Circle calculator: radius, diameter, circumference, area
All four measurements of a circle determine each other. Enter whichever you have.
Radius
10m
- Diameter
- 20
- Circumference
- 62.831 853 07
- Area
- 314.159 265 4
Show the working
- Known value = 10
- What that value is = 1
- Radius = 1 == 1 ? 10 : 1 == 2 ? 10/2 : 1 == 3 ? 10/(2*pi) : sqrt(10/pi) = 10
- Diameter = 2 * 10 = 20
- Circumference = 2 * pi * 10 = 62.8319
- Area = pi * 10^2 = 314.159
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- r = d/2 = C/(2 pi) = sqrt(A/pi)
- C = 2 pi r, A = pi r^2
When you enter an area, the length unit selector is still used for the radius and circumference outputs — the area itself is read in square metres.