Area of a circle calculator
Enter the dimensions of a circle and read off its area, its perimeter and the quantities that follow from them.
Area
78.539 816m²
- Perimeter
- 31.415 927
- Diameter
- 10
- Area to circumference ratio
- 2.5
Show the working
- Radius = 5
- Area = pi * 5^2 = 78.5398
- Perimeter = 2 * pi * 5 = 31.4159
- Diameter = 2 * 5 = 10
- Area to circumference ratio = 5 > 0 ? (pi*5^2) / (2*pi*5) : 0 = 2.5
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
- area = pi r^2
- circumference = 2 pi r
- diameter = 2 r
Anything involving pi is irrational, so the answer is reported as rounded rather than exact however many digits are shown.
Lengths are entered in metres by default; switch any field to feet, inches or millimetres and the conversion is exact.
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