Area Calculator — Rectangle, Circle and Triangle
An area calculator finds how much surface a shape covers, from simple measurements like length, width, radius, base and height.
Area Calculator
What this tool does
Choose a shape, enter its measurements, and the tool applies the correct area formula. Use the same unit for both measurements — for example, metres and metres — and the answer will be in square units, such as square metres.
How to use it, step by step
- Choose rectangle, circle or triangle.
- Enter the first measurement. For a circle this is the radius, not the diameter.
- Enter the second measurement unless you chose a circle.
- Press Calculate and check the unit: square centimetres, square metres, and so on.
Worked example
A rectangle 6 metres long and 4 metres wide has area 6 × 4 = 24 square metres. A circle with radius 3 metres has area π × 3² = about 28.27 square metres. A triangle with base 8 and height 5 has area 0.5 × 8 × 5 = 20 square units.
Common mistakes
- Using diameter instead of radius: for a circle, halve the diameter first. Using the diameter as the radius quadruples the answer.
- Mixing units: 2 metres by 50 centimetres is not 100 square metres; convert to the same unit first.
- Forgetting the half for a triangle: a triangle is half of the matching rectangle, so the 0.5 matters.
Frequently asked questions
What is the difference between area and perimeter?
Area measures surface covered; perimeter measures distance around the edge.
Why is area in square units?
Because you multiply a length by a length, so the unit is squared — for example, metres × metres = square metres.
Can I use it for a room?
Yes, for a rectangular room. For an L-shaped room, split it into rectangles, find each area, and add them.
What value of pi is used?
The full value built into the browser's maths, not just 3.14, so circle answers are as accurate as your measurements allow.