Math
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Radius of a Circle Calculator.
Find circle measurements from one explicitly selected radius, diameter, or circumference input.
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Results update as you type.
Results update automatically as you type.
Find circle measurements from one explicitly selected radius, diameter, or circumference input.
Use Cases
Geometry homework and math practice
Quickly verify radius calculations for school assignments or personal study. Enter the diameter or circumference to check your work and understand the relationship between circle measurements.
Example: If a circle has a diameter of 10 cm, the radius is 5 cm.
DIY and crafting projects
When you need to cut a circle or determine the size of a circular object, use the calculator to find the radius from a known circumference or diameter, ensuring accurate measurements for your project.
Example: A circular table with a circumference of 157 inches has a radius of about 25 inches.
Frequently Asked Questions
- How do I find the radius if I know the diameter?
- Simply select 'Diameter' as the known measurement, enter the diameter value, and the calculator will automatically compute the radius by dividing the diameter by 2. It also shows the circumference for reference.
- Can I calculate the radius from the circumference?
- Yes. Choose 'Circumference' as the known measurement, input the circumference, and the calculator uses the formula radius = circumference / (2π) to find the radius. It also displays the diameter.
- What units does the calculator support?
- The calculator works with any unit of length (e.g., meters, centimeters, inches, feet) as long as you enter the measurement consistently. The result will be in the same unit you input.
Tips & Common Mistakes
Tips
- Ensure you select the correct known measurement type (Radius, Diameter, or Circumference) before entering your value to get accurate results.
- Use consistent units for all measurements. If you enter the diameter in inches, the radius will be in inches as well.
- Remember that the radius is always half the diameter, so you can double-check the calculator's output by dividing the diameter by 2.
- For a quick estimate, you can use the approximation π ≈ 3.14, but the calculator uses a more precise value for accuracy.
Common Mistakes to Avoid
- Selecting the wrong known measurement type, such as entering a diameter value while 'Radius' is selected, which leads to incorrect results.
- Mixing units, like entering the diameter in centimeters but expecting the radius in inches, without converting first.
- Forgetting that the circumference is the distance around the circle, not the diameter, and using the wrong value.
Last updated: August 13, 2026