🪜 Deck Board Calculator
Enter your deck dimensions and board size, and get the number of deck boards, the total decking length and the joists needed.
Deck boards
43
incl. ~10% waste
Total decking
140 m
28 rows
Joists
14
Rows = width ÷ (board width + gap); boards = rows × length ÷ board length, + ~10% waste. Leave a 3-5 mm gap for drainage. 🔒 In your browser.
How the deck board calculator works
The number of board rows is the deck width divided by the board width plus the gap between boards. Multiplying by the deck length gives the total linear decking, and dividing by the board length gives the number of boards. Joists are estimated from the deck length and the joist spacing. Worked example: a 4 m wide deck with 140 mm boards and a 5 mm gap needs 4,000 ÷ 145 ≈ 28 rows; across a 5 m length that is 28 × 5 = 140 m of decking, or about 47 boards if each is 3 m long, before adding ~10% for waste. Joists at 400 mm centres over a 5 m length come to ⌈5,000 ÷ 400⌉ + 1 ≈ 14 joists.
Leave a small gap (about 3-5 mm) between boards for drainage and expansion. Add ~10% for waste and cuts, and buy a few spares, board colour and grade can vary between packs.
Frequently asked questions
How many deck boards do I need?
Divide the deck width by the board width plus the gap to get the number of rows, multiply by the deck length for total decking, then divide by the board length. Add ~10% for waste.
What gap should I leave between deck boards?
About 3-5 mm (⅛–³⁄₁₆ in) for timber, to allow drainage and seasonal movement. Composite boards have their own recommended gaps, check the manufacturer.
How far apart should deck joists be?
Commonly 400-600 mm (16-24 in) on centre, closer for thinner boards or diagonal decking. Follow local building guidance for structural decks.
How much decking waste should I allow?
About 10% for cuts and offcuts, more for diagonal or patterned layouts. Keep a couple of spare boards for repairs.
Does this include the frame?
It estimates the deck boards and joists; posts, beams and fixings depend on your design and ground conditions, so plan those separately.
How do I calculate linear metres of decking?
Multiply the number of board rows by the deck length. The rows are the deck width divided by the board width plus the gap. A 3 m × 4 m deck with 145 mm effective board spacing has about 21 rows, so 21 × 4 = 84 m of decking before waste.
How many screws will I need for a deck?
For two screws at every joist crossing, multiply the number of boards by the number of joists they cross. A deck with 47 boards over 14 joists needs about 47 × 14 × 2 ≈ 1,300 screws, so buy screws by the box and keep a margin.