Circle Calculator
Enter any one measurement of a circle — radius, diameter, circumference or area — and instantly get all the others.
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Save ⟶How to use the Circle Calculator
- 1Enter known value in the form on the left.
- 2Fill in the remaining fields — the result updates automatically as you type.
- 3Review the highlighted result and the supporting breakdown on the right.
- 4Use Copy, Share or Print to save or send your result.
Circle formulas
All circle measurements derive from the radius: diameter d = 2r, circumference C = 2πr, and area A = πr². Given any one value you can recover the rest, which is exactly what this calculator does.
π (pi) ≈ 3.14159 is the ratio of any circle's circumference to its diameter — the same for every circle, which is why these formulas are universal.
Frequently Asked Questions
▸How do I find the area of a circle from its diameter?
Halve the diameter to get the radius, then use A = πr². For d = 10: r = 5, A = π × 25 ≈ 78.54.
▸What is the circumference formula?
C = 2πr or equivalently C = πd. A circle with radius 3 has circumference 2 × π × 3 ≈ 18.85.