Decking calculator
Boards, substructure and fasteners with a cutting layout and 2D plan
How the calculation works
Choose timber or WPC decking, enter the plan (rectangle, L-shape or irregular polygon) and any openings, then the board width, thickness, lengths and gaps. For the substructure set the section, the maximum joist spacing, the support spacing and the lengths. The calculator lays out the rows, packs pieces into the available board lengths allowing for the saw kerf, adds joists at the joints and shows a 2D plan and a purchase list.
Formula and assumptions
Rows = (width − 2 × edge gap + gap) ÷ (board width + gap). Row length = length − 2 × edge gap. Estimate = total row length ÷ board length, plus the reserve. Actual layout: pieces are packed into boards by length (with the kerf) and the whole boards needed are counted. Joists: at most the set spacing, plus edge and extra joists at joints; board–joist crossings × pieces per crossing = fasteners.
Worked example
A 5 × 3 m terrace = 15 m², 145 mm boards, 5 mm gap, 10 mm edge gap: a 150 mm pitch gives 20 rows (19 full and one ripped to 130 mm). Row length is 4.98 m, total 99.6 m. With 3.6 m boards: 99.6 ÷ 3.6 = 27.6, with a 7 % reserve 29.6, so 30 boards. The actual cutting layout also gives 30 boards with about 7.9 % waste.
More worked examples
- A 4 × 3 m terrace, 140 mm board with a 5 mm gap: 3 000 ÷ 145 ≈ 20 rows.
- Total length 100 m, 4 m boards, 7 % reserve: 100 ÷ 4 × 1.07 = 26.75, so 27 boards.
Approximate board length for a 145 mm board with a 5 mm gap (no edge gaps or reserve)
| Area (m²) | Length (m) | 3.6 m boards |
|---|---|---|
| 10 | 66.7 | 18.5 |
| 15 | 100 | 27.8 |
| 20 | 133.3 | 37 |
| 30 | 200 | 55.6 |
| 40 | 266.7 | 74.1 |
What affects real consumption
- Board direction and the plan shape decide the number of rows and how much is lost in cutting.
- Available board lengths and the saw kerf change how many pieces can be cut from one board.
- Joist spacing, the board joint rule and the support spacing set the amount of substructure.
Common mistakes
- Forgetting the gap between boards, which makes the row count too low.
- Ignoring cutting waste on diagonal edges or L-shaped plans.
- Taking the joist section and spacing from memory instead of the manufacturer's instructions.
Frequently asked questions
Why are the estimate and the cutting layout different?
The estimate divides the total length by the board length and does not know how pieces are really cut. The cutting layout packs the actual pieces into boards, so it is the more realistic number to order.
Does the calculator check the load capacity of the substructure?
No. You set the section, spacing and lengths, or take them from the manufacturer's instructions. Load capacity needs a separate calculation.