How the UK decking calculator works
Enter your deck's length and width in metres. The calculator starts with UK presets: treated softwood boards 144 mm wide with a 6 mm gap, joists at 400 mm centres, and the lengths UK merchants stock (2.4, 3.0, 3.6, 4.2, 4.8 and 5.4 m). For composite it uses 140 mm boards in 3.6 and 4.8 m lengths.
It lays out every row, picks the cheapest mix of board lengths, puts joins over joists and staggers them, and adds your allowance for offcuts and mistakes. Enter your merchant's prices to see the cost in pounds. You can change any size, or switch to feet and inches.
UK deck board sizes and lengths
| Board | Typical size | Lengths sold |
|---|---|---|
| Treated softwood | 28 × 144 mm (sold as 32 × 150 mm nominal) | 2.4, 3.0, 3.6, 4.2, 4.8, 5.4 m |
| Composite | 126–200 mm wide, often about 140–150 mm | 3.6 m, 4.8 m (some 2.2 m) |
Sources: JK Timber treated decking, Lawsons composite decking range. The Timber Decking & Cladding Association (TDCA) says timber deck boards are usually 70–145 mm wide and 19–45 mm thick, sold from 1.8 to 6.0 m, and advises against boards wider than 150 mm.
Example: decking for a 4 × 3 m deck
A 4 m long, 3 m wide deck with the UK presets and 10% extra:
- Treated softwood (28 × 144 mm, 6 mm gap): 27 rows. Buy 30 × 3.0 m (90 linear metres) and about 486 screws.
- Composite (140 mm, 5 mm gap): 28 rows. Buy 31 × 3.6 m (111.6 linear metres) and about 252 hidden clips.
- A bigger 6 × 4 m softwood deck: buy 33 × 3.6 m + 27 × 2.4 m (183.6 linear metres).
The calculator also chooses which way the boards run. Running boards the short way often avoids joins altogether.
Gaps and joist spacing
- Gaps: the TDCA says to leave no less than 5 mm and no more than 8 mm between timber boards, 5 mm where boards meet posts, and no more than 2 mm where boards butt end to end (TDCA deck board options). Composite boards use the gap set by their clips, so follow the brand's guide.
- Joists: the TDCA code of practice allows 400 mm maximum joist spacing for 120 × 27 and 145 × 27 mm boards in C16 timber (TDCA code of practice for raised timber decks). Composite makers often ask for 300–400 mm; check yours.
How much does decking cost in the UK?
As a rough guide, Duffield Timber (updated July 2026, prices excluding VAT) puts decking materials at about £25 per m² for treated softwood, £35 per m² for thermo-treated pine, £40 per m² for composite and £60 per m² for hardwood. Supplied and installed, it estimates about £60 per m² for treated softwood, £80 per m² for mid-range softwood and £120 per m² for hardwood, with installers charging £250–£350 a day.
For a 12 m² deck, that's roughly £300 of treated softwood boards or £480 of composite before VAT, framing and fixings. Prices vary by region and supplier, so use real quotes in the calculator's cost panel.
Do you need planning permission for decking?
In England, garden decking is permitted development, with no planning permission needed, if it is no more than 30 cm above the ground, if together with other extensions and outbuildings it covers no more than 50% of the garden, and if none of it is forward of the house's principal elevation (Planning Portal: decking). Stricter limits apply in National Parks, AONBs, World Heritage Sites and for listed buildings, and the rules differ in Scotland, Wales and Northern Ireland.
A raised deck may also need guarding under the Building Regulations (Approved Document K). Check with your council's building control before you build anything high.
Frequently asked questions
How many deck boards do I need for a 4 × 3 m deck?
With 28 × 144 mm treated boards, a 6 mm gap and 10% extra, about 30 boards: 30 × 3.0 m. With 140 mm composite boards, about 31: 31 × 3.6 m. Use the calculator above for your exact size.
How many linear metres of decking per m²?
Divide 1,000 by the board width plus the gap, in millimetres. For a 144 mm board with a 6 mm gap that is about 6.7 linear metres per m². Real decks need a little more for offcuts and joins; the boards per square metre guide has a full table.
What joist spacing should I use for decking in the UK?
400 mm centres is the usual maximum for 27–28 mm softwood boards (TDCA code of practice). Composite boards often need 300–400 mm, depending on the brand.
Does decking need planning permission in the UK?
In England, not if it is no more than 30 cm high, covers no more than 50% of the garden together with other extensions and outbuildings, and isn't in front of the house. Listed buildings and protected areas have stricter rules, and Scotland, Wales and Northern Ireland have their own.