Decking calculator
Count decking boards by row rather than by area, plan the cuts, and size the joists, bearers, posts, fixings and post-hole concrete.
Defaults to metric and the board profiles UK timber merchants actually stock: 144 mm widths, 5 mm gaps and joists at 400 mm centres. Switch the region control to US for feet, inches and lumber sizing.
UK terminology throughout — timber rather than lumber, bearers rather than beams, prices in pounds with a VAT toggle.
Your project
Quick start
Deck size
Rows build up across the width. Joists run across the length.
Boards
The default for UK and US decks. Lightest and cheapest, needs periodic treatment.
The stock length you can buy. The cut plan reuses off-cuts from this length.
5–6 mm for timber that will shrink; composite gaps are set by the manufacturer.
Two screws per board at every joist.
Frame
400 mm centres suit most timber boards; composite usually needs 300–400 mm.
Steps and trim
Estimate updated. 26 boards of Treated softwood decking boards required.
Decking calculator
United Kingdom · Metric units · 30 July 2026
Shopping list
7 lines- 26boardsTreated softwood decking boards144 mm × 28 mm × 3.60 m lengths
- 37mJoists — 47 mm × 145 mm11 joists at 400 mm centres, each 3.00 m (9.84 ft) long
- 9mBearers2 at 1.80 m (5.91 ft) centres, each 4.00 m (13.12 ft) long
- 6postsPosts2 bearers × 3 posts at 1.80 m (5.91 ft) centres
- 486screwsDecking screws2 per board at each of 11 joists
- 5boardsFascia boardsPerimeter of 14.00 m (45.93 ft)
- 2425 kg bagsConcrete for post holes6 holes at 300 mm diameter × 600 mm deep
Estimate results
What you measured
- Deck area
- 12.00 m²
- 129.17 ft²
- Board rows
- 21
- Total board length
- 84.00 m (275.59 ft)
- Joists
- 11
- Posts
- 6
What to buy
10% waste included| Material | Buy | Weight |
|---|---|---|
Treated softwood decking boards 144 mm × 28 mm × 3.60 m lengths Rounding: The calculation requires 23.33 boards. Adding 10% waste gives 25.67. Rounded up to 26 boards. Main material | 26boards | 208 kg |
Joists — 47 mm × 145 mm 11 joists at 400 mm centres, each 3.00 m (9.84 ft) long Rounding: The calculation requires 33 m. Adding 10% waste gives 36.3. Rounded up to 37 m. Structure | 37m | 139 kg |
Bearers 2 at 1.80 m (5.91 ft) centres, each 4.00 m (13.12 ft) long Rounding: The calculation requires 8 m. Adding 10% waste gives 8.8. Rounded up to 9 m. Structure | 9m | 68 kg |
Posts 2 bearers × 3 posts at 1.80 m (5.91 ft) centres Rounding: The calculation requires 6 posts. No waste allowance was added. Rounded up to 6 posts. Structure | 6posts | — |
Decking screws 2 per board at each of 11 joists Rounding: The calculation requires 462 screws. Adding 5% waste gives 485.1. Rounded up to 486 screws. Fixings | 486screws | — |
Fascia boards(optional) Perimeter of 14.00 m (45.93 ft) Rounding: The calculation requires 3.89 boards. Adding 10% waste gives 4.28. Rounded up to 5 boards. Finishing | 5boards | 40 kg |
Concrete for post holes 6 holes at 300 mm diameter × 600 mm deep Rounding: The calculation requires 21.57 25 kg bags. Adding 10% waste gives 23.72. Rounded up to 24 25 kg bags. Base and membrane | 2425 kg bags | 680 kg |
Assumptions behind each line
Treated softwood decking boards
- 21 rows of boards across 3.00 m (9.84 ft), at 144 mm wide plus a 5 mm gap.
- 84 linear metres of board in total.
- Cut plan uses 24 lengths with 97.2% utilisation.
Joists — 47 mm × 145 mm
- Joists run perpendicular to the boards and are spaced along the 4.00 m (13.12 ft) run.
- Joist size and span must be checked against a span table for the load and the spacing used.
