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.
ft
Check against maxSpan check
19'-1" max — OK
Maximum allowable span
19'-1"
Douglas Fir-Larch #1 · 2x10 @ 16" o.c.
Douglas Fir-Larch #1 — All spans
| 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" |
Floor Joist Span Tables
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.
Span values reference IRC Table R502.3.1(1). Always verify with your local building code — jurisdictions may amend IRC tables. Not a substitute for professional engineering judgment.