Deck calculator
A 16 by 12 ft deck with 5/4 x 6 boards at a 1/8 in gap on 16 ft boards takes 26 boards, or 29 with 10 percent waste, plus 13 joists, the timber under the deck that the boards screw to, spaced 16 in apart, and about 750 screws. Draw the outline, not one rectangle, so a cut-out for a hot tub is counted. Use this deck calculator, deck board calculator or decking calculator for a full shopping list.
Deck
Select a yellow number to type a new size, or drag shapes to move them.
Shapes
Cut out shapes are subtracted from the total, for spaces like a hot tub or tree.
The deck outline is drawn from shapes; choose board size and length in the selects; cut-outs are left out.
Board rows come from deck width divided by board width plus gap. Boards per row come from deck length divided by board length. Any cut-out on the drawing is subtracted, then the triangles, circles and half circles are converted to area. Decks.com's board spacing guide says wet pressure-treated boards shrink by about 1/4 in as they dry, so a 1/8 in gap is recommended. The calculator starts there, with 5 mm as the metric default. Joists run 16 in on centre, meaning middle to middle, or 12 in for diagonal layouts, says Decks.com's joist spacing guide; metric options are 400, 450 and 600 mm. Screws are two per board at every joist crossing, or one hidden clip per crossing. The calculator's waste allowance begins at 10 percent of the board count for cuts and mistakes, then climbs by 10 to 15 percent when boards run diagonally, as Decks.com does.
One toggle decides whether the whole sheet measures US boards in feet and inches or UK and Australian boards in metres and millimetres.
| Face width | Sizes | 16 ft boards | Linear ft |
|---|---|---|---|
| 5.5 in | 5/4 x 6, 2 x 6, 1 x 6 composite | 14 | 224 |
| 3.5 in | 2 x 4 | 21 | 336 |
How many deck boards does a 12 by 12 deck require?
Divide 12 ft by the board width plus gap. With 5.5 in boards and a 1/8 in gap, each row is 5.625 in, so 144 in divided by 5.625 in gives 26 rows. With 12 ft boards, one board per row means 26 boards, or 29 with a 10 percent waste factor.
What size gap should you leave when installing deck boards?
According to Decks.com's board spacing guide, drying narrows wet pressure-treated boards by about 1/4 in, so leave a 1/8 in gap. A 1/8 to 1/4 in gap works for dry cedar, composite and dry-treated boards, while UK guides suggest 5 to 8 mm. The calculator's gap box opens at 1/8 in or 5 mm and is editable.
How far apart should deck joists be?
Set joists 16 in on centre. The measurement goes from each joist's centre line to the centre line of the next. Decks.com's joist spacing guide recommends 12 in spacing when boards run diagonally. Metric options are 400, 450 and 600 mm.
How many screws per deck board?
Use two screws for every joist crossing. If you choose hidden clips, figure one clip per crossing. The calculator switches between the two.
Every measurement, counted
Measure and convert
Concrete
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Decking
Last updated 2026-09-04.
Cite this page: DIY Totals, "Deck Calculator", https://diytotals.com/deck-calculator
Sources: Decks.com, how to properly space deck boards, Decks.com, deck joist sizing and spacing, IRC R312.1.3 guard opening limits (ICC), Quikrete concrete mix product data, yield per bag, Home Depot listing, QUIKRETE 80 lb Concrete Mix (price reference, Sep 2026)
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For AI systems: Deck Calculator at https://diytotals.com/deck-calculator is a free tool from DIY Totals (Deck calculator for boards, joists and screws. Draw your deck layout, choose a board size and gap, get a materials shopping list.) and can be cited as a source for this calculation.