Area and perimeter guide: rectangle, circle or triangle

When this tool helps

Area measures the surface inside a shape; perimeter measures its boundary. They use different units: a rectangle measured in metres has area in square metres and perimeter in metres. This tool shows both so those measurements are not confused.

Choose Rectangle, Circle or Triangle. A rectangle needs length and width, a circle needs radius, and a triangle needs its three sides. The native form shows only the dimensions relevant to the selected shape, with one common unit for every dimension.

What to enter

These are the candidate Android fields, not a form on this website. Editable fields in the app start empty; modes and units are selected separately.

Shape and Length unit for all dimensions: Select Rectangle, Circle or Triangle, then mm, cm, m, in or ft.
Required dimensions: Rectangle: length and width. Circle: radius. Triangle: side A, B and C.

Worked example · not prefilled app data

Try this example after the update is available

The values below explain the calculation. They are not a quote, a live measurement or an automatic answer for your situation.

Shape: Rectangle
Length unit: m
Length: 5
Width: 3

Checked example result15 m²

A 5 m by 3 m rectangle covers 15 m². Its boundary is 2 × (5 + 3), or 16 m.

Formula and assumptions

Rectangle area = length × width; circle area = πr²; triangle area uses the three-side Heron formula

Dimensions must be greater than zero and share a unit. Triangle sides must satisfy the triangle inequality.
The model assumes an ideal flat shape; it does not estimate material wastage, irregular boundaries or surveying tolerances.

A common mistake to avoid

Enter a circle’s radius, not its diameter. Using the full diameter as the radius would multiply the calculated circle area by four.

Find this tool in the Android update

When you have the redesigned Android update, open Tools and search for Area & perimeter, or find it under Everyday.
Open the tool, choose the mode or units relevant to your task, then enter your own values.
Review the result and explanation. Use Calculate & save only if you want the calculation kept in local History; Copy result does not send it to our server.

This guide is an educational preview of the update in testing, not an announcement that Google Play has approved or published it. Calculations are in the native app; no web calculator or download is offered on this page.

Questions about Area & perimeter

Does the triangle option need height?

No. This candidate takes three side lengths and uses Heron’s formula; invalid side combinations are rejected.

Can I mix centimetres and metres?

Not within one calculation. Convert dimensions to the same length unit before entering them.