Circle Area Calculator
Find the area of a circle from its radius, diameter or circumference, or work back from an area to the radius, with every measurement shown.
Find the area of a circle from whichever measurement you have: the radius, the diameter (easiest to measure straight across a table or pipe) or the circumference (a tape wrapped around a tree or column). You can also enter a known area to work back to the radius. The calculator returns all four measurements together, plus the area as a multiple of π when you need an exact answer.
How to use this tool
- Choose the measurement you know: radius, diameter, circumference or area.
- Enter its value.
- Pick the unit so the results are labelled; centimetres give an area in cm².
- Read the area, radius, diameter and circumference.
Formula
A = πr² = πd² / 4 = C² / (4π)
r is the radius, d = 2r the diameter and C = 2πr the circumference. Because area grows with the square of the radius, doubling the diameter makes the area four times larger. Working back from a known area, r = √(A / π).
Example
A round table is 120 cm across, so d = 120 cm and r = 60 cm. A = π × 60² = 3600π ≈ 11,309.7336 cm², about 1.13 m². Its circumference is π × 120 ≈ 376.9911 cm, the length of edging banding you would need.
Frequently asked questions
How do I find the area from the diameter?
Halve it to get the radius and use πr², or use πd² / 4 directly. A 10 cm diameter gives π × 25 ≈ 78.54 cm².
Is 3.14 accurate enough for π?
For rough work, yes: it is within 0.05% of π. This calculator uses the full double-precision value, 3.141592653589793.
How much bigger is an 18-inch pizza than a 12-inch one?
A 12-inch pizza has a 6-inch radius, so A = 36π ≈ 113.1 in². An 18-inch pizza has 81π ≈ 254.5 in², which is 2.25 times as much pizza for a diameter only 1.5 times larger.
What unit is the area in?
The square of the length unit you used: centimetres give cm², feet give ft². Convert all lengths to one unit before entering them.
Last updated: 2026-10-30