Area Calculator
Area of a rectangle, circle or triangle, in any unit.
Good to know. Measure twice, order once. Then add 10% for cuts and mistakes.
How it works
In plain words
A rectangle is length times width. A circle is π times the radius squared (the radius is half the diameter). A triangle is half of base times height.
The formula
rectangle: A = length × width circle: A = π × (diameter ÷ 2)² triangle: A = base × height ÷ 2 1 m² = 10.7639 ft²
Worked example
A room 5 m long and 4 m wide has an area of 5 × 4 = 20 m², which is about 215.28 ft².
Sources
- NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B Exact conversion factors for US customary units.
- International yard and pound agreement, 1959 1 yard = 0.9144 m and 1 pound = 0.45359237 kg exactly, so 1 inch = 2.54 cm and 1 mile = 1.609344 km.
Sources checked September 29, 2026. 6 automated tests check its answers on every change.
About the area calculator
Area is the amount of flat space a shape covers, and it is the number you need for flooring, carpet, turf, paint, tiles, fertiliser, rent comparisons and more. It is always measured in square units: square metres (m²), square feet (ft²) and so on.
For a rectangle, including a square, multiply the length by the width. For a circle, measure straight across through the middle (the diameter), halve it to get the radius, then multiply the radius by itself and by π (about 3.14159). For a triangle, multiply the base by the height and halve it. The height must be measured straight up from the base at a right angle, not along a sloping side.
Most rooms are not perfect rectangles. The easiest method is to split an L-shaped or irregular space into rectangles and triangles, work out each one, and add them together. Subtract areas you don’t want to cover, like a kitchen island or a fireplace hearth.
Converting square units catches people out. There are 100 centimetres in a metre, but 10,000 square centimetres in a square metre, because both sides are multiplied. Likewise 1 square metre is 10.76 square feet, not 3.28. The calculator converts for you, whichever unit you measure in.
Questions people ask
How do I calculate the area of a room?
Multiply the length by the width. A 5 m by 4 m room is 20 m². Split L-shaped rooms into rectangles and add them.
How do I convert square metres to square feet?
Multiply by 10.7639. 20 m² is about 215.3 ft².
How do I find the area of a circle?
Halve the diameter to get the radius, square it, and multiply by π. A 3 m circle has an area of about 7.07 m².
Why is there no perimeter for a triangle?
Because base and height alone don’t tell us the lengths of the two other sides. You’d need all three sides.