Maximum floor joist spans for #2 lumber, all four common species, on one page. From IRC Table R502.3.1(2): living areas, 40 psf live load, 10 psf dead load, L/360. A #2 Douglas fir 2x10 at 16" on center spans 15'-7".
Douglas fir-larch, #2
Size
12" o.c.
16" o.c.
19.2" o.c.
24" o.c.
2x6
10'-9"
9'-9"
9'-2"
8'-3"
2x8
14'-2"
12'-9"
11'-8"
10'-5"
2x10
18'
15'-7"
14'-3"
12'-9"
2x12
20'-11"
18'-1"
16'-6"
14'-9"
Hem-fir, #2
Size
12" o.c.
16" o.c.
19.2" o.c.
24" o.c.
2x6
10'
9'-1"
8'-7"
7'-11"
2x8
13'-2"
12'
11'-3"
10'-2"
2x10
16'-10"
15'-2"
13'-10"
12'-5"
2x12
20'-4"
17'-7"
16'-1"
14'-4"
Southern pine, #2
Size
12" o.c.
16" o.c.
19.2" o.c.
24" o.c.
2x6
10'-3"
9'-4"
8'-6"
7'-7"
2x8
13'-6"
11'-10"
10'-10"
9'-8"
2x10
16'-2"
14'
12'-10"
11'-5"
2x12
19'-1"
16'-6"
15'-1"
13'-6"
Spruce-pine-fir, #2
Size
12" o.c.
16" o.c.
19.2" o.c.
24" o.c.
2x6
10'-3"
9'-4"
8'-9"
8'-1"
2x8
13'-6"
12'-3"
11'-6"
10'-3"
2x10
17'-3"
15'-5"
14'-1"
12'-7"
2x12
20'-7"
17'-10"
16'-3"
14'-7"
Floor joists
Max joist span—
Your span —
Joists needed—
Smallest joist that works:—
Size
Max span
From the 2021 International Residential Code, Table R502.3.1(2) (same values as the 2018 IRC): floor joists for residential living areas, 40 psf live load, L/360 deflection, dead load 10 or 20 psf, simple spans of sawn lumber. Sleeping rooms and attics may use the 30 psf table R502.3.1(1), which allows slightly longer spans, so this calculator is conservative for those rooms. Does not cover I-joists, floor trusses, decks, cantilevers, point loads, bearing walls above, or concentrated loads like a hot tub, waterbed or bathtub on a long span. Many floors are stiffer than code minimum by choice to reduce bounce.
⚠ Check all math and verify with a licensed engineer and your local building department before cutting or building. Results are estimates.
Reading the span table
Spans are the clear distance between supports, in feet and inches, from the 2021 IRC (same as 2018). They cover residential living areas only; sleeping rooms and attics use a different table, and a 20 psf dead load (tile over mortar, heavy finishes) shortens deeper joists. Select Structural and #1 lumber span farther: pick the grade in the calculator above.
These are code minimums for strength and deflection. A floor built to the limit can still feel bouncy; dropping one spacing step or going up one size makes it stiffer. Confirm with your building department before you build.
FAQ
How far can a 2x8 floor joist span?
Up to 12'-9" at 16" on center in #2 Douglas fir-larch, 12'-3" in spruce-pine-fir (IRC, living areas).
How far can a 2x12 floor joist span?
Up to 18'-1" at 16" on center in #2 Douglas fir-larch, and 20'-11" at 12" on center.
Disclaimer: Results are estimates for planning and layout only. Check all math yourself and verify with a licensed engineer and your local building department before cutting or building. FramersMath is not a substitute for engineered plans, span tables or code review. See the full disclaimer.