Decking Calculator
Enter your deck dimensions to calculate materials and add everything to your basket in one click
Choose the length closest to or longer than your deck length to minimise waste
Estimated total: £0.00
All quantities include a 10% allowance. Prices are per unit. Contact us if you need help with your order.
How to use the decking calculator
Enter your decking area length and width in metres. The calculator works out the number of decking boards, joists, and screws needed for a standard UK build using 28mm boards on 100×47mm joists at 400mm centres. Add 5–10% on top of the calculated total for cuts, mistakes, and offcuts.
How many decking boards do I need per square metre?
For standard UK 145mm wide decking boards with a 5mm gap, you'll need 6.7 boards per linear metre of deck length. So a 4m × 3m deck (12m²) using 3m boards needs around 20 boards plus 10% wastage. The calculator above gives you an exact figure for your dimensions.
Joist spacing for UK decking
Standard UK decking joist centres:
- 400mm centres — for 28mm decking boards (most common spec). Stiffer feel, less flex.
- 450–500mm centres — for 32mm or composite boards which are stiffer than 28mm timber.
- 600mm centres — only with thick (38mm+) boards or composite boards rated for wider spans.
Going wider than the manufacturer's recommended spacing creates flex in the deck and shortens its lifespan. Stick to 400mm for 28mm boards.
What size joists do I need for decking?
For most domestic UK decks at ground level or on a low frame, use 100×47mm C24 joists. For raised decks, longer spans, or anywhere supporting hot tubs and heavy planters, step up to 150×47mm C24. Always specify pressure-treated for ground-contact or external use.
How many screws per decking board?
2 screws per board per joist crossing. For a 3m board on joists at 400mm centres, that's 8 joist crossings × 2 screws = 16 screws per board. The calculator includes screw quantities in the materials list.
Decking calculator tips
- Add 10% to the total board count — cuts, mistakes, and the occasional warped board.
- Order joists in lengths that minimise cuts — buying 4.8m joists and cutting them in half is cheaper than two 2.4m joists.
- Pressure-treated only for any external decking — untreated softwood will rot within 2–3 years on the ground.
- Allow a 5mm gap between boards to let the deck breathe and drain water. The calculator builds this in.
For decking boards, joists, and treatment, browse our full decking range. Need help working out a complex build? Call 01656 745959 with your dimensions and we'll do the take-off for you.