Circle Calculator
Enter any one measurement of a circle to get all the others.
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Circle formulas
- Diameter = 2 × radius.
- Circumference = 2 × π × radius (or π × diameter).
- Area = π × radius².
- Radius from area = √(area ÷ π).
π (pi) is about 3.14159. The calculator uses it to full precision rather than 22/7 or 3.14, which are only approximations.
Everyday uses
A round table of 1.2 m diameter has a circumference of 3.77 m, which is useful for tablecloths and seating. A round plot or water tank lid of radius 1 m covers 3.14 m². Pizza sizes are a famous example: a 12-inch pizza has more than double the area of an 8-inch one (113 vs 50 in²), because area grows with the square of the radius. All results use the same unit you entered, squared for areas.
Free to useNo software to install — works in any modern browser.
No signup neededOpen the tool and start straight away.
Runs in your browserNothing you enter is sent to a server.
Works on mobileSame tool on your phone, tablet or desktop.
How it works
- Enter — type or paste your values or text.
- Get the result — it is worked out in your browser.
- Adjust — change the inputs to compare, or copy the result.
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