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How Many Decking Boards for a 4.8 x 3 m Deck?

A 4.8 by 3 metre deck laid with 140 mm boards and 5 mm gaps needs 21 boards of 4.8 m (24 with 10% for waste), 12 joists at 450 mm spacing and 504 screws.

Try your own numbers

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 A 4.8 x 3 m deck, boards along the 4.8 m length.

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).

The layout

The boards run along the 4.8 m length and cross joists that run along the 3 m width. Board plus gap is 145 mm, and (3,000 + 5) ÷ 145 = 20.7, so 21 rows. With 4.8 m boards each row is one board.

Joists: 4.8 ÷ 0.45 = 10.7, so 11 spaces and 12 joists, each 3 m long. Two screws at each of the 21 × 12 crossings is 504 screws. Add 10% to the boards for offcuts and a bad board: 23.1, so 24.

Joist spacing. Suppliers commonly give 450 mm as the maximum spacing for 19 to 22 mm boards on a residential deck, and 300 mm for commercial use; the table shows 300, 400, 450 and 600 mm. Follow your board supplier’s installation guide. Bearers, posts, footings, balustrades and stairs are outside this calculator, and many decks need approval.

Common questions

Can I use 2.4 m boards instead?

Yes. Each row then needs two boards (so 42 before waste), with the joints landing over a joist. Change the board length to see.

Do hardwood boards need closer joists?

Often they do not, but it depends on the species and thickness. Follow the supplier’s span table.

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