Every measurement of a circle follows from its radius. Knowing any one value lets you find all the others.
How to use it
Enter the radius. If you know the diameter, halve it first. If you know the area, type it into the result and the radius is solved.
Everyday uses
Size a round table cloth, compare pizza sizes (a 16-inch pizza has about 1.8 times the area of a 12-inch one), or work out how much seed covers a circular lawn.
What each result means
- Area = pi × [Radius]^2
- Diameter = 2×[Radius]
- Circumference = 2×pi×[Radius]
Formula
A = π r²C = 2 π rd = 2 rWorked example
Radius 5 m.
Area = π × 5² ≈ 78.54 m². Circumference = 2π × 5 ≈ 31.42 m. Diameter = 10 m.
Key terms
- Radius
- Distance from the centre to the edge.
- Diameter
- Distance across the circle through the centre.
- Pi (π)
- The ratio of circumference to diameter, about 3.14159.
Frequently asked questions
How do I get radius from circumference?
r = C ÷ 2π.
Why does area grow faster than diameter?
Area depends on the square of the radius, so doubling the diameter quadruples the area.
Can I use different units?
Yes, choose length and area units independently.
What formula does the Circle Calculator use?
A = π r².
Can I work backwards from the result?
Yes. Type the result you want into Area and the calculator finds the matching radius.