How to use the floor joist span calculator
- Choose living or sleeping. Table R502.3.1(2) is 40 psf live load for ordinary residential floors. Table R502.3.1(1) is 30 psf for sleeping rooms and attic joists reached by a stair.
- Pick species, size and spacing (12, 16, 19.2 or 24 inches on center). Only No. 2 grade, 10 psf dead load, L/360 is tabulated here.
- Enter the clear span and compare it with the table. The page also names the smallest size in this set that works.
Deck joists use a different table (AWC DCA 6 / IRC R507.6) on the deck joist span calculator. Lumber actual sizes are on the lumber size calculator. Header and girder spans are on the beam span calculator.
Where the spans come from
The calculator looks up 2021 IRC Tables R502.3.1(1) and R502.3.1(2) as reprinted in the 2021 Seattle Residential Code, Chapter 5, a public PDF. Spans are for Douglas Fir-Larch, Hem-Fir, Southern Pine and Spruce-Pine-Fir, No. 2 grade, at 10 psf dead load. The article does not paste the entire multi-grade table.
A worked example: Southern Pine 2×10 at 16 inches on center
For a living area (40 psf), Southern Pine No. 2 at 16 inches on center, a 2×8 may span 11′-10″, a 2×10 may span 14′-0″, and a 2×12 may span 16′-6″. A 13-foot clear span is within the 2×10 row; a 15-foot span is not, and the next size in this table is a 2×12. The same 2×8 in a sleeping area (30 psf) may span 13′-3″. In the same sleeping table, Hem-Fir No. 2 at 19.2 inches on center, a 2×12 may span 17′-1″ — the 2021 IRC / Seattle SRC tabulated value, not a value from a simplified beam formula.
Sources & assumptions
How this calculator gets its numbers. Full method: our methodology.
- Maximum spans are 2021 IRC Tables R502.3.1(1) (sleeping areas and attic joists, 30 psf live load) and R502.3.1(2) (residential living areas, 40 psf live load), 10 psf dead load, L/360, No. 2 grade only, as reprinted in the 2021 Seattle Residential Code, Chapter 5.
- Species included: Douglas Fir-Larch, Hem-Fir, Southern Pine and Spruce-Pine-Fir. Spacings: 12, 16, 19.2 and 24 inches on center. Sizes: 2×6 through 2×12.
- This is an interior floor-joist table lookup, not AWC DCA 6 / IRC R507.6 deck joists, and not a stamped design. Local amendments and other load cases are not tabulated here.
Sources
- City of Seattle, SDCI, 2021 Seattle Residential Code, Chapter 5 — Floors. Public reprint of 2021 IRC Tables R502.3.1(1) and R502.3.1(2). This calculator uses the 10 psf dead-load, No. 2 grade columns only.
Last reviewed . The adopted code tables and a design professional win.
Frequently asked questions
How far can a 2×10 floor joist span?
For Southern Pine No. 2 in a living area at 16 inches on center, the 2021 IRC table allows 14′-0″ at 10 psf dead load. Other species and spacings differ — use the calculator. This is not a deck-joist span.
What is the difference between this and the deck joist calculator?
This page uses IRC floor-joist Tables R502.3.1(1) and (2). The deck joist span calculator uses AWC DCA 6 / IRC R507.6, which are written for exterior decks.
Why only No. 2 grade?
No. 2 is the common grade in the IRC residential tables and keeps this page from reproducing every grade row. Select Structural and No. 1 spans are longer; see the IRC or a designer if you are using those grades.
Is this a stamped structural design?
No. It is a table lookup. Local amendments, openings, bearing, and loads other than 30 or 40 psf live / 10 psf dead need an engineer or the full code tables.