Decking Calculator
Estimate boards, joists, bearers, posts and concrete for a UK domestic deck.
Your materials list
Enter deck size and we'll build the materials list.
How it works
What this calculator does. It converts a rectangular deck footprint into a full materials list — deck boards, joists, bearers, posts and post-hole concrete — using standard UK dimensions and spacings.
The formula in plain English. Boards = (deck length ÷ board coverage) × (deck width ÷ board length), with waste added on. Board coverage = board width + 5 mm gap. Joists run perpendicular to boards at 400 mm centres. Bearers and posts sit at 1.8 m centres — the trade rule for C16 softwood at typical domestic loading.
A worked example. A 4 m × 3 m deck with 145 mm boards at 3.6 m lengths: coverage 150 mm/board, so 27 rows across 4 m × 1 board per row (3 m ≤ 3.6 m) = 27 boards, +10% waste = 30 boards. Joists at 400 mm centres = 11 joists. Bearers 2, posts 4–6, concrete ~0.18 m³ (15 bags). That's your builder's-merchant shopping list.
Why this matters. Decking is the classic DIY project where people underorder boards and end up with a mismatched pack from a second delivery — production runs vary in colour on both softwood and composite. Getting the count right in one go also lets you compare merchant quotes on like-for-like quantities.
Frequently asked
Complete Guide
How to Plan and Build a Garden Deck: A Beginner's Guide for UK Homeowners
There is something wonderfully satisfying about stepping out of your back door onto a brand-new, perfectly level wooden deck with a morning brew in hand. Whether you want a sunny spot for summer barbecues, an extension of your indoor living room, or simply a low-maintenance way to transform an awkward patch of grass, building a garden deck is one of the most rewarding home improvement projects you can tackle.
Quick takeaways
- A ground-level deck under 30 cm high is usually permitted development in the UK.
- Never lay joists directly onto soil — always sit them on slabs, pedestals or concrete pads.
- Choose between softwood, hardwood and composite based on maintenance appetite, not just price.
- Add a 10% wastage allowance to every timber order to cover cuts and splits.
- Use the Decking, Concrete and Gravel Calculators together to build one accurate shopping list.
This is the first specialist companion to our [Home & DIY Planning Guide](/guides/the-complete-home-diy-planning-guide) — walking you through every stage of a garden deck project so you avoid costly pitfalls and achieve professional-looking results.
1. Is decking right for your garden?
Before you start pricing up timber or borrowing tools from a neighbour, take a realistic look at your outdoor space. Decking can transform almost any garden, but a few layouts need extra thought.
- Sun versus shade. Where does the sun land in the afternoon? If your garden is north-facing and shaded by fences or mature trees, put the deck in the brightest corner rather than tight against the house.
- Sloping gardens. Slight slopes work fine with adjustable pedestals or timber posts. Steep gradients need specialist engineering and secure retaining work — get a pro on the phone.
- Drainage. Water is timber's greatest enemy. If your chosen spot turns boggy after autumn rain, factor in generous ground clearance, weed membrane and a thick layer of gravel.
- Planning permission. For most rear gardens, a ground-level deck is permitted development — as long as it sits no more than 30 cm above ground level, covers under 50% of your garden, and is not forward of the principal elevation. Check with your local authority in conservation areas or on listed properties.
2. Choosing your deck size and calculating materials
One of the most common DIY mistakes is guessing quantities "by eye" and ending up either running back to the timber yard or staring at a mountain of leftover boards.
Mark out your proposed deck using wooden pegs and string. Walk the perimeter, place garden chairs where you plan to sit them, and check whether doors and windows open freely above the proposed height.
Once you have your dimensions — say a comfortable 3 m × 4 m rectangle — you'll need to calculate decking boards, joists and bearers, posts or concrete pads, and a 10% wastage allowance.
Instead of wrestling with spreadsheets, drop your measurements straight into our free [Decking Calculator](/calculators/decking) for an instant breakdown of every board and fixing.
