Decking Calculator

The gaps between the boards do real work — they cover 3.4% of the deck

A 145 mm board laid with a 5 mm gap occupies 150 mm of width, not 145. Over a three-metre span that is twenty boards rather than twenty-one, because the gaps are carrying 3.4% of the width for free. Estimating without the gap over-orders, which is the safer mistake but still a mistake.

How it works

• Adds the gap to the board width to get the true laying pitch.
• Divides the deck width by that pitch and rounds up to whole boards.
• Multiplies by the run and adds a waste margin to give linear metres to order.

pitch = board width + gap
boards = ⌈deck width ÷ pitch⌉
linear metres = boards × deck length × (1 + waste ÷ 100)

Worked example

A three-metre-wide deck in 145 mm boards with a 5 mm gap.

1. pitch = 145 + 5 = 150 mm
2. boards = 3000 ÷ 150 = 20
3. ignoring the gap would give 3000 ÷ 145 = 20.7, rounded to 21
4. the gaps therefore save one board in twenty, about 3.4%
5. each board still has to span the full length, so multiply by the run

Twenty boards, not twenty-one. The gap is not wasted space — it is drainage, expansion room, and a small saving on the order.

Reading the result

• The gap exists for movement, not aesthetics. Timber swells across its width in wet weather, and boards laid tight will cup or lift; the exact gap depends on the material and on how wet the boards are when you lay them, since kiln-dried timber will expand and wet timber will shrink.
• Joist spacing does not change how many boards you need, but it decides whether they will feel solid. Spacing that suits a thick board will let a thinner one flex underfoot, so the two choices have to be made together.
• Waste is not optional on a deck with any angle in it. Square decks waste little; anything with a diagonal, a corner cut or a step generates offcuts that cannot be reused, and 10% is a floor rather than a ceiling.
• Linear metres and board count are different orders. Suppliers quote both, and a board that comes in fixed lengths may force you to buy more than the linear metre figure implies if your run does not divide neatly.

Common questions

How many deck boards do I need for a 5 × 3 m deck?

20 boards, each running the full 5 m, if you use 145 mm boards with a 5 mm gap. Each board plus its gap takes 150 mm, and 3000 ÷ 150 = 20. That is 100 linear metres, or 110 m with a 10% waste allowance. If the boards come in lengths that do not match your run, joins have to land on a joist, which can push the order up further.

How many screws do I need for decking?

Two screws per board at every joist it crosses. A 5 × 3 m deck with 20 boards and joists at 400 mm centres has 13 joists, so it needs 20 × 13 × 2 = 520 screws, roughly 35 per square metre. Buy a box more than the count, since a few always bend or get dropped.

What joist spacing should I use for decking?

400 mm centres is a common choice and the calculator's default, but the board manufacturer's maximum span is the figure to follow; thinner and composite boards often need closer spacing. Spacing does not change the number of boards, only the frame and the fixings: over 5 m, 400 mm centres means 13 joists and 520 screws, while 600 mm means 9 joists and 360 screws.