Circle Calculator

How it is calculated

From the radius r of a circle the tool derives three measures. Area: A = π × r2. Circumference, the distance around the circle: C = 2 × π × r. Diameter, the distance across through the centre: d = 2 × r. Here π is about 3.14159. Enter the radius in any length unit; the circumference and diameter come out in that same unit, and the area in that unit squared (cm gives cm²). The radius must be greater than zero. Each result is rounded to two decimals. If you only know the diameter, halve it first, and if you know the circumference, divide it by 2π to get the radius.

Common mistakes

• Entering the diameter in the radius field, which makes the area four times too large.
• Quoting the area in plain units rather than square units, such as cm instead of cm².
• Mixing up area and circumference: the area grows with the square of the radius, the circumference only in proportion to it.
• Using 3.14 for π in a hand check and comparing with this result, then seeing a small mismatch; the tool uses the full value.
• Entering a radius in one unit and reading the result in another, for example metres in and centimetres out.