What this calculator does
A decking calculator turns a deck's length and width into a materials list: how many boards to buy, how many screws or fixings to have on hand, and roughly what the boards will cost. Getting the board count right depends on the gap left between boards for drainage and expansion, not just the raw floor area, which is easy to forget when eyeballing a quote.
This calculator takes the deck dimensions, the board width and the gap between boards, and works out how many rows of boards span the width and how many board lengths are needed along the length, then multiplies that into a total board count, a fixings estimate and a cost from a price-per-board figure you supply.
The formula
The number of board rows across the deck's width is the width divided by the board width plus the gap left between boards, rounded up. The boards needed along each row is the deck length divided by the length of board you can buy, also rounded up, since a board cannot be joined mid-span without a joist underneath it. Multiplying rows by boards-per-row gives the total board count, which then feeds the fixings estimate and cost.
| Term | Meaning |
|---|---|
| Board width | The width of a single decking board, commonly 90 mm to 140 mm depending on the product. |
| Gap | The spacing left between adjacent boards for drainage and timber movement, typically 3-5 mm. |
| Board length available | The standard length the boards are sold in, which sets how many lengths are needed to span the deck. |
| Fixings | Screws or hidden clips securing each board to the joists beneath it. |
The inputs explained
| Field | What to enter |
|---|---|
| Deck length (m) | The length of the deck, in the direction the boards run along. |
| Deck width (m) | The width of the deck, across the direction the boards run. |
| Board width (mm) | The width of a single decking board, before adding the gap. |
| Gap between boards (mm) | The gap left between boards, added to the board width to work out how many rows fit across the deck. |
| Board length available (m) | The length a single board is sold in; boards longer than the deck are trimmed, boards shorter need joining over a joist. |
| Joists each board crosses | The number of joists each board crosses and needs fixing to. |
| Fixings (screws) per board per joist | The number of screws or fixings used at each joist crossing, per board. |
| Price per board ($) | The price of a single decking board, used to estimate total material cost. |
When to use it
Quoting a deck build
Working out the board count and fixings before ordering avoids a second trip to the timber yard for a handful of extra boards, and gives a rough cost figure to compare against a tradesperson's quote.
Choosing a board width
A narrower board needs more rows to cover the same width, which raises the board count and the number of fixing points; running the numbers at a couple of board widths shows the trade-off before buying.
Budgeting for fixings
Fixings are bought in boxes, and running out partway through a build stalls the job; multiplying the total board count by the fixing points per board gives a number to round up to a box size.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does board count change with deck width?
A fixed 6 m deck length, across a range of deck widths.
| Deck width | Decking boards needed | Deck area | Board rows across the width |
|---|---|---|---|
| 2 m | 28 | 12.00 m² | 14 |
| 3 m | 42 | 18.00 m² | 21 |
| 4 m | 56 | 24.00 m² | 28 |
| 5 m | 70 | 30.00 m² | 35 |
| 6 m | 84 | 36.00 m² | 42 |
| 8 m | 112 | 48.00 m² | 56 |
How does board count change with board width?
A fixed 6 m by 4 m deck, across a range of board widths.
| Board width | Decking boards needed | Board rows across the width |
|---|---|---|
| 90 mm | 86 | 43 |
| 110 mm | 70 | 35 |
| 120 mm | 64 | 32 |
| 140 mm | 56 | 28 |
| 165 mm | 48 | 24 |
| 185 mm | 44 | 22 |
Questions
Why does the calculator round the board count up?
Board rows and board lengths are both rounded up (ceiling), because a partial row or a partial board length is not usable: a deck needs whole boards laid across its full width and length, with the offcut from the last board in each row simply wasted or used elsewhere.
What gap should I leave between decking boards?
Timber decking is commonly spaced 3-5 mm apart to allow for drainage and for the boards to expand slightly with moisture; composite decking often specifies its own gap in the manufacturer's instructions, which can differ from timber.
Does this account for the substructure (joists and bearers)?
No, this calculator covers the decking boards themselves, plus their fixings and cost. The joists and bearers beneath the deck are a separate structural calculation that depends on span tables for the specific timber or steel being used.
How is this different from the fencing and decking materials calculator?
The fencing and decking materials calculator is built around a fence or deck run measured by length and height, counting posts, rails and vertical board rows. This calculator is built around a horizontal deck floor measured by length and width, which is the more common shape of question for a deck surface itself.
For the posts and rails behind a fence or a raised deck edge, see the fencing and decking materials calculator. For the weight of any steel fixings or brackets used in the build, see the steel plate weight calculator.