How to calculate deck area
Last reviewed 1 July 2026
Short answer
For a rectangle, multiply length by width. For anything else, split the shape into rectangles and triangles, work out each one, and add them together. Area tells you how much decking surface you have — it does not tell you how many boards to buy, because that depends on the board width, the gap and the direction the boards run.
Work it out for your project with the decking calculator →
Measure the frame, not the finished surface
Decide first whether your boards will overhang the frame and by how much. A 25–30 mm overhang is common, and on a 4 m × 3 m frame that is an extra 0.05 m² per side — trivial in isolation, but it also changes where the last board lands.
Measure both diagonals before you calculate anything. If they differ, the deck is not square, and the difference tells you how much cutting to expect along one edge.
Splitting awkward shapes
- Sketch the deck and mark every dimension you can physically measure.
- Draw lines to divide it into rectangles and right-angled triangles. Any polygon can be split this way.
- Rectangle area is length × width. Triangle area is ½ × base × height, where the height is measured square to the base.
- Add the pieces. Subtract anything you are not decking, such as a tree well or a hot-tub cut-out.
- Keep the individual pieces written down — you will need each piece's run length later for the board count.
Curves
A curved edge is not worth calculating exactly. Take the rectangle that encloses the curve and treat the curve as removing roughly a third of the corner area for a gentle sweep, or half for a quarter-circle.
For a true quarter-circle of radius r, the removed area is r² − πr²/4, which is about 21% of the enclosing square. Curved decks generate a lot of cutting waste, so the area figure matters much less than the waste allowance.
Why the area does not give you a board count
Two decks with identical areas can need noticeably different quantities. A 6 m × 2 m deck and a 4 m × 3 m deck are both 12 m². Laid with boards running along the length, the first needs 14 rows of 6 m (84 linear metres) and the second needs 21 rows of 4 m (84 linear metres) — the same linear metres but a very different number of stock lengths depending on what your merchant sells.
Add the gaps and the two diverge further. This is why the calculator asks for both dimensions and the board direction rather than just an area.
Note: Use the area for the frame, the rows for the boards
Safety notice: Material planning only