Decks & Fences
Decking Calculator
Enter your deck length and width in metres to get the number of decking boards and joists to buy, using editable standards of 140 mm boards and 450 mm joist centres.
Formula tested · Local units · No sign-up
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Show the calculation methodFormula, conversions, rounding, and assumptions+
Rows of boards = deck width ÷ (board width + gap), rounded up. A common UK softwood decking board is 140 mm wide; with a 3 mm drainage gap each row takes 143 mm of width.
Boards per row = deck length ÷ board length, rounded up — 4.8 m boards on a 6 m deck means 2 boards per row with an offcut.
Boards to buy adds your wastage allowance and rounds up; joists = deck length ÷ joist spacing (450 mm centres is a common standard) rounded up, plus one closing joist.
Real-world example
Worked example: 6 m × 4 m deck, 140 mm boards, 3 mm gap, 4.8 m boards, 450 mm centres
- Row width: 140 + 3 = 143 mm. Rows: 4 ÷ 0.143 = 27.97 → round up to 28 rows.
- Boards per row: 6 ÷ 4.8 = 1.25 → round up to 2 boards.
- Boards to buy: 28 × 2 × 1.10 (10% wastage) = 61.6 → round up to 62 boards.
- Joists: 6 ÷ 0.45 = 13.33 → 14, + 1 closing joist = 15 joists.
- Example cost: 62 boards × £10 = £620; 15 joists × £8 = £120; materials £740; VAT 20% = £148.
Buy 62 boards and 15 joists. With the example prices above the estimate is £888.00 including VAT — enter your supplier's quote.
Before you start
How to measure
- Measure the deck length (the direction the boards run) and width in metres.
- Check your board's actual width — UK softwood decking is commonly 120 mm or 140 mm; grooved and smooth faces share the same width.
- Enter the stock length you'll buy (3.6 m and 4.8 m are common) — planning rows so joints land on joists reduces waste.
Local guidance
Notes for United Kingdom
- Raised decks may need planning permission in the UK — permitted-development rules limit deck height (300 mm is the commonly cited threshold) and how much of the garden a deck may cover; check with your local planning authority before building.
- 450 mm joist centres is a common spacing for standard softwood decking boards; thinner boards or diagonal laying need closer centres — follow the board supplier's guidance.
- Softwood (usually pressure-treated) is the common UK deck board; composite boards use the same calculator — enter the manufacturer's stated width, gap and length.
- VAT at the standard 20% rate applies to most decking supply; trade quotes are often ex-VAT.
Quick reference
Common UK decking planning values
| Item | Common value |
|---|---|
| Board width | 120 or 140 mm |
| Drainage gap | 3–6 mm |
| Board stock lengths | 3.6 m, 4.8 m |
| Joist centres | 450 mm (closer for thin or diagonal boards) |
Planning values only — the board supplier's guidance and your structure govern.
Good to know
Common mistakes to avoid
- Typing board width in centimetres into a millimetre field — 14 instead of 140 makes the row count explode.
- Forgetting the gap between boards — timber needs drainage and expansion room.
- Building a raised deck without checking planning rules — height and coverage limits apply.
- Treating this as a structural design: joist size, span, posts and footings are separate decisions this tool does not make.
Need help?
Frequently asked questions
How many boards for a 4 m × 3 m deck with 140 mm boards?
Rows: 3 ÷ 0.143 = 20.98 → 21 rows. With 4.8 m boards, one board per row covers the 4 m length, and 21 × 1.10 = 23.1 → 24 boards with 10% wastage. Joists at 450 mm: 4 ÷ 0.45 = 8.89 → 9, + 1 = 10 joists.
Do I need planning permission for decking?
Sometimes. Permitted development commonly allows low decks, but raised platforms (the widely cited threshold is 300 mm above ground) and large coverage can need permission, and extra rules apply in conservation areas. Check with your local planning authority.
Why is there always a gap between boards?
The gap (commonly 3–6 mm for timber) lets rainwater drain and gives boards room to move as they take up and lose moisture. Composite manufacturers specify their own gapping — use their figure in the gap field.
Keep planning
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About this calculator
- Written by:
- BuildMeasure Editorial Team
- Technically reviewed by:
- Pending independent technical reviewer (formula unit-tested; see methodology)
- Last reviewed:
- 2026-07-16
- Formula version:
- 1.0.0
- Region reviewed for:
- United Kingdom
- Spotted an error?
- Report a correction
Methodology
- Boards are assumed to run parallel to the deck length; joists run parallel to the width, spaced along the length. All dimensions are converted to metres internally before any arithmetic.
- Rows of boards = deck width ÷ (board width + gap), rounded UP. Boards per row = deck length ÷ board length, rounded UP — part boards count as whole boards because that's what you buy.
- Boards to buy = rows × boards per row × (1 + wastage%), rounded UP. Joists = deck length ÷ joist spacing, rounded UP, plus one closing joist. Exact multiples are not bumped up.
- The cost estimate multiplies the board and joist counts by the prices you enter, then applies the tax rate you enter. No prices are built in.
- The formula is covered by automated unit tests, including hand-calculated worked examples, and is versioned (see formula version on this page).
Sources & standards
- Board dimensions: 120 mm and 140 mm are common UK decking board widths; confirm your supplier's actual size.
- Joist centres: 450 mm centres is common UK practice for standard boards; supplier guidance governs.
This tool provides a material estimate for planning purposes only. It is not a quotation, and it does not size joists, bearers, posts, footings or ledger connections — deck structure is an engineering and permit matter. Confirm the structural design with a qualified person and quantities with your supplier before ordering.