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Circle Calculator
Find circumference, area and diameter from a radius — or from any one of those values.
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How to Use This Circle Calculator
Type a radius, or leave radius blank and type a diameter. The calculator fills circumference, area, diameter and radius.
- Radius — distance from the center to the edge.
- Diameter — a straight line through the center; always twice the radius.
- Circumference — the distance around the circle, 2πr.
Practical Example
A circle with radius 10:
- Diameter = 2 × 10 = 20
- Circumference = 2π × 10 ≈ 62.832
- Area = π × 10² ≈ 314.159
What Your Results Mean
- Circumference — the perimeter of the circle.
- Area — the space inside, in square units of the radius.
- Diameter / radius — the two linear sizes; one always determines the other.
Frequently Asked Questions
What value of π is used?
JavaScript's Math.PI, about 3.141592653589793 — more than enough for everyday measurements.
Can I start from the diameter instead?
Yes. Leave radius empty and enter a diameter; radius is taken as diameter ÷ 2.
What units should I use?
Any, as long as they match. A radius in centimeters gives circumference in centimeters and area in square centimeters.
How is area different from circumference?
Circumference is a length (the outline). Area is the space inside, so it uses square units.
Does a larger radius grow area faster than circumference?
Yes. Area grows with r², so doubling the radius quadruples the area but only doubles the circumference.
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