How many decking boards do I need?
Last reviewed 1 July 2026
Short answer
Divide the span across the boards by the width of one board plus one gap, and round up. That gives the number of rows. Multiply the rows by the length of each run to get total linear metres, then divide by the stock length you can buy. A 4 m × 3 m deck in 144 mm boards with 5 mm gaps needs 21 rows and 84 linear metres — about 18 boards of 4.8 m, or 24 boards of 3.6 m once the off-cuts are accounted for.
Work it out for your project with the decking calculator →
Why board area is the wrong starting point
The instinctive method is to divide the deck area by the area of one board. It is always wrong, and it is always wrong in the same direction — it under-orders.
A decking board does not cover its own width. It covers its width plus the gap next to it. On a 144 mm board with a 5 mm gap, every board is really occupying 149 mm of deck. Across a 3 m span that is 20.2 rows rather than 20.8, but the arithmetic error compounds on larger decks and on narrow boards where the gap is a bigger proportion of the module.
Counting by row also forces you to confront the thing that actually determines cost: how the run length divides into the board length you can buy.
rows = ⌈(span + gap) ÷ (board width + gap)⌉
The four numbers you need
- Span — the deck dimension measured across the boards, i.e. perpendicular to the direction they run.
- Run — the deck dimension the boards run along. Each row has to be made up to this length.
- Board width and gap — the module. Use the actual finished width, not the nominal size; a board sold as 150 mm is often 144 mm.
- Stock length — the lengths your merchant sells. This is what decides how much you throw away.
Worked example: a 4 m × 3 m deck
- Boards run along the 4 m length, so the span is 3 m and the run is 4 m.
- Module = 144 mm board + 5 mm gap = 149 mm.
- Rows = (3,000 mm + 5 mm) ÷ 149 mm = 20.2, rounded up to 21 rows.
- Total board length = 21 rows × 4 m = 84 linear metres.
- In 4.8 m boards: 84 ÷ 4.8 = 17.5, so 18 boards before waste. Each board makes one 4 m row with a 0.8 m off-cut.
- In 3.6 m boards: each row needs a 3.6 m piece plus a 0.4 m piece. You need 21 full lengths plus enough extra to make 21 short pieces — 24 boards in total.
Note: The stock length matters more than the waste percentage
How board direction changes the answer
Turning the boards through 90° changes the span, the run, the number of rows and the number of joists. It is not a cosmetic decision.
On the 4 m × 3 m deck above, running the boards across the 3 m width instead gives 27 rows of 3 m — 81 linear metres, marginally less timber, but 27 rows to fix instead of 21, and the joists now run the other way.
| Boards along the 4 m length | Boards across the 3 m width | |
|---|---|---|
| Span across boards | 3 m | 4 m |
| Rows | 21 | 27 |
| Run length per row | 4 m | 3 m |
| Total linear metres | 84 m | 81 m |
| Boards from 3.6 m stock | 24 | 27 |
| Joists at 400 mm centres | 11 (each 3 m) | 9 (each 4 m) |
Where the joins go
If your run is longer than a board, every row needs a join, and every join has to land on a joist. That usually means adding a joist at the join position, or doubling one up so both board ends have full bearing.
Stagger the joins between adjacent rows rather than lining them up. A line of joins across a deck reads as a seam and is the first thing that moves as the timber weathers.
Related questions
- Should I include the gap on the last board?
- No, but including it in the arithmetic costs nothing and is safer. The final board has no board next to it, so strictly there are one fewer gaps than boards. Adding one extra gap into the numerator, as the formula above does, biases the result upward by a fraction of a board — which is the direction you want to be wrong in.
- Do I need to add waste on top of the row count?
- Yes. Rows tell you the theoretical requirement; waste covers damaged boards, bad ends, cutting mistakes and the odd board too warped to use. 10% is sensible for a rectangular deck and 15% or more for anything with angles or multiple levels.
Safety notice: Material planning only