What is a circle calculator?
A circle calculator uses one known circle measurement to derive the others. If you know the radius, diameter, circumference or area, this page calculates the complete set. For root calculations used in area-to-radius conversions, see the Square Root Calculator.
How does the circle calculator work?
The selected input is converted to radius. From that radius, the calculator uses the standard Euclidean relationships d = 2r, C = 2πr and A = πr². This keeps all result fields based on one internally consistent radius.
What formulas does the circle calculator use?
You can evaluate related powers with the Exponent Calculator.
How do you use this circle calculator?
- Select Radius, Diameter, Circumference or Area.
- Enter one positive measurement.
- Choose an optional unit label.
- Select Calculate Circle.
- Review radius, diameter, circumference and area.
Affiliate disclosure: As an Amazon Associate I earn from qualifying purchases.
For optional supporting equipment or reference material, you can browse Geometry ruler and protractor sets (paid link) on Amazon.com. Sketch or measure the geometry in consistent units. Physical measurements have uncertainty and do not replace exact values supplied in a mathematical problem. No purchase is required to use this calculator.
Circle calculator examples
A circle with radius 5 has diameter 10, circumference about 31.4159 and area about 78.5398. Starting with diameter 10, circumference 31.4159 or area 78.5398 returns the same radius within display precision. For percentages applied to measurements, use the Percentage Calculator.
How are units handled?
The calculator does not convert between unit systems. If you choose centimeters, radius, diameter and circumference are labeled cm while area is labeled cm². If you select No unit, results are shown as pure numeric values.
What are the calculator's limits?
The known measurement must be a positive finite number. Very large measurements can exceed browser number limits, and irrational results involving π are rounded for readable display.
Worked example: Circle
Enter the following values in the Circle Calculator and calculate to reproduce this result.
- Known measurement: Radius
- Radius: 5
- Unit label: cm
Example result: 78.5398163397 cm². This circle has radius 5 cm, diameter 10 cm, circumference 31.4159265359 cm and area 78.5398163397 cm².
Calculation used: r = 5 cm · d = 2r · C = 2πr · A = πr²
How to interpret this example
With radius 5 cm, area is π × 5² = 25π cm². Circumference is 10π cm. Keep squared units for area and linear units for circumference; rounding π too early can explain a small difference from a handheld calculator.
Frequently asked questions about circles
Methodology and calculation scope
The calculator uses JavaScript's Math.PI and double-precision floating-point arithmetic. All calculations describe a Euclidean circle and run locally in the browser.