3. Softwood vs hardwood vs composite
Choosing your decking material is a balance between upfront budget, aesthetic and how much weekend maintenance you're willing to commit to.
| Material | Cost | Maintenance | Lifespan | Best for |
|---|---|---|---|---|
| Softwood (pressure-treated pine) | £15–£30 / m² | Annual clean and stain/oil | 10–15 years | Budget-conscious first-timers |
| Hardwood (Balau, Ipe) | £60–£120+ / m² | Periodic oiling to hold colour | 25+ years | Natural grain and premium look |
| Composite (wood-plastic mix) | £40–£80+ / m² | Occasional soapy wash | 20–25+ years | Splinter-free, low-fuss family gardens |
4. Planning your budget
A great DIY project never breaks the bank, but unexpected costs creep up when you only budget for the visible boards. A realistic deck budget covers:
- Core materials — decking boards, joist timber and fascia trims.
- Subframe foundations — concrete pads, paving slabs or adjustable pedestals (never lay wooden joists directly onto soil).
- Ground preparation — heavy-duty weed membrane and aggregate. Check quantities with our [Aggregate Calculator](/calculators/aggregate). Where you need embedded posts or concrete footings, our [Concrete Calculator](/calculators/concrete) tells you how many bags to order.
- Fixings — high-grade exterior decking screws, joist hangers and corrosion-resistant brackets.
- Tool hire and delivery — jigsaws, mitre saws, spirit levels, PPE and merchant delivery fees.
- Contingency — always add 10% to your total for unforeseen sundries.
If your deck project also involves replacing or extending a boundary, plan the timber for that side at the same time with our [Fence Calculator](/calculators/fence).
5. Step-by-step project overview
This guide is a roadmap, not an exhaustive engineering manual — but knowing the sequence keeps your project smooth and stress-free.
Step 1 — Ground preparation. Clear turf, weeds and organic debris from the marked area. Excavate roughly 100 mm of soil if needed, lay heavy-duty weed membrane and cover with pea shingle or gravel for excellent under-deck drainage.
Step 2 — Setting out supports. Position concrete paving slabs or adjustable feet at regular intervals across the gravel. These carry the outer frame and internal joists, keeping timber safely elevated above damp soil.
Step 3 — Building the frame. Construct a sturdy outer rectangular frame from pressure-treated timber (typically 100 × 47 mm joists). Check the diagonals corner-to-corner — matching diagonals mean the frame is square. Fix internal joists parallel at 400–600 mm centres using galvanised joist hangers.
Step 4 — Laying the deck boards. Lay boards perpendicular to the joists, starting from the edge closest to the house. Keep a consistent 3–5 mm gap between boards for rainwater drainage and thermal expansion. Fix every board to every intersecting joist with two exterior-grade decking screws.
6. Common decking mistakes to avoid
Even seasoned DIYers stumble when they rush the basics. Keep these reminders in mind:
- Forgetting drainage. Never trap water against the house wall or let puddles pool under the frame. Good slope and subframe clearance are essential.
- Inconsistent board spacing. Use dedicated spacers (not nails or tile spacers) so gaps stay uniform from one side of the deck to the other.
- Ignoring boundaries. Measure twice against fences and property lines so you have room to finish edges and fascia neatly.
- Skipping end-grain treatment. Whenever you cut pressure-treated timber, brush end-grain preservative onto the cut to stop rot creeping in.
Conclusion
Building your own garden deck is one of the most rewarding home improvements you can undertake. Take time to measure accurately, choose materials that suit your lifestyle, and map out your budget before buying a single plank.
When you're ready, use our free [Decking Calculator](/calculators/decking) — alongside the [Concrete Calculator](/calculators/concrete), [Aggregate Calculator](/calculators/aggregate) and [Fence Calculator](/calculators/fence) — to take the guesswork out of your materials list. And if you haven't yet, start with the cornerstone [Home & DIY Planning Guide](/guides/the-complete-home-diy-planning-guide) for the wider planning framework this project sits inside.
Related Home & DIY guides
Keep learning with more from the Calcaroo library.