Deck Board Calculator
Works out deck boards from the deck size, board width and gap, and converts that into boards of the length you can buy, plus the screws.
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The formula
rows = ceiling(width ÷ (board width + gap)) boards = ceiling(rows × ceiling(length ÷ stock length) × (1 + waste)) screws = rows × joists × 2
The gap is part of the board’s effective width, which is why a 140 mm board with a 5 mm gap covers 145 mm. The second rounding is the real waste: a 5 m row from 4.8 m stock needs two boards and leaves 4.6 m over.
What to watch out for
Decisions that change the order:
- Board direction. Boards run across the joists. Run them along the longer dimension where you can, so rows are whole stock lengths and there are fewer joints.
- Gap. Timber swells, so 5–6 mm; composite moves with heat, and the manufacturer specifies the gap. Zero gap traps water and rots the edges.
- Screws, not nails. Two per board at every joist, stainless or coated for the timber treatment — ordinary zinc screws react with modern preservatives and stain the deck black.
Frequently Asked Questions
How many deck boards do I need?
Divide the deck width by the board width plus the gap for the number of rows, then work out how many stock lengths each row takes. A 5 × 4 m deck in 140 mm boards with 5 mm gaps is 28 rows, which is 56 boards of 4.8 m with 10% waste — about 62 to order.
What gap should I leave between deck boards?
5–6 mm for timber, which swells when wet. Composite boards move with temperature instead and usually specify 6–8 mm. Boards laid tight buckle and hold water.
How many screws for decking?
Two per board at every joist. 28 rows over 13 joists is about 730 screws, so allow 770–800. Use stainless or coated screws rated for the timber treatment.