Joist Span Calculator
Find the maximum allowable floor or deck joist span from the IRC span tables. Pick species, grade, size, and spacing to get the code max span by joist size and check it against the span you need. Free, no sign-up.
What to calculate next
Tools commonly used alongside this calculation
Deck Beam Span Calculator
Size a built-up deck beam from the IRC R507.5 table. Pick the species and the deck joist span it carries to get the maximum span for double and triple 2× beams and the minimum size for your post spacing. Free, no sign-up.
Rafter Span Calculator
Find the maximum horizontal roof rafter span from the IRC R802.4.1 tables. Pick the roof or ground snow load, species, grade, size, and spacing to get the max span by size and the minimum rafter for your roof. Free, no sign-up.
Concrete Calculator
Calculate concrete for slabs, footings, walls, columns, and sonotubes. Add pours to get total cubic yards, the number of 40/60/80 lb bags, a waste allowance, and ready-mix cost. Free, no sign-up.
Explanation
A joist span is the clear distance a floor or deck joist can cover between its supports without sagging or overstressing. The maximum allowable span is not a single formula — it depends on the joist size, the lumber species and grade, how far apart the joists sit (on-center spacing), and the load. Rather than calculate stress and deflection by hand, the International Residential Code (IRC) publishes prescriptive span tables that have already done that work. This calculator reads those tables for you: pick the application, species, grade, and spacing, and it returns the code maximum span for each joist size and checks it against the span you need to cover.
What controls the allowable span
Four things move the number. A deeper joist (2×10 vs 2×8) spans much farther because bending stiffness grows with the cube of depth. Closer spacing (12″ vs 16″ vs 24″ on-center) lets each joist carry less tributary load, so it spans farther. Stiffer, stronger species and higher grades (Select Structural > No. 1 > No. 2 > No. 3) span farther than weaker ones. And a heavier design load shortens the span. The IRC floor tables are built on a deflection limit of L/360 with the species design values for each grade.
| Species (No. 2) | 2×8 | 2×10 | 2×12 |
|---|---|---|---|
| Douglas Fir-Larch | 12'-7" | 15'-5" | 17'-10" |
| Hem-Fir | 12'-0" | 15'-2" | 17'-7" |
| Southern Pine | 12'-10" | 16'-1" | 18'-10" |
| Spruce-Pine-Fir | 12'-3" | 15'-5" | 17'-10" |
Example: floor joists in a living area, 40 psf live load, 16″ o.c., 10 psf dead load. IRC Table R502.3.1(2).
Floor joists vs. deck joists
Floor joists inside a house use IRC Table R502.3.1: table (1) for sleeping rooms and habitable attics at a 30 psf live load, and table (2) for all other living areas at 40 psf. Exterior deck and balcony joists use a 40 psf live load and the separate IRC Table R507.6, which is given for No. 2 grade and groups the common framing species together. Decks also carry weather exposure, so they are usually built from pressure-treated Southern Pine or naturally durable cedar and redwood.
| Deck species (No. 2, 16″ o.c.) | 2×8 | 2×10 | 2×12 |
|---|---|---|---|
| Southern Pine | 11'-10" | 14'-0" | 16'-6" |
| Douglas Fir-Larch / Hem-Fir / Spruce-Pine-Fir | 11'-1" | 13'-7" | 15'-9" |
| Redwood / Western Cedars | 10'-7" | 13'-0" | 15'-1" |
Once the joists are sized, the rest of the deck follows. The posts bear on concrete footings — size the piers and estimate the mix with the concrete calculator — and if those footings or a grade beam are reinforced, the rebar weight calculator gives the steel takeoff.
Notes and limitations
These tables are prescriptive and cover uniform residential loads only. They assume sawn dimension lumber at L/360 deflection; engineered products (I-joists, LVL, open-web trusses) follow the manufacturer's span charts instead. Concentrated loads — a bathtub, a stone counter, a wall bearing from above — and cantilevers beyond the joist depth need a separate design. Spans longer than the lumber is sold (about 20 ft) require splicing over a beam.
This calculator reads the published IRC span tables and flags the smallest joist size that covers your span. It does not design beams, headers, or hangers, replace a stamped structural plan, or account for snow, point, or seismic loads. Confirm the governing code edition and any local amendments with the authority having jurisdiction.