reduces lumber but limits span significantly
Can I use these tables for deck joists?
No. Deck joists have separate span tables per IRC R507.5 that account for different loading conditions and exposure to weather. Deck joist spans are generally shorter than interior floor joist spans for the same size lumber.
FAQ
How far can a 2x10 floor joist span?
It depends on species, grade, and spacing. A No. 2 Douglas Fir 2x10 at 16 inches on center can span about 16 feet 1 inch for a 40 psf live load floor. At 12 inches on center, it reaches about 17 feet 8 inches. Always check the IRC span tables for your specific lumber.
What is the difference between 16 and 24 on center spacing?
16 inches on center (OC) means joist centers are 16 inches apart — this is standard for most residential floors. 24 OC uses fewer joists but reduces maximum span and can cause subfloor bounce. Most building codes allow 24 OC only with thicker subfloor sheathing (3/4 inch minimum).
What does 'on center' mean?
On center (OC) is the distance measured from the center of one joist to the center of the next. Standard spacings are 12, 16, 19.2, and 24 inches OC. The spacing affects both the maximum span and the subfloor thickness required.