Circle calculator — area and circumference
Enter the radius and the calculator works out the area and the circumference together. Both appear side by side so they do not get mixed up.
A circle of radius 5 has an area of 78.54 and a circumference of 31.42.
Formula
r is the radius. Area is given in square units and circumference in ordinary ones.
How we got there
The calculator uses the full value of π rather than 3.14, so the answer is more precise than one worked out by hand.
How to work out the area and circumference by hand
- Square the radius
For a radius of 5: 5² = 25.
- Multiply by π for the area
π × 25 ≈ 78.54. That is the area, in square units.
- Use 2π r for the circumference
2 × π × 5 ≈ 31.42. The radius is not squared in the circumference.
- If you have the diameter, halve it
The radius is half the diameter. Using the diameter instead would overstate the area fourfold.
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Frequently asked questions
Multiply π by the radius squared: A = π r². For a radius of 5 that is about 78.54.
The circumference is 2π r. For a radius of 5 that is about 31.42.
A circle is the line itself; the disc is the circle together with everything inside it. So a circle has a length and a disc has an area.
Divide it by two and enter it as the radius. A diameter of 10 is a radius of 5.
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