Woodworking & Construction Calculators

Carpentry & framing

Deck Joist Span Calculator (Species, Size, O.C.)

Pick species, joist depth and on-center spacing. The span table is a simplified educational model based on typical SYP #2 deck values, scaled for other species.

Inputs

Units
ft

Direction the ledger runs; joists are counted along this edge.

ft

Results

Max joist span
11.83ft
Joists to buy
13
Mid-span beam needed?
Yes

Yes when deck width exceeds the estimated max span.

Recommended gear

Calculation method

Span comes from a simplified SYP #2 deck model, then species and spacing factors. If the joist span (deck width) is longer than that maximum, the output flags a mid-span beam. Counts assume a single bay along the ledger.

Exact formula

Base spans are SYP #2 at 16 in O.C. Twelve-inch spacing × 1.10, 24 in × 0.85. Cedar × 0.85, spruce × 0.90, Douglas fir × 0.98. Joist count = floor(length / OC) + 1.

Metric decks

Length and width convert to metres. 2× sizes and 16 in O.C. stay as North-American framing labels.

Frequently asked questions

Can I use this instead of the IRC table?

No. It is a planning sketch so you can see whether 2×8 at 16 in O.C. is even in the conversation.

Which way is width?

Width is the joist span, house to outer beam. Length is along the ledger.

What about wet treated lumber?

Wet-service and incising reduce span. Treat this output as optimistic for treated stock.

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