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Decking Calculator

Work out how many decking boards, joists and screws a deck needs, with the layout direction stated.

Deck length (boards run this way)Deck widthJoist spacing (centre to centre)Board widthGapClose-up
Key Boards run the length of the deck, across the joists.
Units
m
m
Typical board. Decking boards are often about 140 mm wide, and narrower ones are sold too. Use the width of your board. Editorial rule of thumb.
mm
mm
Set lengths. Boards are sold in set lengths, so check your supplier’s list. The starting figure just matches the deck.
m
mm

Follow your board maker’s guide.

%
Board rows across the deck21 rows of 1 board
Boards, no waste21
Boards to buy, with waste24 boards
Joists12 × 3 m
Screws504

Boards run along the 4.8 m length, across joists that run along the 3 m width. Rows: (3 m + gap) ÷ (140 mm board + gap) = 20.72, rounded up to 21. Joists: 4.8 ÷ 450 mm spacing, rounded up, plus one = 12. Screws: 21 × 12 × 2 = 504.

Check the joist spacing against your board manufacturer’s guide and local rules.

Fig. 1 Boards, joists and screws for your deck.

Every formula is tested against worked examples. How we check

Joist spacingJoistsScrews
300 mm17714
400 mm13546
450 mm12504
600 mm9378

Fig. 2 The same deck at different joist spacings.

Last checked 10/11/2026. Sources: Parkside Decking Installation Guide, August 2020 (19 to 22 mm boards, joists at 450 mm maximum); Dahlsens decking installation guide (450 mm residential, 300 mm commercial).

Guide

How the amount is worked out

Boards run along the deck’s length, across the joists. The number of board rows = (deck width + gap) ÷ (board width + gap), rounded up. Each row is as many boards as it takes to span the length. Joists run across the width, one every joist spacing along the length, plus one at the end. Screws = rows × joists × screws per crossing.

Example. A 4 m run covered by 140 mm boards with 5 mm gaps has a 145 mm pitch, 27.6 rows, so 28 boards.

Joist spacing

Decking suppliers give 450 mm as the maximum joist spacing for 19 to 22 mm boards on residential decks (300 mm for commercial, per a Dahlsens guide). The tool starts at 450 mm. Hardwood and composite boards, and boards laid diagonally, need their own rules, so follow your supplier’s installation guide and the standard that applies.

Common mistakes

  • Forgetting the gap. Boards need a small gap for drainage and movement, so board width alone undercounts rows.
  • Ignoring the extra joist at the end.
  • Choosing board lengths that leave short offcuts. Joints should land on a joist.

Safety and rules

This is not building or structural advice. Bearers, posts, footings, balustrades and stairs are outside this calculator. Most decks above a certain height or attached to a house need approval, and bushfire areas have extra rules. Before you dig, request plans of underground services from Before You Dig Australia, and check council rules and any approvals that apply to your property.

Common questions

How many deck boards for a 4 m by 3 m deck?

With 140 mm boards, 5 mm gaps and the boards running along the 3 m length in 3 m lengths, a 4 m wide deck needs 28 boards before waste.

How far apart should decking joists be?

Suppliers commonly give 450 mm maximum for 19 to 22 mm boards on residential decks. Check your supplier’s guide.

How many screws per board?

Two screws at each joist crossing is common.

Should I add waste?

5 to 10% covers cut-offs and the odd bad board. Diagonal layouts need more.

Planning a related job?

Worked examples

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