Radius Calculator

Last Updated on March 2, 2026 | 2 : 52 pm by Anas Brittany

Use this radius calculator to find the radius of a circle from the diameter, circumference, or area. Enter any known value and the calculator will instantly compute the radius using the correct circle formula.

What Is the Radius of a Circle?

The radius is the distance from the center of a circle to any point on its edge. It is one of the most important measurements used to calculate other circle properties such as area, diameter, and circumference.

Radius Formulas

You can find the radius using different formulas depending on what information you know.

From Diameter

r = d ÷ 2

The diameter is twice the radius.

From Circumference

r = C ÷ (2π)

This formula comes from the circumference equation:
C = 2πr

From Area

r = √(A ÷ π)

This comes from the area formula:
A = πr²

Example Calculations

If the diameter is 10:
r = 10 ÷ 2 = 5

If the circumference is 31.4:
r = 31.4 ÷ (2π) ≈ 5

If the area is 78.5:
r = √(78.5 ÷ π) ≈ 5

Why Radius Matters

The radius is used in many calculations, including:

• Circle area and circumference
• Geometry and trigonometry
• Engineering and design
• Construction and measurement
• Physics and motion calculations

Frequently Asked Questions

What is the difference between radius and diameter?

The diameter goes across the entire circle through the center. The radius is half of the diameter.

Can the radius be calculated from area?

Yes. Use the formula r = √(A ÷ π).

What value of π should I use?

For most calculations, use π ≈ 3.1416 or the π button on a calculator.