How Many Decking Boards Do I Need?
To work out how many composite decking boards you need, measure your deck area in square metres (length × width) and multiply by two — with our boards, two 3.6m boards cover 1m². Then add around 10% extra for cutting and waste. For a 20m² deck that’s 40 boards, plus roughly four spare, so around 44 in total.
That’s the quick answer. Below we’ll show you the simple formula, how to include the fixing clips and joists you’ll also need, and the fastest way to get an exact figure using our free 3D decking calculator.
How Many Decking Boards Are There Per Square Metre?
Two of our 3.6m composite decking boards cover 1 square metre. So a single board covers about half a square metre. This applies across both our budget-friendly Forest Range and our capped Premium boards, as all our boards share the same 3.6m length.
Keep this rule in mind and the maths becomes very simple: square metres × 2 = number of boards.
The Simple Formula (With a Worked Example)
Here’s the step-by-step:
- Measure your area. Multiply the length by the width of your deck in metres. A deck 5m long and 4m wide is 5 × 4 = 20m².
- Multiply by two. 20m² × 2 = 40 boards.
- Add 10% for waste. Cutting, board ends and the odd mistake mean you should never order the bare minimum. 40 + 10% = 44 boards.
For an irregular garden, split the space into rectangles, work out each one, then add them together. If your design runs boards diagonally or you have lots of angles and steps, add closer to 15% for waste rather than 10%, as diagonal cuts create more offcuts.
Don’t Forget the Fixing Clips
Boards aren’t screwed through the face — they’re held down with hidden clips, which gives that clean, screw-free surface. Each 3.6m board needs 12 fixing clips, and these are bought separately.
So for our 44-board example: 44 × 12 = 528 clips. You’ll also want a pack of starter clips to secure the very first board along each edge. Our calculator adds the right number of clips to your order automatically, so you don’t have to work this out by hand.
What About the Joists and Subframe?
Your boards sit on a subframe of joists, and composite decking needs joists spaced more closely than timber — typically around 300mm apart — to fully support the boards and stop any flex. The number of composite joists you need depends on the direction you run your boards and your joist spacing, so it’s worth planning the subframe before you order. Our installation guide walks through subframe layout, joist spacing and expansion gaps step by step.
How Much Extra Should I Add for Waste?
As a rule of thumb, add 10% to your board count for a standard rectangular deck, or around 15% for diagonal, angled or multi-level designs. It’s always cheaper to order a little spare than to pause a job waiting on two extra boards — and a spare board is handy to keep for any future repairs, since colours can settle slightly over time.
Remember, too, that composite decking expands and contracts slightly with the weather, so leaving the correct expansion gaps at board ends is part of a proper installation — another reason not to cut it too fine on quantities.
The Easiest Way: Use Our Free Decking Calculator
If you’d rather skip the sums, our free 3D decking calculator does it all for you. Enter your measurements and it works out the exact number of boards, clips, joists and accessories, and prices the whole system in one place. You can also use our decking visualiser to see how different colours will look in your space before you buy. It’s the quickest way to go from “roughly how much do I need?” to a complete, accurate order — with no risk of running short.
Once you know your quantities, it’s worth reading up on how much composite decking costs so you can budget the full project, boards, clips, joists and all.
Frequently Asked Questions
How many decking boards do I need per square metre?
Two of our 3.6m composite decking boards cover one square metre, so each board covers about half a square metre. To get your total, multiply your deck area in square metres by two, then add around 10% for waste.
How many decking boards do I need for a 20m² deck?
For a 20m² deck you’ll need about 40 boards (20 × 2), plus roughly 10% extra for cutting and waste — so around 44 boards in total. You’ll also need about 528 fixing clips (12 per board).
How many clips do I need per decking board?
Each 3.6m board needs 12 fixing clips, and they’re bought separately from the boards. Our decking calculator adds the correct number to your order automatically.
How much extra decking should I order for waste?
Add around 10% for a standard rectangular deck, or closer to 15% if your design uses diagonal boards, lots of angles or multiple levels, as these create more offcuts.
What’s the easiest way to work out how much decking I need?
Use our free 3D decking calculator: enter your measurements and it works out the exact number of boards, clips and joists and prices the full system, so you don’t have to do the maths yourself.
