Decking calculator
Decking boards are sold by length, so the useful number is the total running metres: the area divided by the board width, plus waste.
Your measurements
Result
- Area to order (incl. waste)
- 16.50 m²
- Total board length
- 137.5 m
The substructure (joists, adjustable pedestals) is not included.
How the calculation works
- Area to order = deck area + waste (10% by default).
- Board length (m) = area to order ÷ board width (in metres).
Worked example
For a 15 m² deck with 10% waste you order 16.5 m². With 12 cm wide boards: 16.5 ÷ 0.12 = 137.5 m of boards.
What the calculator assumes
- The 5–7 mm gaps between boards are ignored, which leaves a small extra reserve.
- 10% covers cut ends and staggered joints.
When to adjust the result
- The substructure is not included: joists are usually 40–50 cm apart for timber and about 30–40 cm for composite, on pedestals or pads.
- Timber is treated before fitting and then once a year; composite needs no oil but expands more.
What makes a deck last
Most decks fail at the substructure, not the boards. Joists need to stay dry: keep them off the ground on pads or posts, leave airflow underneath, and use treated timber rated for ground contact where it is close to the soil. Fit joist tape on top of the joists to keep water out of the screw holes.
Leave gaps between boards (typically 5–8 mm for timber, as specified for composite) so water drains and boards can swell. Lay boards with a slight fall away from the house, use stainless or coated deck screws, and pre-drill near board ends to avoid splitting. Composite boards follow their own joist-spacing and expansion rules — always check the manufacturer's installation guide.
Frequently asked questions
How many decking boards do I need?
Divide the total board length by the length of one board — for example 137.5 m ÷ 4 m boards = 35 boards.
Timber or composite decking?
Timber is cheaper and natural but needs yearly care; composite is low-maintenance but expands more with heat.
How far apart should deck joists be?
Commonly about 40 cm for diagonal or thin boards and up to 50–60 cm for thicker timber laid square to the joists. Composite systems often require closer spacing; follow the product guide.
Timber or composite decking?
Timber costs less and is easier to cut but needs oiling or staining; composite needs little maintenance but is heavier, can get hot in the sun and has strict installation rules.