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Math & Scientific

Circle Calculator

Calculate area, circumference, diameter, and radius of a circle with this easy-to-use calculator. Get instant results and formulas explained.

Circle measurements
Radius
5.0000
Diameter
10.0000
Circumference
31.4159
Area
78.5398

All measurements use the same unit as your input, except area, which is squared.

Results update as you type.

Circle Calculator

Use this calculator to find a circle’s radius, diameter, circumference, and area when you know any one of those measurements. It is useful for geometry homework, landscaping, craft layouts, machining, cooking pans, and design work.

How to use this calculator

  1. Choose what you know: Radius, Diameter, Circumference, or Area.
  2. Enter the Value as a positive number.
  3. Read the calculated Radius, Diameter, Circumference, and Area.

The calculator keeps length units consistent. If the radius is in inches, circumference and diameter are in inches, and area is in square inches.

Circle formulas

The radius is the distance from the center to the edge. The diameter crosses the circle through the center.

  • diameter = 2 × radius
  • circumference = 2 × π × radius
  • circumference = π × diameter
  • area = π × radius²

For example, a circle with radius 5 has diameter 10, circumference about 31.4159, and area about 78.5398. For related geometry tools, see the square footage calculator, slope calculator, and scale converter calculator.

Tips for real projects

Round only at the end, especially for material estimates. For physical work, add tolerance for cutting, seams, edging, or waste.

Frequently asked questions

What circle measurements can this calculator find?
Enter a radius, diameter, circumference, or area, and the calculator returns the radius, diameter, circumference, and area.
What units should I use for circle calculations?
Use one consistent length unit. Radius, diameter, and circumference use that unit, while area uses the squared version of the unit.
How do I find circumference from radius?
Multiply 2 by pi and by the radius. A radius of 5 gives a circumference of about 31.4159.
How do I find area from diameter?
Divide the diameter by 2 to get the radius, then multiply pi by the radius squared.

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