Decking Calculator
Work out how many deck boards and joists you need for a rectangular deck, including gaps and waste.
Boards are assumed to run along the deck length. Check the board maker’s required gap and maximum joist span, and your local building rules for the frame, beams and posts.
About this tool
Enter the deck size, the board width and length, the gap between boards and your joist spacing. The calculator works out how many rows of boards cover the deck, how many boards to buy with a waste allowance, the total board length, and how many joists the frame needs.
Frequently asked questions
Divide the deck width plus one gap by the board width plus the gap to get the number of rows, then multiply by the deck length. A 4 × 3 m deck with 145 mm boards and 5 mm gaps needs 21 rows of 4 m — 21 boards of 4.8 m, or 24 with 10% waste.
Follow the board maker's instructions. Trex, for example, recommends a 3/16 in (about 5 mm) gap side to side for its composite boards and 1/8 in end to end on days warmer than 40 °F. Gaps let water drain and allow the boards to move.
16 in (about 400 mm) on centre is the usual maximum for boards laid at 90° to the joists, and 12 in (300 mm) gives a stiffer feel. Boards laid diagonally need closer joists — TimberTech specifies 12 in on centre at 45°.
Divide the deck length by the joist spacing, round up and add one for the end joist. A 4 m deck at 400 mm centres needs 4 ÷ 0.4 + 1 = 11 joists; a 16 ft deck at 16 in centres needs 13.
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