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Circle Calculator

Enter the radius, diameter, circumference or area of a circle and get the other three at once, with every formula and substitution shown step by step.

  • Free, no sign-up
  • Runs in your browser
  • Math
Enter one value
A label only; nothing is converted. Area is shown in square units.
Fill in exactly one field and leave the other three empty.

Read the guide How to Find the Area of a Circle (and Its Circumference)

How to use the calculator

Type a value into one of the four fields: Radius, Diameter, Circumference or Area. Leave the other three empty and press Calculate. The result card lists all four measurements, and the details panel shows each formula with your numbers substituted in.

You can pick a unit label such as cm, m, in or ft. It is only a label: nothing is converted. Lengths are shown in the unit you chose and the area in the matching square unit, for example cm². Use Clear to start again.

The formulas

Everything about a circle follows from its radius r, the distance from the center to the edge:

  • Diameter: d = 2r
  • Circumference: C = 2πr, which is the same as πd
  • Area: A = πr²

When you enter something other than the radius, the calculator first works the radius out by reversing one of these:

  • From the diameter: r = d / 2
  • From the circumference: r = C / (2π)
  • From the area: r = √(A / π)

π (pi) is the ratio of any circle's circumference to its diameter, about 3.141592654. The calculator uses the full double-precision value and shows results with up to 10 significant digits.

Worked example

A round table top has a radius of 5 cm.

  1. Diameter: d = 2 × 5 = 10 cm.
  2. Circumference: C = 2 × 3.141592654 × 5 ≈ 31.41592654 cm.
  3. Area: A = 3.141592654 × 5² = 3.141592654 × 25 ≈ 78.53981634 cm².

Going the other way, a circle with an area of 100 m² has a radius of √(100 / π) ≈ 5.641895835 m and a circumference of about 35.44907702 m.

Good to know

  • Doubling the radius doubles the diameter and circumference but multiplies the area by four, because the radius is squared.
  • If you measure around a pipe or tree trunk with a tape, enter that as the circumference to get its diameter.
  • Make sure all of your measurements use the same unit. To change units, for example from square feet to square meters, use a separate area or length converter.

Frequently asked questions

What is the difference between radius and diameter?

The radius runs from the center of the circle to its edge. The diameter runs straight across the circle through the center, so it is always exactly twice the radius.

How do I find the area if I only know the circumference?

First find the radius with r = C / (2π), then use A = πr². You can also combine the two steps into A = C² / (4π). For a circumference of 10, the area is about 7.957747155. Enter the circumference in the calculator and it does both steps for you.

Does the unit setting convert my values?

No. The unit is only a label added to the results so they are easier to read. If you enter a radius in inches, the circumference is in inches and the area is in square inches.

Why are the results rounded?

π is an irrational number with endless decimal digits, so most circle results cannot be written exactly as decimals. The calculator shows up to 10 significant digits, which is far more precise than any physical measurement.