The four measurements
A circle is described by one number. Radius, diameter, circumference and area are all linked through π, so knowing any one lets you find the rest.
Radius and diameter measure length. Circumference is the distance around the edge, also a length. Area measures the space inside, so it is in square units.
A worked example
Suppose a round table has a radius of 5 in whatever unit you like. The diameter is 10. The circumference is 2π × 5, about 31.416. The area is π × 5², about 78.540.
Starting from an area of 100 instead, the radius is about 5.642, the diameter 11.284 and the circumference 35.449. The calculator does this reverse step for you.
Everyday uses
You might use it to find how much fabric covers a round table, how much fence goes around a circular bed, how much paint a circular sign needs, or how much concrete a round column takes.
Remember that doubling the radius quadruples the area, because the radius is squared. A pizza with twice the diameter has four times the topping.
Relating the measurements
The ratio of circumference to diameter is the same for every circle, and that constant is π. It is why a wheel with a 0.7 m diameter travels about 2.2 m in one turn, and why a circular track with a 100 m diameter is about 314 m around.
Area and circumference grow at different rates. Doubling the radius doubles the circumference but quadruples the area, so a bigger circle holds proportionally much more inside than it takes to enclose it.
Good to know
Use the same unit for every length. Area comes out in that unit squared and is not converted. For round concrete columns, see the concrete calculator.
Questions people ask
How do you find the area of a circle?
Area = π × radius². For a radius of 5, the area is π × 25, which is about 78.54. If you know the diameter, halve it to get the radius first.
How do you find the circumference?
Circumference = 2 × π × radius, or π × diameter. A circle with radius 5 has a circumference of about 31.42.
How do I find the radius from the area?
Divide the area by π and take the square root. An area of 100 gives a radius of about 5.642, because 100 ÷ π is 31.83 and its square root is 5.642.
What is the difference between radius and diameter?
The radius runs from the centre to the edge. The diameter runs across the circle through the centre, so it is twice the radius.
What value of π is used?
The full-precision value of π (3.14159265…), not the rounded 3.14 or 22/7. That keeps results accurate for large circles and engineering work.