Deck Framing Calculator
Works out the joists, beams, posts and footings for a deck frame from the size and the spacings you choose. It counts and measures; the sizes of the timbers come from a span table for your grade and code.
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The formula
joists = floor(length ÷ spacing) + 1 + 2 rims beams = ceiling(width ÷ joist span) + 1 − ledger posts = beams × (floor(length ÷ post spacing) + 1)
A deck is a grid: joists one way at close centres, beams the other way at the joist span, posts under the beams at the beam span. Each spacing you choose trades member count against member size, and the size is what the span table decides.
What to watch out for
Three things decide whether the deck stays up:
- The ledger. Bolted through to the house structure with flashing above it, never nailed to cladding. Ledger failure is the most common deck collapse.
- Member sizes from the table. A 2 m joist span, a 1.8 m beam span and a 400 mm spacing each map to a minimum section in the deck span table for your timber. Shorter spans mean smaller timbers.
- Footings below the frost line where there is one, on undisturbed ground, with the post on a bracket above the concrete rather than cast into it.
Frequently Asked Questions
How many joists for a deck?
Deck length divided by the joist spacing, plus one, plus two rim joists. A 5 m deck at 400 mm centres is 13 field joists plus 2 rims, about 16 with waste.
How far apart should deck posts be?
1.8–2.4 m along a beam is typical, and it depends on the beam size — a bigger beam spans further. The span table for your timber gives the limit for each section.
What size joists for a deck?
That depends on the span, spacing and timber grade, from a span table — not from a count. As a rough indication, 47 × 150 mm C24 at 400 mm centres spans about 2.4 m, but check the table that applies to your timber and location.