Deck Board Calculator
Deck board calculator — enter your deck size and the board width, length and spacing gap to get the number of decking boards to buy, with a waste allowance. Works for timber and composite decking. Runs in your browser.
Deck Board Calculator
How to Use the Deck Board Calculator
Measure the deck
Enter the deck length and width in metres.
Enter board size
Add the board width, length and the drainage gap.
Set the waste
~10% straight; more for diagonal patterns.
Read the count
See boards needed, rows and boards per row.
Counting Boards, Not Just Area
Decking is one of those jobs where a simple area calculation quietly misleads you, because boards are bought as fixed-length pieces and laid in rows with a deliberate gap between them. This calculator counts the way you actually build: it divides the deck width by the board width plus the drainage gap to find how many rows of boards you need, divides the deck length by the board length to find how many boards make up each row, multiplies the two, and then adds a waste allowance for the cuts and end joints. The result is the real number of boards to order, not a square-metre figure you would have to convert and guess at.
The gap between boards is small but important. Decking is laid with a three-to-eight-millimetre gap so rainwater drains freely and the boards can expand and contract through the seasons without buckling — composite decking in particular needs a larger, manufacturer-specified gap and end-gap. Because each board plus its gap covers slightly more width than the board alone, the gap subtly reduces how many rows you need, and the calculator accounts for it directly. The default board here is a common 140-millimetre-wide, 3.6-metre-long profile, but you should enter the exact dimensions and recommended gap of the boards you are buying.
A few build realities sit alongside the board count. Boards run perpendicular to the joists below, so each board is supported across several of them; joist spacing is typically 400 to 600 millimetres for timber and often closer for composite, but that sub-frame — joists, bearers, posts and fixings — is a separate calculation from the surface boards this tool counts. Where a board cannot span the full deck, pieces meet end-to-end over a joist, and you stagger those joints between rows for strength and looks, which creates offcuts and is part of why the waste allowance exists. Allow about ten percent for a straight rectangular deck and more for diagonal or picture-frame layouts. Treat the figure here as your board order, keep a couple of spares, and plan the sub-frame separately. As with every RECATOOLS tool, the whole calculation runs in your browser, so your measurements never leave your device.
Decking is counted in boards and gaps, not square metres — the drainage gap and staggered end joints both change the order.
10 Facts About Decking
Decking boards need a 3–8 mm gap for drainage and movement.
Composite boards expand more, so follow the maker’s gap.
Run boards perpendicular to the joists.
Joists are usually spaced 400–600 mm apart.
A typical board is 140 mm wide.
Allow 10% waste for cuts and end joins.
Hidden fasteners give a cleaner finish than screws.
Leave a gap at the ends for thermal movement.
Diagonal decking patterns waste more boards.
This calculator runs in your browser — nothing is uploaded.
Frequently Asked Questions
- The calculator divides the deck width by the board width plus the gap between boards to find how many rows you need, divides the deck length by the board length to find how many boards make up each row, multiplies the two, and adds a waste allowance. The result is the total number of boards to buy.
- Decking is laid with a small gap — typically 3 to 8 millimetres — so that rainwater drains away and the boards can expand and contract without buckling. That gap means each board effectively covers a little more width than the board alone, which slightly reduces how many rows you need. Composite decking usually needs a larger gap than timber.
- Around 10 percent for a simple rectangular deck laid straight. Increase it for diagonal or herringbone patterns, which produce more offcuts, and for decks with lots of edges or cut-outs around posts. Rounding up to whole boards already adds a little slack, but the waste percentage protects against the cuts.
- Boards should run perpendicular to the joists beneath them, so each board is supported across several joists. This affects the structure rather than the board count, but it is worth planning before you buy, because changing direction changes which dimension the board length has to span.
- Use the actual dimensions of the boards you are buying. A very common timber and composite deck board is about 140 millimetres wide and supplied in lengths like 3.6 metres, which are the defaults here. Check the product specification, as widths of 120 and 145 millimetres and various lengths are also sold.
- No — this calculator counts the surface deck boards only. The supporting joists, bearers, posts and fixings are a separate calculation based on your joist spacing (commonly 400 to 600 millimetres for timber, often closer for composite). Plan the sub-frame separately once you know the deck size.
- Where a single board cannot span the full deck length, boards meet end-to-end over a joist, and you should stagger these joints between rows for strength and appearance. Staggering creates offcuts, which is part of why the waste allowance exists. The calculator assumes you cut from full boards and reuse where practical.
- The board-count maths is the same, but composite decking typically requires a slightly larger expansion gap and specific end-gap rules from the manufacturer, and it is often fixed with hidden fasteners. Enter the composite board’s width, length and recommended gap and the count will reflect it.
- It is a solid estimate for a rectangular deck. Diagonal layouts, picture-frame borders, cut-outs and staggered joints all affect the true number, which is why the waste allowance matters. Use the figure to order with confidence and keep a couple of spare boards.
- Completely free, with no account or limit. It works offline once the page has loaded and collects no data.
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