Circle Calculator
Enter the radius, diameter, circumference or area of a circle and get the other three, with the working shown.
About this tool
Know one measurement of a circle? Choose whether you have the radius, the diameter, the circumference or the area, enter the value and get the other three instantly. The calculator uses d = 2r, C = 2πr and A = πr², and shows each step — handy for homework, round tables and tablecloths, garden beds, pipes, pools and comparing pizza sizes. Works in any unit; the area comes out in square units of what you entered.
Frequently asked questions
Multiply π by the radius squared: A = πr². A circle with radius 5 cm has an area of π × 25 ≈ 78.54 cm². If you only know the diameter, halve it first — a 10 cm diameter gives the same 78.54 cm².
Use C = 2πr or C = πd. A circle with a diameter of 10 cm has a circumference of π × 10 ≈ 31.42 cm. π (pi) is about 3.14159 — the ratio of every circle’s circumference to its diameter.
Reverse the formulas: r = C ÷ (2π) and r = √(A ÷ π). A circumference of 100 cm gives a radius of 100 ÷ 6.2832 ≈ 15.92 cm. An area of 50 m² gives a radius of √(50 ÷ 3.14159) ≈ 3.99 m.
Often, because area grows with the square of the diameter. A 30 cm pizza has an area of about 706.9 cm², while two 20 cm pizzas together have 2 × 314.2 = 628.3 cm² — the single large pizza is about 12% bigger.
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