Bearers
- Bearers run parallel to the boards and carry the joists.
- Assumed to be twice the joist thickness. Size them from a span table for your post spacing.
Posts
- Post spacing drives the bearer size. Closer posts allow a smaller bearer section.
- Post height and fixing method depend on the ground and the deck height, which are not estimated here.
Decking screws
- Two screws per board at every joist crossing, which is standard practice for face fixing.
- Use stainless or coated fixings; plain steel stains treated timber.
Fascia boards
- One row of fascia around the full perimeter. Deep frames may need two rows.
Concrete for post holes
- Hole volume π × r² × depth = 42.4 litres per post.
- No deduction is made for the post itself, which gives a little spare.
- Hole depth must reach below the frost line and suit the ground conditions.
Board and joist layout
How this was calculated
Deck area
4.00 m × 3.00 m
= 12.00 m² (129.17 ft²)
Board rows
(3.00 m (9.84 ft) + 5 mm gap) ÷ (144 mm board + 5 mm gap)
= 21 rows
- Boards are counted by row, not by area. Dividing deck area by board area ignores the gaps and under-orders.
Total board length
21 rows × 4.00 m (13.12 ft) run
= 84.00 m (275.59 ft)
Boards required
The calculation requires 23.33 boards. Adding 10% waste gives 25.67. Rounded up to 26 boards.
= 26 boards of 3.60 m
- Cut plan: 24 lengths, 2.4 m of off-cut, 97.2% utilisation.
Joists
(4.00 m (13.12 ft) ÷ 400 mm) + 1
= 11 joists, 33.00 m (108.27 ft) total
Bearers
(3.00 m (9.84 ft) ÷ 1.80 m (5.91 ft)) + 1
= 2 × 4.00 m (13.12 ft) = 8.00 m (26.25 ft)
Posts
2 bearers × ((4.00 m (13.12 ft) ÷ 1.80 m (5.91 ft)) + 1)
= 6 posts
Screws
21 rows × 11 joists × 2
= 462 screws before waste
Weight and delivery
Approximate weight
1.13 tonnes
Likely delivery
tipper or grab lorry
- Board lengths of 3.60 m will not fit in a car. Check the merchant's delivery vehicle can reach your access.
- The boards alone weigh about 208 kg (458 lb).
- 1.13 tonnes needs a tipper or grab lorry. Confirm access width, overhead clearance and where the driver can tip.
- Check that the delivery route can take the weight — block paving, drainage covers and soft verges are easily damaged.
Important: Boards will need joining
Safety and compliance
Material planning only
This estimate is for material planning only. Structural dimensions, foundations and compliance requirements should be checked by a qualified professional.
Deck framing must be designed, not guessed
Joist and bearer sizes, post fixings, footing depth and any connection to a building are structural decisions. Raised decks and decks attached to a house need particular care — a badly fixed ledger is one of the most common causes of deck collapse.
Check whether approval is required
Decks over 300 mm (12 in) above ground, decks attached to a dwelling, and decks covering more than half the garden often need approval. Confirm the requirements with your local authority Building Control team before you start.
Data behind this estimate
| Assumption | Value | Source |
|---|---|---|
Treated softwood decking and framing Freshly treated timber is heavier than kiln-dried stock and can exceed the range. | 550 kg/m³range 450–650 | TRADA / BS EN 338Reviewed 2026-07-01 |
Deck joist spacing 400 mm (16 in) centres suit most 32–38 mm softwood boards laid square to the joists. | 400 mm centresrange 300–600 | Timber Decking and Cladding AssociationReviewed 2026-07-01 |
Wet concrete Normal-weight concrete. Lightweight mixes are considerably lighter. | 2,400 kg/m³range 2,300–2,500 | British Standards InstitutionReviewed 2026-07-01 |
Bagged ready-to-use concrete yield Derived from the published 80 lb (36.3 kg) ≈ 0.60 ft³ (0.0170 m³) yield. | 0 m³ per kg of dry mixrange 0–0.001 | Bagged concrete manufacturers (Quikrete, Sakrete)Reviewed 2026-07-01 |
Default waste allowance Covers cutting, breakage, setting-out error and the odd unusable unit. | 10 %range 5–15 | How Much NeededReviewed 2026-07-01 |
Cost
Enter the prices your supplier quoted. This site does not hold or estimate supplier prices, so totals appear only once you fill these in.
