How to calculate floor joists
The number of joists depends on how far they spread along the floor and how tightly they're spaced:
Joists = (floor length × 12 ÷ on-center spacing) + 1
The “+ 1” is the joist at the far end. Each joist spans the other dimension, and two rim (band) joists cap the ends, running the full length. On top of the field joists you add doubled joists under partition walls and around stair or floor openings, plus mid-span blocking.
Approximate joist spans
Rough maximums for ordinary residential floors at 16″ on-center — always confirm with a span table or local code, since species, grade, and load change the answer.
| Joist size | Approx. max span (16″ OC) |
|---|---|
| 2×6 | ~9 ft |
| 2×8 | ~12–13 ft |
| 2×10 | ~16 ft |
| 2×12 | ~18 ft |
Good to know
- Spacing changes stiffness. 16″ on-center is standard; drop to 12″ for tile or bouncy floors, and 24″ where the span table and code allow.
- Block long joists. Add blocking or cross-bridging at mid-span on runs over ~8 ft to stop twisting.
- Double up under loads. Joists under parallel walls, and the trimmers around openings, are doubled — extra boards beyond the count here.
- Hang it right. Joist hangers and the correct structural fasteners matter as much as the joists themselves.