Deck Calculator
Deck boards, joists and screws for a deck of any size.
Good to know. Joists 40 cm apart are the difference between a deck and a trampoline.
Approximate. Real-world values vary.
How it works
In plain words
The number of rows is the deck width divided by one board plus its gap. Each row needs the deck length divided by the board length, rounded up. Joists run across the boards, one every joist spacing plus one at the end.
The formula
rows = ⌈width ÷ (board + gap)⌉ boards = rows × ⌈length ÷ board length⌉ × (1 + waste) joists = ⌈length ÷ spacing⌉ + 1 · screws = rows × joists × 2
Worked example
360 cm ÷ (14.5 + 0.5) cm is 24 rows of one 4.8 m board each. With 10% for waste, that is 27 boards. Joists every 40 cm along 4.8 m need 13, and the boards 624 screws.
Sources
No outside data or rates: this is plain arithmetic.
3 automated tests check its answers on every change.
About the deck calculator
A deck is boards laid across joists, the beams underneath. This calculator counts both from the size of the deck, plus the screws to fix them, so you can price the materials before you order.
Enter the length in the direction the boards run and the width across them. The number of rows is the width divided by one board plus the gap between boards. If a board is shorter than the deck, each row needs more than one, joined over a joist; boards that match the deck length save on cuts and joins. Add 10% for waste, more for a diagonal pattern.
Joists usually sit 40 cm (16 in) apart, centre to centre. Composite boards and boards laid diagonally often need 30 cm (12 in). Each board is fixed with two screws at every joist it crosses, which is where the large screw count comes from; hidden fixing clips replace them on many composite decks.
This covers the deck surface and joists only. Beams, posts and footings depend on the height and span of the deck, and on local building rules. Raised decks usually need a permit and proper structural sizing.
Questions people ask
How many deck boards do I need?
Divide the deck width by the board width plus the gap for the rows, and multiply by the boards per row. Add 10% for waste.
How far apart should deck joists be?
Usually 40 cm (16 in) centre to centre, or 30 cm (12 in) for composite boards and diagonal patterns.
How big a gap should be left between deck boards?
About 3 to 6 mm (⅛ to ¼ in) for drainage and movement. Wet timber shrinks as it dries, so it can be laid tight.