Area Calculator guide
Area Calculator: Area and Perimeter of 2D Shapes
Use our area calculator to work out the area and perimeter of rectangles, triangles, circles, trapeziums and other shapes, in any units.
By the AllCalculator editorial teamUpdated 16 October 20261 min read
Area is the amount of flat space inside a shape. Our Area Calculator works it out for 12 shapes, with the perimeter too.
How to use the calculator (Area Calculator)
- Choose a shape.
- Enter its measurements and choose your units.
You'll see the area in square units, square metres and square feet, and in hectares and acres for big areas.
Area formulas
| Shape | Area |
|---|---|
| Rectangle | length × width |
| Square | side² |
| Triangle | ½ × base × height |
| Circle | πr² |
| Trapezium | ½(a + b) × height |
| Parallelogram | base × height |
| Rhombus or kite | ½ × diagonal 1 × diagonal 2 |
| Ellipse | πab |
| Sector | (angle ÷ 360) × πr² |
A worked example
With the calculator's default settings, a rectangle 8cm by 5cm has an area of 40cm² and a perimeter of 26cm.
Frequently asked questions
What if I only know a triangle's three sides?
Choose "Triangle (three sides)" and the calculator uses Heron's formula.
How do I measure a room?
For floor areas, flooring and carpet, use our Square Meter Calculator, which handles several rooms.
Why are the units squared?
Area measures how many unit squares fit inside a shape, so a 1cm by 1cm square has an area of 1cm².
For 3D shapes, use our Volume Calculator and Surface Area Calculator.
Sources and further reading
- GOV.UK: National curriculum in England – mathematicsgov.uk
- Wolfram MathWorld: Areamathworld.wolfram.com
Links open official and independent websites in a new tab.
This guide is general information to help you understand your options. It isn't financial, tax, legal or building advice and doesn't take your personal circumstances into account. Figures were checked when the guide was last updated and can change, so confirm anything important with GOV.UK, HMRC, a regulated adviser or a qualified tradesperson.