Joist Span Calculator

SPF 2×10 @ 16″ o.c. spans 13′-0″. Southern Pine 2×10 @ 16″ o.c. spans 14′-0″. At 24″ o.c. (the cheapest option) SPF still gets you 10′-7″ — long enough for most single-bay decks. 2×10 is the most common deck-joist size in the US.

Up to 18′-0″ for Southern Pine 2×12 @ 12″ o.c. — the IRC prescriptive table caps at 18′. Anything longer requires engineered lumber (LVL, PSL, or I-joists) or a structural engineer. SPF 2×12 @ 16″ o.c. spans 15′-1″ — solid for most large decks.

16″ on-center is the default for most deck framing. Use 12″ when you need extra span, you're using composite decking that requires tighter support, or your boards are running diagonally. 24″ is the cheapest but is only suitable for short spans with thicker decking. Never go above 24″ o.c. for residential deck framing.

Spruce-Pine-Fir (SPF) is the lightest, weakest, and most commonly stocked dimensional lumber at big-box stores. Southern Pine (SP) is the strongest dimensional softwood and is the dominant pressure-treated species east of the Mississippi. The span difference is about 8–12% — meaningful when you're at the limit of a table. Always check the grade stamp on each piece (S-P-F, S-Pine, DF-L, Hem-Fir).

Yes. IRC tables assume 40 psf live load. Set the live load to 50, 60, or 70 psf for light, moderate, or heavy snow zones — the calculator derates the tabulated maximum span by sqrt(40 / load_psf). For ground snow loads above 70 psf or when the deck supports a hot tub, get a stamped engineering review.

Yes — up to 1/4 of the allowable back-span (IRC R507.6.1). Example: a 2×10 SPF @ 16″ o.c. is rated for 13′ span, so you can cantilever up to 3′-3″ past the last beam. Set the cantilever input on this calculator to verify your design. Note: cantilever does NOT extend the joist's allowable back-span — it's an additional run on the end.

Tributary load is the slice of the deck each joist carries — half the spacing on either side. At 16″ o.c. each joist is responsible for 16″ × (40 LL + 10 DL) = 67 plf. Load tables for engineered lumber are quoted in plf, so converting your design to plf lets you cross-check against an LVL chart if you go beyond the 18′ prescriptive max.

IRC prescriptive tables stop at 18′ — the limit for 2×12 Southern Pine @ 12″ o.c. Beyond that you need either (a) a stamped engineer's design, (b) engineered lumber such as LVL, PSL, or I-joists with manufacturer load tables, or (c) a beam in the middle to break the run into two shorter spans. Adding a beam is almost always the cheapest fix.

The numbers are pulled directly from IRC 2021 R507.6 / AWC DCA-6 — the same table your inspector uses. But always confirm with your local Authority Having Jurisdiction (AHJ): some jurisdictions amend the IRC, require stamped plans, or use a different code edition. The calculator gives you a code-compliant prescriptive design; submit the printable PDF with your permit application.

Joist span capacity scales roughly with depth squared. A 2×10 has 9.25″ depth vs a 2×8's 7.25″ — that's about 60% more bending capacity, which translates to ~25% more span. One nominal size up almost always solves a marginal failure. Tighter spacing (16″ → 12″) buys you another ~12%. The cheapest fix depends on your project; the calculator finds it for you.

Ledger-attached is cheaper (one less beam + posts on one side) but requires correct ledger flashing and code-compliant lag bolts or thru-bolts to the house rim joist. Freestanding is more expensive but eliminates the most common deck-failure mode (water-damaged ledger). For span calculation the difference is minor — just remember to account for cantilevers off the ledger if you're using one.

Composite manufacturers (Trex, TimberTech, Fiberon) typically require 16″ o.c. joist spacing for perpendicular installs and 12″ o.c. for diagonal. The joist span itself doesn't change — it's still IRC R507.6. But your spacing might be locked at 16″ or 12″, which changes which combinations are available. Always check the manufacturer's installation guide before finalizing framing.

Yes. Use the action bar above the results — Save PDF generates a clean, permit-ready document with your inputs, the IRC compliance result, and the relevant code reference. Print works too if you prefer browser-side. Copy Link gives you a shareable URL with all inputs preserved.