Decking Board Count
Board width plus drainage gap is the pitch of one run. Divide the short span of the deck by that for the number of runs, then multiply by the length for the total metres to buy.
Your numbers
Boards needed
21
Result
84m
Area
12m²
Formula
Boards needed = Width × 100 ÷ (Board width + Post spacing)
Timber swells when wet, so leave a 3–6 mm gap. Laid tight, the boards push against each other and lift.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Length (m)The measurement across, side to side. | 4 | 0 and up |
| Width (m)The measurement front to back. | 3 | 0 and up |
| Board width (cm)The width of a single board; 90, 120 and 140 mm are common. | 14 | 1 and up |
| Post spacing (cm)The spacing from one post to the next. | 0.5 | 0 and up |
Step by step
Quick reference table
Results when only Width (m) changes and everything else stays put.
| Width (m) | Boards needed | Result (m) | Area (m²) |
|---|---|---|---|
| 1.5 | 11 | 44 | 6 |
| 2.25 | 16 | 64 | 9 |
| 3 | 21 | 84 | 12 |
| 4.5 | 32 | 128 | 18 |
| 6 | 42 | 168 | 24 |
What each result means
| Result | At default values |
|---|---|
| Boards neededHow many runs of board you need to lay. | 21 |
| Result (m)What the formula gives back — the answer this page is for. | 84 |
| Area (m²)The space a shape covers — length squared. | 12 |
Common mistakes
Timber swells when wet, so leave a 3–6 mm gap. Laid tight, the boards push against each other and lift.
Glossary
- Length
- The measurement across, side to side.
- Width
- The measurement front to back.
- Board width
- The width of a single board; 90, 120 and 140 mm are common.
- Post spacing
- The spacing from one post to the next.
- Boards needed
- How many runs of board you need to lay.
- Result
- What the formula gives back — the answer this page is for.
- Area
- The space a shape covers — length squared.
Frequently asked questions
Q. How is Decking Board Count calculated?
Boards needed = Width × 100 ÷ (Board width + Post spacing) — Board width plus drainage gap is the pitch of one run. Divide the short span of the deck by that for the number of runs, then multiply by the length for the total metres to buy.
Q. Can you walk through an example?
With Length 4m, Width 3m, Board width 14cm, Post spacing 0.5cm, the answer is Boards needed 21.
Q. What do I need to enter?
Enter Length, Width, Board width, Post spacing. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Width (m) moves the answer to Width (m) 1.5 → Boards needed 11 and Width (m) 6 → Boards needed 42. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
Timber swells when wet, so leave a 3–6 mm gap. Laid tight, the boards push against each other and lift.
Related calculators
Keep your length units consistent across inputs — mixing centimetres and metres throws results far off.