How to find the area of a shape
- Choose the shape.
- Enter its measurements, all in the same unit.
- Read the area in square units, with the formula used and the perimeter where it applies.
Formulas
| Shape | Area | Perimeter |
|---|---|---|
| Rectangle | l × w | 2(l + w) |
| Square | s² | 4s |
| Triangle | ½ × b × h | sum of sides |
| Circle | πr² | 2πr |
| Trapezoid | ½(a + b)h | sum of sides |
| Parallelogram | b × h | 2(b + slanted side) |
| Ellipse | πab | about π[3(a + b) − √((3a + b)(a + 3b))] |
| Sector | (θ ÷ 360) × πr² | 2r + (θ ÷ 360) × 2πr |
The ellipse perimeter has no exact simple formula; the calculator uses Ramanujan’s approximation, which is accurate to a tiny fraction of a percent for everyday shapes.
Measuring tips
Measure in one unit throughout: mixing feet and inches is the most common source of wrong answers, so convert 10 ft 6 in to 10.5 ft first. For a room that isn’t a simple rectangle, split it into rectangles and triangles, work out each and add them. For flooring or paint, add 5 to 10% for cuts and waste.