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Circle Calculator

Calculators Free Updated 25 Sep 2026
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ⓘ About this tool 🔄 Last updated 25 Sep 2026 ✓ Reviewed by TopToolAI Team

Enter any one measurement of a circle, radius, diameter, circumference or area, and get all four, worked out step by step.

How to use Circle Calculator

Step-by-step guide to the boxes below. Open it only if you need help.
  1. 1
    Radius Text

    Distance from the centre to the edge. Enter any one of the four boxes and the rest are calculated.

    Example: 5

  2. 2
    Diameter Text

    Distance straight across the circle. It is twice the radius.

  3. 3
    Circumference Text

    Distance around the circle (the circumference).

  4. 4
    Area Text

    The flat space inside the circle.

  5. 5
    Press “Calculate”

    Click the button under the boxes. If something is missing or not valid, a short message tells you which box to fix.

  6. 6
    Read your result

    Your answer appears under the button, with a short note in plain words where it helps. Change any box and press again to compare different numbers.

Go to the calculator

Fill in exactly one box and leave the other three empty.

Embed this calculator on your website

Want this calculator on your own site? Copy the code below and paste it into the HTML of any page or post. It works the same as it does here. If you do not mind, leave the small link under it in place.

Circle Calculator FAQs

What is the relationship between radius and diameter?
The diameter is always exactly twice the radius. If you know one, the other takes one step to find.
Why does the area use radius squared?
A circle's area formula, πr², comes from adding up the area of thin rings from the centre out to the edge. It is one of the oldest results in geometry.
How do I calculate the area of a circle?
Area = π × r². A radius of 5 gives about 78.54.
How do I calculate the circumference?
Circumference = 2 × π × r, or π × diameter. A radius of 5 gives about 31.42.

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