Circle Calculator
Enter any one measurement of a circle - radius, diameter, circumference, or area - to calculate the rest.
Last updated: September 5, 2026
Frequently asked questions
What is the circumference of a circle with radius 5?
About 31.42 - circumference = 2 × π × 5 ≈ 31.42.
What is the area of a circle with radius 5?
About 78.54 - area = π × 5² ≈ 78.54.
How do I find the radius from the circumference?
Divide the circumference by 2π: radius = circumference ÷ (2×π).
How do I find the radius from the area?
Divide the area by π, then take the square root: radius = √(area ÷ π).
How do I find a circle's diameter if I know its area?
First find the radius with radius = √(area ÷ π), then double it for the diameter. For an area of 78.54: radius = √(78.54/π) ≈ 5, so diameter = 10.
What is the diameter of a circle with a circumference of 31.42?
About 10 - diameter = circumference ÷ π = 31.42 ÷ π ≈ 10, since circumference always equals π times the diameter.
Why does area grow faster than circumference as radius increases?
Circumference scales linearly with radius (2πr) while area scales with radius squared (πr²) - so doubling the radius only doubles the circumference but quadruples the area, which is why larger circles hold disproportionately more area relative to their edge length.