Decking Board Calculator

Decking is two fencepost problems stacked on top of each other. Across the deck, boards leave gaps between them but not after the last one — and the gaps are not optional, because timber moves and a deck laid tight buckles in the first wet winter. Along the deck, joists at 400mm centres over four metres is eleven joists, not ten.

Length, in metres

The way the boards run.

Width, in metres

Across the boards.

Board width

Gap between boards, in millimetres

Five is usual. Timber moves, and a tight deck buckles.

Joist centres

Board length as sold, in metres

3.6m is common; longer means fewer joins.

Screws at each crossing

Two is standard.

21 boards across, 11 joists along

42 lengths of 3.6m — 151.2m of board for a 12m² deck, with 462 screws. The last board rips down to 100mm; if that looks mean, open the gaps by a fraction instead.

✓ One fewer gap than there are boards — 21 boards with 20 gaps of 5mm between them — there is no gap after the last board
✓ The boards and gaps reach across the deck — 21 boards and 20 gaps measure 3.04m across a 3m deck
✓ Nothing wider than a board and a gap is wasted — the last board is ripped down to 100mm
✓ The joists reach the far end — 11 joists at 400mm centres span 4m of a 4m deck
✓ There is enough board length for every run — 42 lengths of 3.6m makes 151.2m, for 84m of board run
✓ A screw at every crossing — 21 boards across 11 joists at 2 screws each is 462

What this generator does

Divides the width by a board plus a gap, allows for there being one fewer gap than boards, counts joists at the chosen centres plus one to close the run, and multiplies the crossings out into screws.

How to use this tool

Type the length the boards will run and the width across them.
Pick the board width and the gap you want between boards.
Choose the joist centres and the length of board you can buy.
Read the boards, the joists, the screws and the rip on the last board.

Understanding the controls

Length: The direction the boards run, in metres. This decides how many lengths of board each run takes and how many joists sit underneath.
Width: Across the boards, in metres. This is what the boards and gaps have to add up to.
Board width: The common sizes. A narrower board means more boards, more gaps and more screws for the same deck.
Gap between boards: Five millimetres is usual. It is there so the timber can move; a deck laid tight will lift.
Joist centres: How far apart the joists sit. Closer centres make a firmer deck and take more timber; 400mm is a common compromise.
Board length as sold: What the merchant stocks. A length that divides the deck neatly saves both waste and joins.
Screws at each crossing: Two is standard, one either side of the board. This is what turns the crossings into a box of screws.

Common use cases

Ordering timber for a garden deck without a second trip
Working out how much of the last board has to be ripped down
Comparing 400mm and 600mm joist centres on the order
Counting screws before starting rather than halfway through
Checking a quote against the arithmetic

How this generator works

Across the deck, each board takes its own width plus one gap, except the last, which has no gap after it — so the number of boards is the width plus one gap, divided by a board plus a gap, rounded up. The overshoot is what the last board is ripped down by, and it can be as much as a board plus a gap, which is why a sliver sometimes appears and the honest advice is to open the gaps a fraction instead. Along the deck, the joists are the same identity in the other direction: the spans divide the length, and one more joist closes it. Everything is then added back up and compared with the deck it came from.

Randomness and fairness

Nothing is random. The same deck always gives the same order.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

Joist spans and sizes depend on load and on the timber, which this does not size — it counts joists at the centres you choose.
Boards are counted as running the full length; a deck with staggered joins uses the same metres but cuts differently.
The frame's outer edges, noggins and trimmers are not counted.
Composite boards want different gaps from timber; follow the maker's figure rather than five millimetres.

Privacy and your data

The quantities are worked out in your browser. No deck measurements are transmitted or stored.