Circle Radius Calculator
Know the diameter, circumference, or area but need the radius? This calculator reverses the standard circle formulas to extract the radius from any known measurement. Enter one value and get all circle properties back.
1. Select Known Metric
2. Enter Value & Unit
Results
Circle Radius Calculator
The radius is the master measurement of a circle. Every other property — diameter, circumference, area — derives from it. Hover over the parts below to see these relationships.
When you need the radius
Many geometry formulas require the radius as input. The area formula (A = πr²), circumference formula (C = 2πr), and sphere volume formula (V = 4/3 πr³) all start with r. If you only have a tape-measured diameter or a computed area, this radius calculator gets you the starting value.
Radius in real-world applications
Engineers size round pipes and bearings by radius. Architects design curved walls with a center point and radius. Surveyors mark circular boundaries from a stake (the center). In each case, the radius is the construction parameter, and our circle radius calculator online helps derive it from field measurements.
Radius Formula
The simplest radius formula divides the diameter by 2. Click each part to explore.
Click each part to explore
Alternative formulas: r = C / (2π) from circumference, r = √(A / π) from area.
How to Calculate Radius
Three steps to find the radius from any known value. Enter a diameter below to see it work.
Identify Your Known Value
d, C, or ADetermine which measurement you have: diameter, circumference, or area. Each requires a different formula to find the radius.
Apply the Right Formula
r = d/2 or C/(2π) or √(A/π)From diameter: r = d/2. From circumference: r = C/(2π). From area: r = √(A/π). Pick the one that matches your known value.
Get the Radius
rThe result is your radius in the same linear unit as your input measurement (for diameter and circumference) or in the root unit (for area).
Formula Variations
Three ways to find the radius, depending on which measurement you start with.
The simplest conversion. The diameter passes through the center and equals twice the radius. Halving it gives you r.
Example
Find the radius from a known diameter. Change the value to try your own.
The diameter of the circle is 14 centimeters.
Frequently Asked Questions
Common questions about finding and using the radius of a circle.