Instant Calculations
Get all major circle measurements immediately from one value.
Geometry Calculator
Circle Calculator
Calculate the radius, diameter, circumference and area of a circle from any one known measurement. The calculator gives instant results with formulas and appropriate squared units.
Enter a Known Circle Value
Select the measurement you already know and enter its value.
What Is a Circle Calculator?
A Circle Calculator is a geometry tool that determines the radius, diameter, circumference and area of a circle. You only need to provide one known value. The calculator converts that value into the circle's radius and then uses standard geometry formulas to calculate the remaining measurements.
This tool is useful for students, teachers, engineers, designers, construction workers and anyone who needs quick circle measurements without performing each calculation manually.
Multiple Units
Use millimeters, centimeters, meters, inches, feet and more.
Reliable Formulas
Calculations use the standard mathematical value of π.
How to Use the Circle Calculator
Follow these steps to calculate the dimensions of your circle.
Select a Measurement
Choose whether your known value is the radius, diameter, circumference or area.
Enter the Value
Type a positive numerical value and select the appropriate measurement unit.
Calculate the Circle
Press the calculation button to see all related circle measurements and the applied formula.
Circle Formulas
Circle calculations are based on the radius and the mathematical constant π, which is approximately equal to 3.14159265.
Diameter Formula
The diameter is twice the length of the radius.
d = 2rCircumference Formula
Circumference measures the total distance around a circle.
C = 2πrArea Formula
Area measures the total space enclosed inside the circle.
A = πr²| Measurement | Symbol | Formula Using Radius | Unit Type |
|---|---|---|---|
| Radius | r | r | Linear unit |
| Diameter | d | 2r | Linear unit |
| Circumference | C | 2πr | Linear unit |
| Area | A | πr² | Square unit |
Circle Calculation Example
Suppose a circle has a radius of 5 centimeters. Its diameter is 10 centimeters because the diameter is twice the radius.
The circumference is approximately 31.4159 centimeters, calculated using 2 × π × 5. Its area is approximately 78.5398 square centimeters, calculated using π × 5².
Why does area use square units?
Radius, diameter and circumference describe length, so they use regular linear units such as centimeters. Area measures a two-dimensional surface, so it uses squared units such as cm², m² or ft².
Frequently Asked Questions
You only need one known circle measurement: radius, diameter, circumference or area. The calculator determines all remaining values automatically.
Square the radius and multiply the result by π. The area formula is A = πr².
Divide the diameter by two. For example, a diameter of 20 centimeters gives a radius of 10 centimeters.
Circumference is the distance around a circle, while area is the amount of surface enclosed inside the circle.
The calculator uses JavaScript's built-in Math.PI value, which provides greater precision than manually using 3.14.
Yes. Select “No unit” to receive numerical results without a measurement unit.