Woodworking & Construction Calculators

Carpentry & framing

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

Pick species, joist depth and on-center spacing. Spans come from the prescriptive deck joist table (IRC / AWC DCA 6) for No. 2 lumber at 40 psf live + 10 psf dead load (about 1.9 + 0.5 kN/m²), which groups species as southern pine; Douglas fir, hem-fir and SPF; and cedar or redwood.

Inputs

Units
ft

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

ft

Terms of Service

Results

Please tick the box above to display the Result.

Calculation method

Span is looked up in the prescriptive table for the species group, joist depth and spacing. Douglas fir and SPF share a row because the table groups them with hem-fir. If the joist span (deck width) is longer than that maximum, the output flags a mid-span beam. Joists are counted along the ledger with one at each end: a 16 ft deck at 16 in O.C. needs 13, and a deck that is not a whole number of spacings gets one more to close the end.

Exact formula

Max span = table value for the species group, joist size and O.C. spacing (No. 2, wet service, 40 psf live + 10 psf dead, no snow). Southern pine at 16 in O.C.: 2×6 9 ft 0 in, 2×8 11 ft 10 in, 2×10 14 ft 0 in, 2×12 16 ft 6 in. Joist count = ceil((length_in − 1.5) / OC) + 1. A beam is flagged when deck width > max span.

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?

Use it to plan, not to get a permit. The spans follow the common prescriptive table for typical loads, but your local code, snow load and the beam and posts decide.

Which way is width?

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

What about wet treated lumber?

The table already assumes No. 2 lumber in wet exterior service. Lower grades, heavier loads such as a hot tub or snow, and cantilevers need shorter spans.

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