Radius Calculator
The radius is the distance from the center of a circle to its edge. When you can only measure around the circle, straight across it, or know its area, enter that here to get the radius.
- Diameter
- 14.0056 in
- Circumference
- 44 in
- Area
- 154.062 in²
Saved setups
Save a set of inputs you reuse — your usual rate, your loan, your room sizes — and load it back in one tap.
Your recent calculations
Results you calculate here are kept on this device so you can come back to them.
Formula
From diameter: radius = diameter ÷ 2
From circumference: radius = circumference ÷ (2 × π)
From area: radius = √(area ÷ π)
How to use it
- Choose what you know: circumference, diameter or area.
- Enter the value and its unit.
- Read the radius; the other measurements are listed below it.
Worked examples
A tree trunk 44 inches around: radius 7.003 in, which is 0.1779 m
- Radius
- 7.0028 in
- Diameter
- 14.0056 in
A circle 10 m around
- Radius
- 1.5915 m
A circle with an area of 50 m²
- Radius
- 3.9894 m
Measuring around instead of across
For a tree trunk, column or large pipe it is far easier to wrap a tape measure around than to find the center. Divide that circumference by 6.2832 to get the radius. A trunk 44 inches around has a radius of 7 inches.
Questions people ask
What is the radius of a circle with a circumference of 44 inches?
44 ÷ (2 × π) = 7.00 inches.
What is the radius of a circle with an area of 100 square inches?
√(100 ÷ π) = 5.64 inches.
Is the radius always half the diameter?
Yes, for every circle. A 12-inch diameter always means a 6-inch radius.
Related calculators
Circle CalculatorRadius, diameter, circumference and area from any one of them.Diameter CalculatorDiameter of a circle from its radius, circumference or area.Circumference CalculatorDistance around a circle from its radius, diameter or area.Area of a Circle CalculatorArea of a circle from its radius, diameter or circumference.Arc Length CalculatorLength of a circular arc from its radius and central angle.