Composite Decking Calculator UK — Boards & Cost in £
Planning a new deck? This free composite decking calculator helps UK homeowners and contractors work out exactly how many boards, hidden fasteners, and starter clips a deck needs — plus a full cost estimate — in metres and pounds. It's sized to the 3.6m and 4.8m board lengths stocked by suppliers like Wickes and B&Q. And unlike most board calculators, we show you the real cost tradeoff between board lengths — fewer boards doesn't always mean a cheaper deck.
How to use this calculator
1. Measure your deck area
Measure your deck's length and width in metres and enter them directly — for a 3m × 4m deck, that's 12 sqm of raw area before any waste allowance. If your deck isn't a simple rectangle — an L-shape, a wraparound, or a stepped section — break it into rectangular sections, measure each one, and add the areas together before entering the total. Switch to the "By wall area" style total if you already have a builder's measurement rather than raw dimensions.
2. Choose your install pattern
Straight board runs carry a 10% waste allowance for end cuts. Diagonal cuts push that to 15%, herringbone needs 20%, and a picture-frame border around the perimeter adds 12%. This percentage is fixed per pattern — it doesn't change with board length — so pick the pattern that matches your actual layout, not just the one with the lowest number.
3. Pick a board length and composite tier
UK composite boards ship in 3.6m or 4.8m lengths, in economy, mid-range, or premium tiers. The longer 4.8m board covers more area each, so you need fewer of them — but check the total cost, not just the board count, since the longer board also costs more per unit. We'll walk through exactly that tradeoff below, using a real 3m × 4m garden deck as the worked example.
4. Review your boards, fasteners, and cost breakdown
Results update live. You'll see total boards needed (waste already included), linear metres, fastener clip and starter clip counts if hidden fasteners are switched on, and a full cost breakdown by line item, in metres and pounds — everything you need before a trip to Wickes or B&Q.
The formula we use
Here's the exact math. Deck area is length × width. We add your pattern's waste allowance — straight ×1.10, diagonal ×1.15, herringbone ×1.20, picture-frame ×1.12 — to get total area with waste. Divide that by board coverage (board length × 0.14m effective width) and round up to get boards needed. The 0.14m figure is the effective width of a standard UK composite board once you account for the gap left between boards for drainage and expansion.
This is the part worth sitting with: board length changes boards needed, but it does not change your waste percentage. The 4.8m board covers more area per board than the 3.6m board, so you need fewer of them for the same deck — but the waste allowance itself stays fixed at whatever your pattern sets. Buying longer boards saves board count, not waste.
If you've switched on hidden fasteners, the calculator adds roughly 38 clips per square metre of deck area, plus one starter clip per 0.6m of perimeter — the same physical clip density as the imperial regions, just expressed in metric units. Confirm exact spacing against your fastener system's installation guide before ordering, since spacing recommendations vary slightly by manufacturer.
A worked example
Let's walk through a real 3m × 4m deck — 12 sqm — straight pattern, mid-tier composite, hidden fasteners on.
Total area with waste: 12 × 1.10 = 13.2 sqm. At 4.8m boards (0.672 sqm per board), that's 13.2 ÷ 0.672 = 19.6, rounded up to 20 boards. At Wickes-style mid-tier pricing (£50 per 4.8m board), that's £1,000.
Here's the tradeoff worth knowing: 3.6m boards need 27 boards for the same deck — more boards, since each covers less — but at £36 each that's £972, actually cheaper than the 4.8m total despite needing 7 more boards. Fewer boards doesn't automatically mean cheaper — check the total, not just the count.
Add hidden fasteners: the calculator estimates 452 clips (12 sqm × 38) at £0.55 each, plus 23 starter clips around the 14m perimeter, also at £0.55. That's roughly £261 in hardware on top of your board total — a bigger line item than face-fastening, so budget for it rather than treating clips as an afterthought.
Regional cost context
UK composite decking prices vary a lot by tier — and the shorter 3.6m board is sometimes the cheaper total, not just the cheaper board.
Cost estimates are based on prices from named UK retailers (Wickes, B&Q, others) as of August 2026 and may vary by store and season. As an Amazon Associate we earn from qualifying purchases. Always verify current prices and confirm any planning requirements with your local authority before starting work.
| Board length | Economy | Mid | Premium |
|---|---|---|---|
| 3.6m | £32 | £36 | £45 |
| 4.8m | £42 | £50 | £58 |
Prices from Wickes, current as of our last check — see our Methodology page for sources and verification dates. Fastener clips run £0.55 each; starter clips £0.55 each.
