Circle Calculator
Find area, circumference, radius or diameter of a circle from any one measurement.
What the Circle Calculator does
Every measurement of a circle follows from the radius. Area grows with the square of the radius, so doubling the radius quadruples the area — worth remembering when sizing pizzas, pipes or tanks.
Formula
Area = πr²Circumference = 2πr = πdr = C ÷ 2πr = √(A ÷ π)
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| What do you know? | one of 4 options | Yes | — |
| Value | number | Yes | Accepts more than 0. |
How to use it
- Choose What do you know?.
- Enter Value.
- Select Calculate.
Worked example
A circular table has a 1.4 m diameter. How much cloth covers the top?
- Known
- Diameter
- Value
- 1.4
r = 0.7 m, Area = π × 0.49 ≈ 1.539 m².
Reading the result
- Area grows with the square of the radius, so doubling the radius quadruples the area — a 16-inch pizza is nearly twice the food of a 12-inch one.
- Every other measurement follows from the radius, which is why entering any one of radius, diameter, circumference or area determines the rest.
Common mistakes
- Using the diameter where the formula expects the radius, which overstates the area fourfold.
- Comparing circles by their diameter when the quantity that matters is area.
Frequently asked questions
Is a 16-inch pizza twice a 12-inch one?
More than that in area terms per inch of diameter: 16² ÷ 12² = 1.78, so it holds about 78% more pizza.