Circle Calculator
Instantly calculate a circle’s radius, diameter, circumference, and area from any single known value. Free, accurate geometry tool for school, work, or DIY projects.
Calculate Any Circle Measurement
Enter any one known value — radius, diameter, circumference, or area — and this tool will instantly work out the rest.
Circle Details
Choose what you know, then enter its value
Circle Evaluation
All key measurements calculated instantly
Common Circle Sizes
A quick lookup of area and circumference for common radius values (unitless — multiply by your unit squared for area).
| Radius | Diameter | Circumference | Area |
|---|---|---|---|
| 1 | 2 | 6.28 | 3.14 |
| 5 | 10 | 31.42 | 78.54 |
| 10 | 20 | 62.83 | 314.16 |
| 25 | 50 | 157.08 | 1,963.50 |
| 50 | 100 | 314.16 | 7,853.98 |
| 100 | 200 | 628.32 | 31,415.93 |
Circle FAQ
Everything you need to know about circle geometry and measurement formulas.
The area of a circle is calculated using the formula A = pi times radius squared (A = πr²). If you only know the diameter, first divide it by two to find the radius before applying the formula.
The circumference of a circle is calculated using the formula C = 2 times pi times radius (C = 2πr), or equivalently C = pi times diameter (C = πd).
The diameter of a circle is always exactly twice the length of its radius. The radius is the distance from the centre of the circle to its edge, while the diameter spans the full width through the centre.
This calculator uses the full precision value of pi available in JavaScript (approximately 3.14159265358979), which is more than accurate enough for virtually any practical or academic calculation.