How to use it
- Choose a shape from the list.
- Enter the dimensions it asks for, all in the same unit.
- The area appears, with the perimeter or arc length when the shape has a simple one.
Formulas
| Shape | Area | Perimeter |
|---|---|---|
| Rectangle | w × h | 2(w + h) |
| Square | s² | 4s |
| Triangle | ½ × b × h | not shown (needs all sides) |
| Circle | πr² | 2πr |
| Trapezoid | (a + b) ÷ 2 × h | not shown (needs the slanted sides) |
| Parallelogram | b × h | not shown (needs the slanted sides) |
| Ellipse | πab | Ramanujan approximation |
| Sector | (angle ÷ 360) × πr² | 2r + arc |
The arc of a sector is (angle ÷ 360) × 2πr, with the angle in degrees.
Worked examples
- Rectangle 5 × 3. Area 15, perimeter 16.
- Circle with radius 3. Area 9π ≈ 28.2743, circumference 6π ≈ 18.8496.
- Sector, radius 6, 60°. Area 6π ≈ 18.85, arc length 2π ≈ 6.28, perimeter 12 + 2π.
Common mistakes
- Mixing units. A width in metres and a height in centimetres give a meaningless result.
- Using the slanted side as the height of a triangle or parallelogram.
- Entering the diameter as the radius. Halve the diameter first, or use the circle calculator.