Area Calculator
Calculate the area of a rectangle, triangle, circle or trapezoid, with results in metric and imperial units.
Formula
Each shape uses its standard geometric formula.
- Rectangle: A = L × W
- Triangle: A = ½ × base × perpendicular height
- Circle: A = π × r², where r = diameter ÷ 2
- Trapezoid: A = ½ × (a + b) × h, where h is measured perpendicular to the parallel sides
Worked example
A trapezoidal plot with parallel sides of 10 m and 6 m, 8 m apart.
- A = ½ × (10 + 6) × 8
- A = ½ × 16 × 8
- A = 64 m²
64 m², which is about 689 square feet.
Assumptions and limitations
- Triangle area uses base and perpendicular height, not the sloping side length.
- Circle area uses the diameter, since that is what gets measured in the field far more often than the radius.
- Trapezoid height is the perpendicular distance between the parallel sides, not the length of a sloping edge.
- All shapes are plane figures. Sloping ground has a larger surface area than its plan area.
What this calculator does not do
- Does not calculate areas of irregular polygons directly — split them into these shapes.
- Does not convert plan area to surface area on a slope.
- Does not handle curved boundaries other than a full circle.
Practical tips
- Measure diagonals on anything you assume is rectangular. If the two diagonals differ, the shape is not square and the simple formula will be slightly out.
- For an irregular site boundary, split it into triangles from one corner. Triangles handle any polygon and each only needs a base and a perpendicular height.
- Plan area is not surface area on a slope. A roof or embankment covers more material than its footprint suggests.
Frequently asked questions
How do I calculate the area of an irregular shape?
Divide it into rectangles and triangles, calculate each one, and add them. Triangles taken from a single corner will cover any straight-sided polygon.
How do I convert square metres to square feet?
Multiply by 10.7639. One square metre is about 10.76 square feet. The calculator shows both.
Do I use the slope length or the horizontal distance?
For plan area, the horizontal distance. For surface area — which is what you need for materials on a slope — you need the slope length, which is longer.
Why does the circle calculator ask for diameter?
Because diameter is what people actually measure in the field. Radius is easy to halve wrongly under pressure, so the tool takes the measured value directly.
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All surveying & geometry calculators
Estimating tool, not a design. This calculator works out quantities from the dimensions you enter. It does not size any structural member, verify a design, or confirm compliance with any building code. Check every result against your project specification and applicable local standards, and involve a qualified engineer where the work requires one.