Math

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Diameter to Radius Calculator.

Convert a circle diameter into its radius and circumference.

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Set your values

Results update as you type.

Result: radius: 5.000000 · circumference: 31.415927

Result

radius: 5.000000 · circumference: 31.415927

Use Cases

Quick geometry calculations

Ideal for students or professionals who need to find the radius or circumference of a circle from its diameter without manual math.

Example: Enter a diameter of 12 units to get radius 6 and circumference ~37.7.

Design and construction planning

Useful for architects, engineers, or DIY enthusiasts to determine dimensions for circular objects or spaces.

Example: If a circular table has a diameter of 4 feet, the radius is 2 feet.

Frequently Asked Questions

How do I convert diameter to radius?
The radius is half the diameter. Simply divide the diameter by 2. For example, if the diameter is 10 units, the radius is 5 units.
What is the formula for circumference from diameter?
The circumference is π times the diameter. Using the calculator, you can get the circumference directly by entering the diameter.
Can I use any unit for the diameter?
Yes, the calculator accepts any unit you choose. The radius and circumference will be in the same unit as the diameter.

Tips & Common Mistakes

Tips

  • Ensure the diameter is a positive number; negative values are not valid for a circle.
  • Use consistent units for all measurements to avoid confusion.
  • For precise results, use a high-precision value of π if needed, but the calculator provides a standard approximation.
  • Double-check your input to avoid errors in the calculated radius and circumference.

Common Mistakes to Avoid

  • Forgetting to divide the diameter by 2 to get the radius.
  • Using the radius instead of the diameter when entering the value.
  • Mixing units (e.g., entering diameter in inches but expecting radius in centimeters).

Last updated: August 13, 2026