Decking Calculator
Use this decking calculator to work out how many deck boards, joists and screws you need. Enter the size of your deck and the board you plan to use: it allows for the gap between boards and adds 10% for cuts. It also estimates the installed cost, which in the UK is about £70 to £130 per m² in 2026 depending on the material.
- Softwood, installed£70 – £90 per m²
- Composite, installed£90 – £110 per m²
- Joist centres400 mm
Updated November 2026 · Works in metric and imperial units
Calculate your decking
Your result
Enter your measurements, then press Calculate to see the area in m², sq ft and sq yd.
You changed your details. Press Calculate to update the estimate.
How many decking boards do I need?
Divide the deck width by the board width plus the gap to get the number of rows, then multiply by the number of boards needed to cover the length of each row. Add 10% for cuts and offcuts. For example, a 4 m × 3 m deck with 120 mm boards and 5 mm gaps needs 24 rows; with 3.6 m boards each row needs 2 boards, so 48 boards plus 10% = 53 boards.
Decking board sizes in the UK
| Board | Width | Thickness | Lengths |
|---|---|---|---|
| Softwood (treated pine) | 120, 125 or 150 mm | 28 – 32 mm | 2.4, 3.0, 3.6, 4.2, 4.8 m |
| Composite | 140 – 150 mm (some 200 mm) | 25 – 32 mm | 2.4 – 4 m |
Leave a gap of about 5 mm between boards for drainage and movement, or follow the composite manufacturer’s clip system.
Decking joist spacing and sizes (UK)
- Joist spacing: 400 mm centres is the standard for timber decking. Many composite boards need 300 to 400 mm.
- Joist size: 47 × 100 mm C24 timber for low decks, 47 × 150 mm for longer spans.
- Posts: 100 × 100 mm posts set about 700 mm deep at around 1.5 m spacing for raised decks; low decks can sit on pads.
- Planning: decking more than 30 cm above the ground needs planning permission in England.
Decking cost per m² (2026)
| Material | Installed, per m² | Boards only, per m² |
|---|---|---|
| Softwood | £70 – £90 | £26 – £53 |
| Composite | £90 – £110 | £35 – £105 |
| Hardwood | £110 – £130 | £35 – £88 |
Raised decks with steps and balustrades cost considerably more.
Frequently asked questions
How many decking boards do I need for 10 m²?
With 120 mm boards and 5 mm gaps you need about 8 rows per metre of width. A 4 m × 2.5 m deck with 3.6 m boards needs about 44 boards including 10% for cuts.
How much decking do I need?
Measure the deck area in m², then use the calculator to convert it into boards, joists and screws for your board size.
What is the standard joist spacing for decking in the UK?
Joists are usually fixed at 400 mm centres. Composite decking often needs 300 to 400 mm; check the manufacturer’s instructions.
How much does decking cost per m² in the UK?
Installed decking costs about £70 to £90 per m² for softwood, £90 to £110 for composite and £110 to £130 for hardwood in 2026.
Do I need planning permission for decking?
Not usually for a low deck. Decking more than 30 cm above ground, or covering more than half the garden with other buildings, needs permission in England.
Where our prices come from
We compare published 2026 UK cost guides and trade price data, then use the middle of the range. Figures are for budgeting only and are not a quote. See how we price.