How Much Decking for a 4m x 3m Deck?

A 4 × 3 m deck (12 m²) needs 31 boards of 3.0 m in 144 mm width, including 10% for cuts, laid in 21 rows over about 11 joists. That is roughly 462 screws. Change the board width, length and joist spacing below.

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Example job shown. Change the numbers to match yours.

Decking boards to buy

31 boards

At your chosen length, with waste

What to buy

Joists11 joists
Screws (2 per joist crossing)462 screws
Bags of cement (25 kg)4 bags

Rows of boards across the width: 21 rows
Total board length (with waste): 92.4 m
Deck area: 12.0 m²

Framing and fixings

JoistsEach as long as the deck width: 11 joists
Screws (2 per joist crossing): 462 screws

Concrete for the footings

Footing concrete (with 10% extra)238 litres for 6 footings: 0.238 m³
Cement if you mix 1 : 2 : 4 yourself: 75 kg
Bags of cement (25 kg)Plus sharp sand and 20 mm aggregate, or buy bagged post-setting mix: 4 bags

Assumptions used: 10% waste added; 144 mm boards with a 5 mm gap; joists 400 mm apart; 2 screws per joist crossing; footings 6 of 300 × 300 × 400 mm with 10% extra concrete, 1 : 2 : 4 mix.
Boards run across the joists. If the boards are shorter than the deck, plan the butt joints to fall over a double joist, and stagger them. Joist size, posts and bearers depend on the span and are not included; only the concrete for the footings is. Use the concrete calculator for a slab or pad.

Before you buy: round up to whole packs, check the size printed on the label, and add a little spare. Your supplier has the final say.

Decking for a 4 × 3 m deck, by board width

The deck is 12 m². Narrower boards mean more rows and more boards, and wider gaps mean fewer. These counts include 10% for cuts and use boards of 3.0 m with 5 mm gaps:

Board width	Rows of boards	Boards to buy	Total length (m)
144 mm (the usual width)	21	31	92.4
120 mm	24	36	105.6
95 mm	30	44	132.0

Boards by length you buy

Boards are sold in standard lengths. Pick the length that wastes the least off-cut for your span (144 mm boards):

Board length	Boards to buy
2.4 m	39
3 m	31
3.6 m	26
4.8 m	24

About a 4 × 3 m deck

This size suits a deck for a dining set and a barbecue. The structure needs about 11 joists at 400 mm centres, with posts or pads, a damp-proof strip on the tops of the joists and good ventilation underneath. A deck more than 300 mm above the ground generally needs planning permission in England. Check with your council. The decking materials calculator adds the joists, posts, screws and tools.

Want to change anything else? The calculator above is the full Decking Calculator (UK): edit any number and the results update instantly.

Tools & materials for this project

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Decking screws (60 mm)Decking boards (144 mm)Decking oil Weed control membrane Decking spacers

Frequently asked questions

How many decking boards do I need for a 4m x 3m deck?

A 4 × 3 m deck is 12 m². With 144 mm boards, 5 mm gaps and 10% extra you need 31 boards of 3.0 m (92.4 m of board in total), laid in 21 rows.

How many joists for a 4m x 3m deck?

At 400 mm centres you need about 11 joists. Closer spacing (300 mm) is a good idea for 19 mm boards on a deck that gets heavy use, and wider spacing risks bounce.

How many screws for a 4m x 3m deck?

About 462 decking screws, allowing two per board at every joist crossing plus a margin. Buy a box or two more than the count.

Related sizes and guides

Related sizes

Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.

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