UK standard-rate VAT is 20%. Builders' merchant prices are often quoted excluding VAT.
No prices entered yet. Everything else in this estimate — quantities, weight, delivery and the shopping list — works without them.
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How this calculator works
Every figure below is derived rather than looked up, so you can check it against your own numbers. Nothing is hidden behind a black box.
Board rows, not area
A deck is covered by whole board widths separated by gaps. The number of rows is the span across the boards, plus one gap, divided by the module of one board plus one gap, rounded up.
Dividing the deck area by the board area is the common shortcut and it always under-orders, because it treats the gaps as if they were covered by timber.
rows = ⌈(span + gap) ÷ (board width + gap)⌉
Board direction
Which way the boards run changes everything downstream: the number of rows, the length of each run, the number of joists and how much cutting there is. Both options are offered because the answer is genuinely different.
Cutting plan
The board quantity is the greater of two numbers: total linear length divided by the stock length plus waste, and the number of stock lengths a first-fit-decreasing cutting plan actually consumes when off-cuts are reused.
The second number is often higher. A 4 m run cut from 3.6 m boards leaves a 0.4 m piece from every board, and you cannot make a 4 m row out of leftovers.
Frame
Joists run perpendicular to the boards and are counted along the board run at the spacing you set, plus one for the final joist. Bearers run parallel to the boards beneath the joists, and posts are counted along each bearer.
Sizes are not designed here. Joist depth, bearer section and post fixing all depend on span and load and must come from a span table or an engineer.
joists = ⌊run length ÷ spacing⌋ + 1
Fixings
Face-fixed decking takes two screws per board at every joist crossing. Hidden fastener systems take one clip per crossing, though some need two at board ends — check the system before ordering.
screws = rows × joists × 2
Post-hole concrete
Each hole is treated as a cylinder: π × radius² × depth, using 300 mm × 600 mm in the UK and 12 in × 30 in in the US. No deduction is made for the post itself, which leaves a small margin.
Common questions
- How many decking boards do I need?
- Divide the span across the boards by the width of one board plus one gap, then round up to get the number of rows. Multiply by the length of the run for total linear metres, and divide by the board length you can buy. A 4 × 3 m deck in 144 mm boards with 5 mm gaps needs 21 rows and 84 linear metres.
- What gap should I leave between decking boards?
- 5–6 mm (about 3/16 in) for timber that has been stored dry, and less for timber that arrives wet since it will shrink. Composite gaps are set by the manufacturer and are often larger, with a separate end gap for expansion.
- How far apart should deck joists be?
- 400 mm (16 in) centres suit most 28–32 mm softwood boards laid square to the joists. Composite boards usually need 300–400 mm, and any board laid diagonally needs the spacing reduced further.
UK product sizes and conventions
Board profiles commonly stocked in the UK
- 144 × 28 mm softwood in 3.0 m, 3.6 m and 4.8 m lengths — the most widely available profile.
- 120 × 28 mm softwood, a narrower option that needs more rows and more screws.
- 145 × 32 mm heavy-duty softwood for wider joist spacing.
- 145 × 23 mm composite in 3.6 m lengths, usually requiring 300–400 mm joist centres.
UK frame conventions
- Joists at 400 mm centres for softwood boards laid square to the joists.
- C16 or C24 graded, treated softwood, typically 47 × 145 mm for joists.
- Bearers at 1.2–1.8 m centres depending on the joist section.
- Post holes commonly 300 mm diameter × 600 mm deep, filled with a bagged concrete mix.
Buying and VAT
- Timber merchant prices are frequently quoted excluding VAT — check before comparing with a DIY shed.
- The VAT toggle applies the 20% standard rate to the material subtotal and any delivery figure you enter.
- Boards over 3 m will not fit in most cars. Check the merchant's delivery vehicle can reach your access.