Decking Calculator
Enter the size of the deck, the board width and the joist spacing. You get the number of boards, joists and screws, with a waste allowance. Metric or imperial.
A guide to quantities. Add framing for edges, posts and foundations separately and follow the board maker’s advice on spacing and fixings.
Worked examples
Small garden deck
4 m by 3 m deck, 140 mm boards 3.6 m long, 5 mm gap, joists at 400 mm, 10% waste, 2 screws a crossing
Deck area: 12 m² (129.2 sq ft) Rows of boards across the width: 21 Boards needed (3.6 m long, with 10% waste): 26 Total length of boards: 92.4 m Joists needed, each 3 m long: 11 Total length of joists: 33 m Screws needed (2 at each crossing, with 10% waste): 509
Boards run along the 4 m length.
Deck in feet and inches
13 ft by 10 ft deck, 5.5 in boards 12 ft long, 0.2 in gap, joists at 16 in, 10% waste, 2 screws a crossing
Deck area: 12.08 m² (130 sq ft) Rows of boards across the width: 22 Boards needed (12 ft long, with 10% waste): 27 Total length of boards: 314.6 ft Joists needed, each 10 ft long: 11 Total length of joists: 110 ft Screws needed (2 at each crossing, with 10% waste): 533
Imperial sizes are converted to the same calculation.
About Decking Calculator
This decking calculator works out how many boards, joists and screws you need for a deck. Enter the length and width of the deck, the width and length of the boards, the gap between them and the joist spacing. It adds a waste allowance and shows the area, the rows of boards and the total lengths of timber.
You can work in metres and millimetres or in feet and inches. Boards are assumed to run along the length of the deck, with joists across them. The result is a guide to quantities, so add framing for edges, posts and foundations, and follow the manufacturer’s advice for your boards.
How to use Decking Calculator
- Choose metric or imperial.
- Enter the deck length and width and the board length.
- Enter the board width, the gap and the joist spacing.
- Set the waste allowance and screws per crossing, then read the quantities.
Common uses
- Pricing a deck before you buy timber
- Checking how many boards to order
- Counting screws for a job
- Comparing narrow and wide boards
Frequently asked questions
How many decking boards do I need?
Divide the width of the deck by the board width plus the gap to get the rows, then multiply by the deck length and divide by the board length, and add waste. The tool does this for you.
How far apart should joists be?
It depends on the boards. 400 mm centres is common for 28 mm timber boards, and closer spacing is often needed for thinner boards, composite boards or diagonal laying. Check the maker’s instructions.
How much waste should I allow?
10% is a common allowance for straight laying. Allow 15% or more for a diagonal pattern or an awkward shape.
How many screws are needed?
Normally two screws at every point where a board crosses a joist. The tool counts the crossings and adds the waste allowance.