Deck Joist Span Calculator
Pick your lumber, size and spacing. Get the max span the code table allows.
100% in your browser - no upload, no sign-up.
Max joist span
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In feet: -
✅ Your planned span is within the table maximum for this joist. Good to go - still verify against your local code.
⚠️ Enter your planned span above to check it against the table maximum.
⛔ Your planned span exceeds the table maximum - reduce it, move up a joist size, tighten the spacing, or add a beam.
⚠️ This is a table lookup, not an engineering design. A deck is a load-bearing structure - always confirm joist size, span and spacing against your locally adopted code (IRC R507 / DCA6) and get a permit. Values are for No. 2 grade at 40 psf; heavier loads (hot tubs, snow) or lower grades reduce the span.
⚠️ Keep joists 16 in apart - most composite decking sags on 24 in.
How to use this deck joist span calculator
- Pick your lumber species - Southern Pine spans farther than SPF.
- Choose the joist size (2×6 to 2×12) and the on-center spacing (12, 16 or 24 in).
- Read the maximum span, and enter your planned span to see if it's within the table.
How deck joist spans are determined
These spans come straight from the AWC DCA6 prescriptive residential deck tables (mirrored in IRC R507.5), for No. 2 grade lumber under a 40 psf live load plus 10 psf dead load. Span is measured from the face of one support to the next. A bigger joist spans farther, and tighter spacing spans farther for the same joist - a 2×8 Southern Pine joist reaches about 13 ft 1 in at 12 inches on-center but 11 ft 10 in at 16 inches. Southern Pine spans the farthest of the common species; SPF the least, with Douglas Fir-Larch and Hem-Fir in between (this tool uses the conservative SPF value for that group). The tables already build in the deflection limit, so no separate calculation is needed - but the values assume standard residential loads, not concentrated loads like a hot tub.
Frequently asked questions
- How far can a 2×8 deck joist span?
- A No. 2 Southern Pine 2×8 spans up to about 13 ft 1 in at 12 inches on-center, 11 ft 10 in at 16 inches, and 9 ft 8 in at 24 inches, under a standard 40 psf load. SPF and other softwoods span a bit less. Pick your species, size and spacing above for the exact table value.
- What size joists do I need for my deck?
- Work backward from the span you need. Find the shortest joist that reaches it at your chosen spacing: if you need 12 feet at 16 inches on-center, a Southern Pine 2×8 (11 ft 10 in) falls short, so you'd step up to a 2×10 (14 ft). Closer spacing also buys span. Enter your planned span above and adjust size and spacing until it passes.
- Does joist spacing affect the span?
- Yes - the tighter the spacing, the farther a joist can span, because each joist carries less load. The same 2×10 Southern Pine joist spans about 16 ft 2 in at 12 inches on-center, 14 ft at 16 inches, and 11 ft 5 in at 24 inches. Composite decking and heavy loads often call for 12-inch spacing, which both stiffens the surface and lengthens the allowable span.
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