Decking calculator
Enter the deck length and width in metres, the board width and gap in mm, the stock board length and joist spacing, and see the boards and joists to order.
Estimates only. Check the manufacturer's coverage rates.
How this is calculated
Boards run along the deck length. The number of rows is the deck width divided by the board width plus the gap, rounded up; leave a small gap between boards for drainage and expansion. Decking boards vary in width between products, so check yours and enter the actual size rather than assuming the default.
The total board metres are the rows times the deck length plus the wastage allowance, which covers cuts and offcuts at the ends of rows. Dividing by the stock board length and rounding up gives the number of boards to buy. Where a row needs more than one board, the boards that need joining should meet on a joist so both ends are supported.
Joists run along the width, spaced at regular intervals along the length. The count is the deck length divided by the joist spacing, rounded down, plus one, and the total joist metres are shown so you can order lengths. Joists must be spaced to suit the board thickness, so check the manufacturer's maximum spacing; 400 mm is common. This page does not estimate fixings.
- Joist spacing must suit the board thickness; check the manufacturer's maximum spacing (400 mm is common).
- Boards that need joining should meet on a joist.
- This page does not estimate fixings.
Frequently asked questions
What gap should I leave between decking boards?
Leave a small gap for drainage and expansion; 5 mm is the default here.
How far apart should joists be?
Joists must be spaced to suit the board thickness, so check the manufacturer's maximum spacing. 400 mm is common.
Does this include screws and fixings?
No. This page estimates boards and joists only.