Break it into shapes
Almost every area on a building site is a handful of rectangles with the odd triangle or curve. An L-shaped living room is a big rectangle with a corner missing. A gable end is a rectangle with a triangle on top. A round paved area is a circle. Measure each piece, work each one out, and add them up. The calculator does the arithmetic for as many pieces as you fill in and ignores the rest.
rectangle = length × width triangle = base × height ÷ 2 circle = π × radius² trapezoid = (a + b) ÷ 2 × height
| Shape | Measure | Example | Area |
|---|---|---|---|
| Rectangle | Length and width | Bedroom 4 × 3.5 | 14.00 m² |
| L-shape | Outer rectangle less the missing corner | Living 7 × 5, corner 3 × 2 missing | 29.00 m² |
| Triangle | Base and square height | Gable 10 wide, 2.1 high | 10.50 m² |
| Triangle, three sides | All three sides | Garden bed 5, 6, 7 | 14.70 m² |
| Circle | Diameter | Fire pit paving 3 m across | 7.07 m² |
| Trapezoid | Two parallel sides and the gap | Driveway 3 m at the gate, 5 m at the garage, 12 m long | 48.00 m² |
Triangles you cannot square
A triangle's area is half the base times the height, but the height has to be measured square to the base, which is hard on a garden bed with no right angles. Measure the three sides instead. Heron's formula, which the calculator uses, gets the area from the sides alone: halve the sum of the sides to get s, then take the square root of s × (s − a) × (s − b) × (s − c). If the three sides could not form a triangle, because one is longer than the other two together, the result is zero and one of the measurements is wrong.
Square metres to lineal metres
Decking, flooring and cladding are sold by the lineal metre of board, so the area has to be converted. Divide the area by the cover width of one board in metres: its width plus the gap for decking, or the exposed face for lapped weatherboards. A 90 mm deck board with a 4 mm gap covers 94 mm, so 20 m² is 213 lineal metres. Enter the cover in the last field and the calculator does it, after waste.
Worked example
An L-shaped deck: an outer rectangle 8 × 5 m with a 3 × 2 m corner cut out for the house, plus a triangular corner 2 × 2 m where it meets the path. L-shape 40 − 6 = 34 m², triangle 2 m², total 36 m². With 10 percent waste, 39.6 m². In 90 mm boards at a 4 mm gap that is 421 lineal metres to order.
Common questions
Area of an L-shaped room?
Outer rectangle less the missing corner, or two rectangles added.
Triangle from three sides?
Heron's formula: s = half the sum of the sides, area = √(s(s−a)(s−b)(s−c)).
Square metres to lineal metres?
Area ÷ cover width in metres. 20 m² at a 94 mm cover is 213 lm.
Circle from the diameter?
π × (diameter ÷ 2)². A 3 m circle is 7.07 m².
Geometry only. Measure on site, not from the plan, and add the waste allowance that suits the material.