Circle Calculator
Calculate radius, diameter, circumference and area from one known circle measurement
Circle area
How we calculated it
About the Circle Calculator
A circle can be described from its radius, diameter, circumference or area, and each of those measurements can be converted into the others. This calculator is useful when a drawing or real object gives you only one dimension.
Diameter is twice the radius, while circumference and area depend on π. Keep track of whether the number you know is a length around the circle or a measurement through it.
How to Use the Circle Calculator
- Enter the circle measurement you already know, such as radius, diameter, circumference or area.
- Choose the correct input unit.
- Calculate the remaining circle properties.
- Use linear units for radius, diameter and circumference and square units for area.
How the Calculation Works
Diameter d = 2r. Circumference C = 2πr = πd. Area A = πr².
When circumference or area is known, the formulas are rearranged to solve for radius before deriving the remaining values.
Worked example
Circle Calculator Worked Example
Circle with radius 5 cm
Diameter = 2×5 = 10 cm. Circumference = 2π×5 ≈ 31.42 cm. Area = π×5² = 25π ≈ 78.54 cm².
Result guide
Understanding Your Circle Calculator Result
Circumference is a boundary length while area measures the surface inside the circle. They therefore use different units and should not be compared as if they were the same type of measurement.
Before you rely on the answer
Assumptions and Limitations
- The shape is an ideal circle.
- The entered dimension is measured consistently.
- Displayed decimal values of π-based results may be rounded.
Practical tip
Using This Result in Practice
If you measured straight across the widest part of a circular object, that is the diameter, not the radius. Entering it as radius would double every linear result and quadruple the area.
Related calculations
Useful Calculations to Continue With
If you only need a slice of the circle, use the Circle Sector Calculator for sector area and angle relationships.
If the question concerns only part of the circumference, use the Arc Length Calculator.
Common questions
Circle Calculator FAQ
What is the relationship between radius and diameter?
The diameter is twice the radius, so radius = diameter ÷ 2.
Why does circle area use radius squared?
Area grows in two dimensions, so doubling radius produces four times the area.
Is circumference the same as perimeter?
Yes in concept. Circumference is the specific name for the perimeter of a circle.