Decking Screw Calculator
Work out exactly how many decking screws you need for your project. Enter your deck size and board width below — we’ll do the maths, including a 10% allowance for spares and mistakes.
How to calculate decking screws
The number of screws a deck needs comes down to three things: how many boards you’re laying, how many joists each board crosses, and how many screws you drive at each crossing. The standard practice is two screws per board at every joist it sits on.
The formula is: boards × joist crossings × screws per crossing. The number of boards is your deck width divided by the board width plus the gap between boards. Joist crossings is your deck length divided by the joist spacing, plus one for the end joist. Most builders then add around 10% for offcuts, mis-drives and a few spares.
Decking screw length by board thickness
| Board thickness | Recommended screw length |
|---|---|
| 19–21 mm | 50–60 mm |
| 22–25 mm | 60–65 mm |
| 28–32 mm | 70–80 mm |
| 38–50 mm (sleeper/structural) | 100–120 mm |
Rule of thumb: the screw should penetrate the joist by at least twice the board thickness without poking through the underside.
Joist spacing and screw count
Closer joist spacing means more crossings and more screws. 450 mm centres is the most common spacing for timber decking in Australia; 300 mm is used for diagonal or wider-span boards, and 600 mm only for thicker, stiffer boards. Tighter spacing uses noticeably more screws, so it’s worth setting this accurately above.
Frequently asked questions
How many screws do I need per decking board?
Two screws at each joist the board crosses. A 6 m board over joists at 450 mm centres crosses about 14 joists, so roughly 28 screws per board.
How many decking screws per square metre?
For 140 mm boards at 450 mm joist spacing with two screws per crossing, it’s roughly 35–40 screws per square metre. Narrower boards or tighter joists push that higher.
Should I use two screws or one per board?
Two is standard — it stops boards cupping and twisting. One screw per crossing is only used for very narrow boards (under 70 mm).
Do I need to pre-drill?
For most softwoods and many hardwoods, no. Near board ends or in dense hardwoods, a quick pilot hole helps prevent splitting.
How much extra should I buy?
Around 10% over the calculated figure covers mis-drives, stripped heads and the odd dropped screw. This calculator already adds that buffer to the total.