Decking calculator
Estimate a timber deck from its dimensions, board width and length, gaps, joist spacing, fixings and cutting allowance.
Timber deck material estimate
Enter the deck dimensions and board specifications to estimate boards, joists and fixings.
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How does this tool work?
A timber or composite deck is ordered in boards of stock length, joists and boxes of screws, not in square metres. This calculator takes the deck length and width, board width, gap between boards and joist spacing to count the rows, boards to buy, joists and their total length, plus a screw count consistent with the number of board-joist crossings.
Boards run along the length. The number of rows is (width + gap) ÷ (board width + gap), rounded up: with 145 mm boards and a 5 mm gap, a 4 m wide deck needs 27 rows.
Linear metres (rows × deck length) are divided by the stock length of one board, increased by the cutting allowance and rounded up to whole boards. This assumes offcuts from one row can be used to finish another.
Joists run perpendicular to the boards, across the width: their number is length ÷ spacing, rounded up, plus one. Screws equal rows × joists × screws per crossing. The purchased area is boards ordered × stock length × board width.
Limits: the deck is a rectangle with a single board direction, without herringbone patterns or a picture-frame border. Joists are counted at the exact deck width, without splices or doubled joists at board butt joints. Pedestals, posts, bearers, joist tape and hidden clips are not calculated. Spans, sections and ventilation rules follow local guidance (IRC and AWC DCA 6 in the US, TDCA guidance in the UK, NF DTU 51.4 in France) and the manufacturer: the tool checks no span.
Deck coverage depends on the usable board width and the gap between boards, while stock lengths determine offcuts. A board quantity calculation does not size the supporting structure: joists, spacing, spans and fixings must be considered separately.
How to use this tool
Measure the deck
Enter the length along the boards and the width along the joists, in metres.
Describe the board
Enter its actual width in millimetres, the planned gap between boards and the stock length available from your supplier.
Set the substructure
Enter the joist spacing recommended for the board thickness and species, then the number of screws per crossing.
Read the materials list
Adjust the cutting allowance, then note boards, joist linear metres, screws and purchased area.
Boards, joists and screws
Boards are counted in rows, joists in gaps and screws at each crossing.
Board rows
R = ⌈(W + g) ÷ (b + g)⌉
W deck width, b board width, g gap.
Boards to buy
N = ⌈R × L ÷ Lb × (1 + allowance)⌉
L deck length, Lb board length.
Joists and screws
J = ⌈L ÷ spacing⌉ + 1; screws = R × J × screws per fixing
Joists carry the boards at right angles.
Boards are assumed to run along the length and joists across the width. Pedestals, posts, bearers and joist tape are not counted.
Use cases
Deck on joists and pedestals
Prepare the list of boards, joists and screws for a 15 to 30 m² deck before checking timber merchant prices.
Choosing board length
Compare 3 m, 4.2 m or 5.1 m boards to see which one keeps pieces and offcuts down for your deck length.
Matching joist spacing to thickness
Move from 500 mm to 400 mm spacing for thinner boards and see the effect on joists and screws.
Examples
6 × 4 m deck with 145 mm boards
4.005 ÷ 0.150 = 26.7, so 27 rows of 6 m. 162 ÷ 3 = 54 boards, 59.4 with allowance, rounded to 60. 6 ÷ 0.40 = 15 gaps, so 16 joists of 4 m.
Input
6 × 4 m, 145 mm boards, 5 mm gap, 3 m boards, 40 cm joist spacing, 10% allowance, 2 screws per fixingResult
27 rows, 162 m of boards, 60 boards, 16 joists (64 m), 864 screws, 26.1 m² bought5 × 3 m deck with 120 mm boards
Narrower boards mean more rows and screws: 24 × 11 × 2 = 528 screws for 15 m².
Input
5 × 3 m, 120 mm boards, 6 mm gap, 4.2 m boards, 50 cm joist spacing, 10% allowance, 2 screws per fixingResult
24 rows, 120 m of boards, 32 boards, 11 joists (33 m), 528 screws, 16.1 m² boughtTips and best practices
- Where possible, choose a board length that divides the deck length: 6 m covered with 3 m boards leaves far fewer offcuts than with 4 m boards.
- Plan a doubled joist at every board butt joint: add them to the result when the deck is longer than the boards.
- The gap between boards depends on the timber moisture at installation: ask the supplier for tropical hardwood or pressure-treated softwood.
- Stainless screws come in boxes of 100, 200 or 500: round the count up to the next box and keep a margin for the edges.
Sources and references
Frequently asked questions
How much allowance should I add for the boards?
It depends on the layout and the number of cuts. The allowance is adjustable: increase it for a complex shape and reduce it only when the cutting plan is under control.
Does the calculator size the joists?
No. It estimates their number and length from a spacing. Section size, spans and supports must be checked against the construction system you use.
Why is the number of screws only indicative?
It depends on the fixing system, the screws per crossing, the edges and the installation details. The result helps you prepare a quantity, not a final fixing plan.
What joist spacing should I use?
It depends on board thickness and species: the thinner the board, the closer the joists. Timber trade guides and manufacturers give the maximum span; 400 to 500 mm is common for boards around 21 to 28 mm thick, and composite boards often need closer spacing.
What gap should I leave between boards?
A few millimetres, often 3 to 8 mm depending on species, moisture content at installation and board width. Wet timber shrinks as it dries, very dry timber swells: follow the supplier’s advice.
How many screws per square metre?
With two screws at every board-joist crossing, the screw count is rows × joists × 2. In the first example that is 36 screws per m²; buy one extra box for edges and mistakes.
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