Decking Calculator
Boards, lineal feet, and fasteners for the deck — figured the way the yard sells them, with the gap in the math and the waste where it belongs.
Joist count is a count, never a size — span and sizing come from the plans and the lumber tables, not a calculator. Fasteners are a planning allowance of two per board at every joist crossing.
How the math works
Each board covers its face width plus one gap. Courses across the width = deck width ÷ (board width + gap). Boards per course = deck length ÷ the stock length you buy, rounded up — offcuts are why the waste factor exists. Fasteners are a planning allowance of two screws or clips per board at every joist crossing; joist count is floor(width × 12 ÷ spacing) + 1.
Diagonal layouts do not change the course math much, but they eat boards at the ends — that is what the waste factor is for. Raise it to 15% for a 45-degree layout.
A worked example
A 20 ft × 12 ft deck with 5.5 in boards on a 1/4 in gap: the course pitch is 5.75 in, so 26 courses. Two 12 ft boards per course makes 52 boards exact; at 10% waste, buy 58 (696 stock lineal feet, 520 installed). Joists at 16 in spacing count 10, and the fastener allowance lands at 832.
Questions from the field
How many deck boards do I need? Width ÷ (board width + gap) gives the courses; multiply by the boards per course and add waste. A 20 × 12 ft deck with 5.5 in boards takes 52 boards exact, 58 to buy at 10% waste.
How much gap belongs between deck boards? About 1/4 in for most composite and pressure-treated layouts — enough for drainage and swelling. The gap is part of the coverage math, which is why it is an input and not a footnote.
Does this calculator size my joists? No. It counts joists at your spacing. Span and sizing come from the plans and the lumber span tables.
How many screws does a deck take? Plan on two fasteners per board at every joist crossing. The example deck lands at 832 — buy the bucket size above your number, not below it.
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