How this calculator works
Deck boards are laid side by side across the joists, so the deck area divided by the width of one board gives the total length of board needed. That length, plus the waste allowance, is divided by the stock length and rounded up. The joist count comes from the span divided by the spacing plus one for the end joist, and the fastener count assumes two screws or clips at every joist crossing.
Formula
linear length = deck area ÷ board width; boards = ceil(linear length × (1 + waste%) ÷ board length); joists = floor(deck length ÷ joist spacing) + 1; fasteners = 2 × joists × boards.
Worked example
A 20 × 12 ft deck with 5.5 in boards: 240 ft² ÷ 0.4583 ft = 523.6 linear ft. At 16 ft boards and 10% waste, 576 ft is needed, so 36 boards. Joists at 16 in along the 20 ft span give 16, and two fasteners per crossing gives 1152.
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Frequently asked questions
What gap should I leave between boards?
About 1/8 in to 3/16 in for dry wood, and check the manufacturer for composite. The gap lets water drain and gives each board room to swell; it is too small to change the board count by more than a few percent.
Which dimension do the joists run?
The calculator treats the deck length as the joist span, because joists usually run under the shorter boards. If your joists run the other way, swap the two dimensions so the joist count still comes out right.
Do hidden fasteners change the count?
They come one clip at each joist crossing between boards, so the total is close to the screw figure. Buy a little extra for clips broken or dropped.