The Area Calculator works out the area of the most common geometric shapes — rectangle, square, triangle, circle, trapezoid, parallelogram and ellipse — in one place. It is useful for homework, DIY and construction estimates, flooring and paint quantities, and any time you need a quick, accurate area without looking up the formula.
How it works
You pick a shape, enter its dimensions, and the calculator applies that shape’s standard geometric formula. The tool is unit-agnostic: whatever unit you type the lengths in, the area comes back in that unit squared (metres in → metres² out).
| Shape | Formula | Key dimensions to measure |
|---|---|---|
| Rectangle | width × height | Two sides at right angles |
| Square | side × side | Any one side |
| Triangle | ½ × base × height | Base length and perpendicular height |
| Circle | π × radius² | Radius (half the diameter) |
| Trapezoid | ½ × (a + b) × height | Both parallel sides and the perpendicular height between them |
| Parallelogram | base × height | Base and perpendicular height (not slant height) |
| Ellipse | π × a × b | Semi-major axis (a) and semi-minor axis (b) |
Worked examples
Circle, radius 5 m: Area = π × 5² = π × 25 ≈ 78.54 m²
Rectangle, 4 m wide by 3 m tall: Area = 4 × 3 = 12 m²
Trapezoid, parallel sides 6 m and 4 m, perpendicular height 3 m: Area = ½ × (6 + 4) × 3 = ½ × 10 × 3 = 15 m²
Triangle, base 8 m, perpendicular height 5 m: Area = ½ × 8 × 5 = 20 m²
Common measurement mistakes to avoid
- Triangle height must be perpendicular — for an acute triangle, the height is the vertical distance from the base to the opposite vertex, not the length of a slanted side. Measure or calculate the perpendicular height, not the hypotenuse.
- Parallelogram height is not slant height — the formula uses the perpendicular distance between the two parallel sides, which is less than the slant side length.
- Circle radius not diameter — if you measured the full width of a circular space, halve it before entering. A circle of diameter 10 m has radius 5 m.
- Unit consistency — if width is in metres and height is in centimetres, convert both to the same unit first. The result will be in that unit squared.
Practical uses
The area formula comes up constantly outside the classroom:
- Flooring and tiles — measure the room as a rectangle (or break an L-shaped room into two rectangles), add 10–15% for cuts and waste, and match against the pack coverage stated on the box.
- Paint coverage — wall area minus window and door areas gives the surface to paint. Most paint tins state coverage in m² per litre.
- Garden beds and turf — irregularly shaped beds can often be approximated as a trapezoid or a triangle plus a rectangle.
- Fabric cutting — sewing patterns often state fabric requirements in m²; enter the pattern dimensions to see whether your fabric piece is large enough.
Every calculation runs entirely in your browser and nothing is uploaded.