How to use the floor joist span calculator
- Species and grade: read them from the grade stamp. No. 2 is the most common framing grade. Southern pine is stamped SP or SYP, and spruce-pine-fir is stamped S-P-F.
- Size and spacing: 16" O.C. is standard, and 19.2" and 24" are common with engineered subfloor.
- Dead load: use 10 psf for a normal wood floor. Use 20 psf for heavy finishes like tile on a mortar bed, stone, or a plaster ceiling below.
- Your span: the clear distance between supports, such as foundation wall to center beam. The result shows whether it fits and the smallest joist that would at each spacing.
Example
No. 2 Douglas fir-larch 2×10s at 16" O.C. with a 10 psf dead load can span 15'-7". In No. 2 SPF the same joist spans 15'-5", so a 14' span has 1'-5" to spare. Move to 24" O.C. and SPF 2×10s drop to 12'-7", so you'd need 2×12s.
What "span" means here
The table gives the clear span between the faces of supports, measured horizontally. Joists also need bearing at each end. The IRC requires at least 1 1/2" on wood or steel and 3" on masonry or concrete. A joist running continuously over a center beam is still sized for each span on its own.
When you need more than the table
- Bearing walls above or point loads from posts need extra joists, blocking or a beam sized by the code or an engineer.
- Openings for stairs and chimneys need doubled headers and trimmers per IRC R502.10.
- Cantilevers are limited to the joist's depth unless they meet Table R502.3.3(1) or (2).
- Engineered joists (I-joists, LVL, trusses) use the maker's span tables, not this one.
FAQ
How far can a 2x10 floor joist span?
At 16" O.C. with a 10 psf dead load, No. 2 2×10s span 15'-7" in Douglas fir-larch, 15'-5" in SPF, 15'-2" in hem-fir and 14'-0" in southern pine, per IRC Table R502.3.1(2).
How far can a 2x12 floor joist span?
No. 2 Douglas fir-larch 2×12s at 16" O.C. span 18'-1" with a 10 psf dead load. SPF spans 17'-10", hem-fir 17'-7" and southern pine 16'-6".
Why does southern pine span less than SPF?
Design values for No. 2 southern pine were lowered in 2013, so newer code tables give it shorter floor spans than older tables did. Use the table from the code edition your town has adopted.
What is the difference between 30 psf and 40 psf joist tables?
The IRC allows a 30 psf live load for sleeping rooms and attics, and 40 psf for other living areas. This calculator uses the 40 psf table, which is conservative for bedrooms.