Circle Calculator
Enter any one measure of a circle — radius, diameter, circumference or area — and get the other three instantly, with every formula shown with your numbers substituted in.
Result
This circle calculator solves the whole circle from any single measurement. Pick which value you know — radius, diameter, circumference or area — type the number, and it returns the other three values immediately. Every step is spelled out: each formula is printed with your numbers substituted in, so you can copy the working straight into homework or a report.
The math follows the four classic identities: the diameter is twice the radius (d = 2r), the circumference is C = 2πr (equivalently πd), and the area is A = πr². When you start from circumference or area the tool inverts those formulas for you, computing r = C ÷ 2π or r = √(A ÷ π) before filling in the rest. Results are rounded to six decimal places for display.
When the input is a radius or diameter, the calculator also prints the exact answer in terms of π. For a radius of 5 the circumference is exactly 10π and the area is exactly 25π — the form most geometry teachers ask for, since it carries no rounding error at all. The decimal equivalents (31.415927 and 78.539816) appear right next to it.
Use it to find how much edging a round flower bed needs, the area a circular table covers, the diameter of a tree trunk from a tape measurement around it, or just to check geometry answers. Everything runs locally in your browser — nothing is sent to a server — and the copy button grabs all four measures at once.