Math

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Circle Calculator.

Calculate a circle's area, circumference, and diameter from radius.

On-device calculationNo signup
01

Set your values

Results update as you type.

Area: 78.539816

Area

0.000000
Circumference: 31.415927

Circumference

0.000000
Diameter: 10

Diameter

0.000000

Results update automatically as you type.

Use Cases

Geometry homework and math practice

Quickly verify your manual calculations for a circle's area, circumference, and diameter when you know the radius.

Example: Check your answer for a circle with radius 5 cm.

DIY and crafting projects

Determine the amount of material needed for circular objects, such as fabric, wood, or paper, by calculating the area or circumference.

Example: Find the circumference of a circular tabletop with radius 3 feet.

Frequently Asked Questions

How do I use the Circle Calculator?
Enter the radius of the circle in the provided field (in any unit). The calculator will automatically compute the area, circumference, and diameter using the standard formulas: area = πr², circumference = 2πr, and diameter = 2r.
What units can I use for the radius?
You can enter the radius in any unit (e.g., meters, centimeters, inches, feet). The results will be in the same unit squared for area, and in the same unit for circumference and diameter.
Can I calculate the radius from the area or circumference?
No, this calculator only takes the radius as input. It does not have fields for area or circumference to reverse-calculate the radius.

Tips & Common Mistakes

Tips

  • Ensure the radius is a positive number. Negative values are not valid for a circle.
  • Use consistent units for the radius to get results in the same unit system.
  • For large radii, the calculator will handle the numbers, but be mindful of the unit (e.g., kilometers vs. meters).
  • Remember that the area is always in square units (e.g., square meters), while circumference and diameter are in linear units.

Common Mistakes to Avoid

  • Entering the diameter instead of the radius. The radius is half the diameter.
  • Forgetting to square the radius when calculating the area, leading to an incorrect result.
  • Using different units for the radius and expecting results in a different unit without conversion.

Last updated: August 13, 2026