A 2x8 floor joist can span up to 12'-9" at 16" on center in #2 Douglas fir-larch, per IRC Table R502.3.1(2) for living areas (40 psf live load, 10 psf dead load). In spruce-pine-fir it's 12'-3"; at 24" on center it drops to 10'-5".
Species (#2 grade)
12" o.c.
16" o.c.
19.2" o.c.
24" o.c.
Douglas fir-larch
14'-2"
12'-9"
11'-8"
10'-5"
Hem-fir
13'-2"
12'
11'-3"
10'-2"
Southern pine
13'-6"
11'-10"
10'-10"
9'-8"
Spruce-pine-fir
13'-6"
12'-3"
11'-6"
10'-3"
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 chart
Spans are the clear distance between supports, from the 2021 IRC Table R502.3.1(2) (same as 2018): residential living areas, 40 psf live load, 10 psf dead load, deflection limited to L/360. Sleeping rooms and attics use a different table with longer spans; heavy finishes like tile over a mortar bed need the 20 psf dead load column, which shortens deeper joists.
Tighter spacing buys span: the same 2x8 goes 14'-2" at 12" o.c. Select Structural and #1 lumber go farther than #2, which is what most yards stock. Pick your species, grade and spacing in the calculator above, and confirm with your building department before you build.
FAQ
Can a 2x8 span 14 feet?
Not as a floor joist in #2 lumber at 16" on center under the IRC table: the limit is 12'-9" in Douglas fir-larch. Step up to a 2x10 (15'-7" at 16") or tighten spacing to 12" (14'-2").
What size joist do I need for a 13 ft span?
A #2 Douglas fir-larch 2x8 at 16" on center covers up to 12'-9" for a living-area floor. Use the calculator above with your species and spacing; it also lists the smallest size that works at each spacing.
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.