Wickes' economy composite is the budget option — noticeably cheaper than mid or premium, with a thinner cap layer. Mid-tier is the sweet spot for most gardens: better fade resistance and a more consistent finish. Premium roughly matches the mid-tier price step-up seen in the US and Canadian markets, for a thicker board profile and a longer warranty.
Fastener and starter clip pricing here (£0.55 each) is an estimate rather than a directly sourced figure — treat it as reasonably confident but not retailer-verified, per our Methodology page. All board prices reflect our last direct Wickes check; expect a few pounds of regional swing at B&Q or elsewhere.
Shop on Amazon: Trex Enhance decking
Do you need a permit or approval?
UK decking rules run through Permitted Development, and the thresholds are more specific than a single height number.
Planning Portal's rule: decking counts as permitted development — no planning permission needed — if it's no more than 30cm above the ground, and combined with other extensions and outbuildings, covers no more than 50% of your garden. There's a third condition worth knowing too: none of the decking can sit forward of the wall that forms the principal elevation of your house, so you can't deck over a front garden under permitted development.
Quick heads-up: if you're in a National Park, an Area of Outstanding Natural Beauty, or your home is listed, tighter rules apply, and decking within a listed building's curtilage generally isn't permitted at all. Always confirm with your local council's planning department before you start — this calculator estimates materials, not planning compliance.
Common mistakes to avoid
- Ordering board count from raw area, no waste allowance. This almost always ends in a mid-project supply run.
- Assuming the longer board is always cheaper overall. As shown above, 3.6m boards can cost less in total than 4.8m boards, despite needing more of them.
- Forgetting joists and substructure aren't included. Budget 15-30% on top if you're building from scratch.
- Picking the wrong install pattern. Herringbone at straight's 10% waste rate will leave you well short.
- Decking over 30cm high or covering more than half the garden without checking Permitted Development rules first.
- Assuming your board's real width matches the 140mm nominal figure exactly. Actual profiles vary slightly by brand — this calculator uses a standard effective width, so treat the board count as a solid planning estimate, then confirm exact coverage against your chosen product's spec sheet.
- Planning a garden with terracing? See our retaining wall block calculator. Planning to stain or paint your deck? Use our paint coverage calculator.
Advertisement
Frequently asked questions
How much decking do I need per square metre?
It depends on your install pattern's waste allowance, not a flat multiplier. For a straight layout, budget your raw area plus 10% for cuts; diagonal needs 15%, herringbone 20%, and a picture-frame border 12%. For a 12 sqm deck laid straight, that's 13.2 sqm of effective coverage needed. Divide by your board's coverage area (board length × 0.14m effective width) to get the exact board count — use the calculator above for your dimensions.
How much does composite decking cost per m² in 2026?
Composite decking boards alone run roughly £70-97 per square metre in 2026, depending on tier — about £70/sqm for economy, £83/sqm for mid-tier, and around £97/sqm for premium, based on a 3m × 4m deck with 4.8m boards. Add hidden fasteners and you're looking at roughly £22 more per square metre. This covers boards and fasteners only — joists and substructure typically add another 15-30% if you're building from scratch.
Do I need planning permission for decking?
Usually not, under Permitted Development, if your decking is no more than 30cm above the ground and, combined with other extensions and outbuildings, covers no more than 50% of your garden. There's a third condition too: none of the decking can sit forward of your house's principal elevation, so you can't deck over a front garden this way. National Parks, AONBs, and listed buildings all have tighter rules. Confirm with your local council before you start.
How many decking boards for a 3m × 4m deck?
For a 3m × 4m deck (12 sqm) laid straight, you'd need 20 boards if you use 4.8m boards, or 27 boards if you use the shorter 3.6m length — that's total area with a 10% waste allowance, divided by each board's coverage. Interestingly, the 3.6m option can come out cheaper overall despite needing more boards, since the per-board price doesn't scale evenly with length. Use the calculator above to compare both for your exact deck.
What joist spacing do I need for composite decking?
This calculator doesn't size your substructure, but most UK composite manufacturers recommend joist centres of 300-400mm for boards laid straight, tightening to around 300mm for diagonal or herringbone patterns, since angled boards cross fewer joists per run. Always check your specific product's installation guide before framing, since exact spacing varies by manufacturer and board thickness — getting this wrong risks a bouncy or sagging deck regardless of how accurate your board count is.
Advertisement
Cost estimates are based on prices from named UK retailers (Wickes, B&Q, others) as of August 2026 and may vary by store and season. As an Amazon Associate we earn from qualifying purchases. Always verify current prices and confirm any planning requirements with your local authority before starting work.
Last updated: August 2026. Estimates are for planning purposes only. Verify with local suppliers before purchase, and check local codes and permit requirements before starting work.