Area Calculator
This area calculator finds the area of seven common shapes and shows the formula with your numbers. A rectangle 5 m by 4 m has an area of 20 m²; a circle with a 3 m radius has an area of about 28.27 m². Pick a shape, choose your units and enter the measurements.
- Rectanglelength × width
- Triangle½ × base × height
- Circleπ × r²
Updated November 2026 · Shows the formula and working
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Area formulas for common shapes
| Shape | Formula | Example |
|---|---|---|
| Rectangle | length × width | 5 × 4 = 20 |
| Square | side² | 3² = 9 |
| Triangle | ½ × base × height | ½ × 6 × 4 = 12 |
| Circle | π × radius² | 3.14159 × 3² = 28.27 |
| Trapezium | ½ × (a + b) × height | ½ × (4 + 6) × 3 = 15 |
| Parallelogram | base × perpendicular height | 7 × 3 = 21 |
| Ellipse | π × a × b (semi-axes) | 3.14159 × 4 × 2 = 25.13 |
| Sector | (angle ÷ 360) × π × radius² | (90 ÷ 360) × 3.14159 × 4² = 12.57 |
How to calculate the area of a triangle
Multiply the base by the perpendicular height and halve it. The height must be measured at right angles to the base, not along a sloping side. If you only know the three sides, use Heron’s formula: work out s = (a + b + c) ÷ 2, then area = √(s(s − a)(s − b)(s − c)). For a 3, 4, 5 triangle, s = 6 and the area is √(6 × 3 × 2 × 1) = 6.
How to calculate the area of a circle
Square the radius and multiply by π (about 3.14159). If you measured the diameter, halve it first. A circle 10 cm across has a radius of 5 cm, so its area is 3.14159 × 25 = 78.54 cm². A sector is a slice of a circle, so its area is the same calculation multiplied by the angle as a fraction of 360°.
Converting units of area
Area units convert by the square of the length conversion. One metre is 100 cm, so one square metre is 100 × 100 = 10,000 cm².
| From | To | Multiply by |
|---|---|---|
| m² | cm² | 10,000 |
| m² | ft² | 10.764 |
| ft² | m² | 0.0929 |
| m² | yd² | 1.196 |
| hectares | m² | 10,000 |
| acres | m² | 4,046.86 |
Frequently asked questions
How do you work out area?
For a rectangle, multiply the length by the width. A 5 m by 4 m room is 20 m². Other shapes have their own formulas, shown in the table on this page.
What is the formula for the area of a trapezium?
Add the two parallel sides, halve the total and multiply by the height: ½ × (a + b) × h. With sides of 4 and 6 and a height of 3, the area is 15.
How do I find the area of an irregular shape?
Split it into rectangles, triangles and circle parts, work out each area and add them together. Subtract any cut-out areas.
How many square feet are in a square metre?
One square metre is about 10.764 square feet. One square foot is about 0.0929 m².
What is the area of a circle with a diameter of 1 metre?
The radius is 0.5 m, so the area is 3.14159 × 0.25 = about 0.785 m².
Sources and method
We use official UK guidance and recognised formulas, and test each calculator against worked examples. Results are estimates to help you plan, not professional advice. See how we work.