Circle Calculator (area, circumference)
From the radius, get a circle's diameter, circumference and area.
Result
Diameter
10.0000
Circumference
31.4159
Area
78.5398
How it works
One number rules the whole circle: from the radius r follow the diameter 2r, the circumference 2πr and the area πr². The constant π ≈ 3.14159 is the same for every circle ever drawn — it is the ratio of any circle's circumference to its diameter. Mind the powers: circumference grows linearly with the radius, but area grows with its square. Double the radius and the border doubles while the surface quadruples — the reason a 30 cm pizza is four times the food of a 15 cm one, not two. Working backwards is just as easy: from a diameter, halve it; from a circumference, r = C/(2π). Measuring around a tree trunk or a pipe with a tape and dividing by 2π is the practical way to get a radius you cannot measure directly.
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Frequently asked questions
How do I find the area of a circle?
Multiply π by the radius squared: area = π·r².
What is the circumference formula?
Circumference = 2·π·r (or π × diameter).
If I double the radius, how much bigger is the area?
Four times bigger — area grows with the square of the radius. A 30 cm pizza has 4× the surface of a 15 cm one; triple the radius and the area is 9× larger.
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