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Circle Calculator: Radius, Diameter, Circumference, Area

This free circle calculator finds every measurement of a circle from just one. Give the radius, the diameter, the circumference or the area, and read all four values with the formulas.

By Debmedia Technologies LLP · Kolkata, India · Updated

What Is It and When Do You Need It?

Every measurement of a circle is tied to its radius. If you know one, you can find all the others. People need it for round tables, pipes, wheels, pools and school problems.

Choose which value you know and type it in.

How to Use This Tool

  1. Enter the value or values asked for in the boxes.
  2. Choose the unit label if there is a choice.
  3. Press Calculate.
  4. Read every result in the table and copy what you need.

How the Calculation Works

The tool first turns your value into the radius: r = d ÷ 2, r = C ÷ 2π or r = √(A ÷ π). Then it computes diameter = 2r, circumference = 2πr and area = πr². All results use the exact value of pi and show up to 10 significant digits.

Worked Example

A circle with radius 5 has a diameter of 10, a circumference of 31.4159 and an area of 78.5398. A circle with an area of 100 has a radius of 5.6419.

Common Uses

  • Round tables, rugs and gardens.
  • Pipes, wheels and gears.
  • Maths homework.
  • Craft and design.

Tips and Common Mistakes

  • Measure the diameter across the widest point.
  • Use the same unit throughout.
  • Area is in square units, not plain units.
  • Measure carefully: a small error in a length is squared in an area and cubed in a volume, so a 5 percent error in a side can become a 10 percent error in the area or a 15 percent error in the volume.
  • Round only at the end. If you copy a result into another calculation, copy the full number from the tool rather than a rounded one, so small errors do not build up.

Next, you may need the sphere calculator, arc length calculator, ellipse calculator, cylinder calculator.

Everything runs in your browser. Nothing you type is uploaded, stored or sent to any outside service.

Frequently Asked Questions

Diameter = 2r, circumference = 2πr and area = πr². Given any one value, the others follow.
The ratio of a circle’s circumference to its diameter, about 3.14159265. It is the same for every circle.
The same unit as your input, and area in that unit squared.
Divide the area by π and take the square root: r = √(A ÷ π).

Disclaimer: results are calculated in your browser from the numbers you enter. Check important results before you rely on them for money, health, safety or legal decisions.