Deck Calculator
Work out how much decking a deck needs — boards from the size, width and gap, and joists from the spacing.
A deck is boards and joists: boards across the width, joists along the length. This calculator counts both from the deck size, the board width, the gap and the spacing.
It counts joists — it does not size their spans. That is structural, and the safety notice on this page applies.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Measure the deck length and width.
- Enter the board width and the gap between boards.
- Enter the joist spacing (usually 16 in).
- Set wastage at 10% and add a price per board for a cost estimate.
Worked example
| Deck | 16 ft × 12 ft = 192 ft² |
|---|---|
| Boards (5.5 in + ¼ in gap) | ⌈(144 in ÷ 5.75 in) × 1.10⌉ = 28 boards, each 16 ft long |
| Joists (16 in spacing) | ⌈192 in ÷ 16 in⌉ + 1 = 13 joists |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
How many deck boards do I need?
Divide the deck width by the board width plus the gap, and add wastage. A 16 × 12 ft deck with 5.5 in boards needs 28 boards of 16 ft. Enter your board size and gap above.
How many joists do I need?
Divide the deck length by the joist spacing and add one. A 16 ft deck at 16 in spacing needs 13 joists. This calculator counts joists — it does not size their span, which is structural.
What gap should I leave between deck boards?
About ¼ in (6 mm) for drainage and expansion. Composite boards need the gap the manufacturer states — check it, because it varies by product and temperature.
What size joists do I need?
That depends on the span, the load and the timber — it is structural sizing, not a material count. Use span tables with a qualified professional, and check local building codes before building.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.