How to use this calculator
- Enter the deck length (the direction the boards run) and width.
- Enter your board width, the gap, and the length boards are sold in.
- Enter the joist spacing the board maker recommends.
The formula
Rows = deck width ÷ (board width + gap), rounded up
Boards = rows × deck length × (1 + extra %) ÷ board length, rounded up
Joists = deck length ÷ spacing, rounded down, + 1
Screws ≈ rows × joists × 2
Each row of boards runs the full length of the deck, joining over joists where boards are shorter than the deck. Working in total length means offcuts from one row can start another.
Two screws where every board crosses a joist is the usual fixing pattern. Hidden-fastener systems use their own clips instead.
Worked example
A 4 m × 3 m deck with 140 mm boards and 5 mm gaps has 21 rows.
With 3.6 m boards and 10% extra, buy 26 boards. At 400 mm centres you need 11 joists and about 462 screws.
Tips before you buy
- Stagger board joints so they don't all land on the same joist.
- Leave a gap at the house wall and follow the board maker's ventilation advice.
- Use screws rated for outdoor use and, for hardwood, pre-drill.
Frequently asked questions
›How far apart should deck joists be?
Commonly 400 mm (16 in) centres for softwood boards; many composite boards need 300–400 mm. Always follow the board maker.
›How many screws per deck board?
Two at every joist the board crosses.
Sources
- Method: rows × deck length ÷ board length; fixing: two screws per board per joist (common practice).
Formulas last reviewed: September 29, 2026. How we check our formulas