How many decking boards do I need per square metre?

Divide 1 by the board width plus the gap, in metres. For 86 mm boards with 3 mm gaps that's 1 ÷ 0.089 = 11.2 linear metres of board per square metre, before cutting.

How far apart should deck joists be?

450 mm is the common spacing for 19 mm hardwood and 22 mm treated pine boards (the span tables allow up to 500 mm for standard grade 19 mm hardwood). Thinner pine boards need closer joists, and composite boards follow the maker's instructions, often 400 to 450 mm.

How many screws do I need for a deck?

Two per board at every joist it crosses, including both boards at a butt join. The calculator counts every crossing on the layout.

What gap should I leave between decking boards?

About 3 mm for hardwood up to 90 mm wide, 4 to 5 mm for wider seasoned hardwood and 5 mm for treated pine. Composite boards use the gap in their instructions or the one their clips set.

Free calculator

Decking calculator

Rows = (deck width + gap) ÷ (board width + gap), rounded up. Linear metres = rows × deck length. Fix every board twice at each joist, with joists at 450 mm, and land every butt join on a joist.

Your job

Deck size

Along the boards

Across the boards

Boards run across the joists. Keep the deck 10 mm clear of the house wall.

Boards

Board width

Gap

Joists at

Gaps by board: 3 mm for hardwood up to 90 mm wide, 4 to 5 mm for wider hardwood, 5 mm for treated pine. Check the span for your board thickness before going past 450 mm joists.

Lengths you can get

Every butt join lands on a joist and joins never line up on neighbouring rows. Short ends are cut from offcuts before another board is bought.

Oil

Oil the deck

Coverage

Coats

A typical decking oil spreads about 10 m² a litre per coat; two coats is the usual advice. Check your tin.

Optional

Your price per metre of board

Ask your timber yard. The greyed number is an example only and is never used in your result.

Joists run under the boards. Orange marks are butt joins, each on a joist; the striped row is ripped to fit.

Boards to buy

82boards

20 × 4.8 m + 21 × 3.6 m + 21 × 3.0 m + 20 × 1.8 m

Deck area

23.76m²

41 rows of 86 mm boards, 3.0 mm gaps

Board laid

270.6m

Bought 270.6 m, offcut 0.0 m

Fixings

1,394

Two per board at every joist

Joists

450 mm centres, 57.6 m of joist

Decking oil

4.8L

2 coats at 10.0 m²/L

The working, with your numbers

1. Rows(3.60 + 0.0030) ÷ (0.086 + 0.0030), rounded up= 41
2. Board laid41 rows × 6.60= 270.60 m
3. Joists6.60 ÷ 0.450, rounded up, + 1= 16
4. Fixings2 × every board-joist crossing= 1,394
5. Oil23.76 × 2 ÷ 10.0= 4.8 L

Typed those measurements in by hand?MetreMate measures them straight off your PDF plans. Free to start.

The formula, in plain words

1Rows = (deck width + gap) ÷ (board width + gap), rounded up. The last row is ripped to fit.
2Board laid = rows × deck length. Each row is cut from the lengths you can get so every join lands on a joist, and neighbouring rows join on different joists.
3Boards to buy: the longest pieces first, with short ends cut from offcuts before another board is bought.
4Joists = deck length ÷ joist spacing, rounded up, plus one. Fixings = 2 at every board-joist crossing.
5Oil = deck area × coats ÷ spread rate (m² per litre).

Worked example

A 6.6 × 3.6 m deck in 86 mm hardwood boards with 3 mm gaps, running the 6.6 m way, joists at 450 mm, two coats of oil at 10 m² per litre.

1.Rows: (3.6 + 0.003) ÷ (0.086 + 0.003) = 40.5, so 41 rows. Board laid: 41 × 6.6 = 270.6 m.
2.Rows alternate 3.6 + 3.0 m and 1.8 + 4.8 m, so joins sit on joists 1.8 m apart: 82 boards, no offcut.
3.Joists: 6.6 ÷ 0.45 = 14.7, so 15 spaces and 16 joists.
4.Oil: 23.76 m² × 2 ÷ 10 = 4.752, about 4.8 litres.

Answer: 82 boards (270.6 m), 16 joists and about 4.8 litres of oil.

How to measure it

Measure the deck along the way the boards will run, and across them.
Allow a 10 mm gap where the deck meets the house wall.
Pick your board, or type its width, gap and joist spacing from the product's instructions.
Untick the board lengths your yard doesn't stock.

Where the numbers come from

Every default in the calculator is a number a supplier or manufacturer publishes. We list the source next to it so you can check.

What	Value	Source
Joists at 450 mm maximum spacing	450 mm	Timber Queensland Technical Data Sheet 4, Residential Timber Decks (2014) (checked 2026-10-01)
Two fixings per board at every joist crossing	2 fixings	Timber Queensland Technical Data Sheet 4, Residential Timber Decks (2014) (checked 2026-10-01)
Gap between hardwood boards up to 90 mm wide (minimum)	3 mm	Timber Queensland Technical Data Sheet 4, Residential Timber Decks (2014) (checked 2026-10-01)
Gap for seasoned hardwood boards 91 to 140 mm wide: 4 to 5 mm	5 mm	WoodSolutions Technical Design Guide 21, Domestic Timber Deck Design (2020) (checked 2026-10-01)
Gap between softwood (treated pine) boards	5 mm	Timber Queensland Technical Data Sheet 4, Residential Timber Decks (2014) (checked 2026-10-01)
Side gap between 137 × 23 mm composite boards	4 mm	ModWood Fixing Instructions (2021) (checked 2026-10-01)
Decking sold in set lengths from 1.8 to 6.0 m (1.8, 2.4, 3.0, 3.6, 4.2, 4.8, 5.1, 5.4, 5.7, 6.0)	6 m longest	Product listing: 90 × 22 mm Decking H3 Treated Pine, lengths 1.8 to 6.0 m (checked 2026-10-01)
Composite boards come in 5.4 m lengths	5.4 m	ModWood decking calculator (board length 5.4 m) (checked 2026-10-01)
Decking oil spread rate; two coats recommended	10 m² per litre	Cabot's Natural Decking Oil product page (checked 2026-10-01)

Manufacturers named here are the source of the number, not a recommendation. Confirm quantities with your supplier.

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