Circle Calculator. Enter any one known value — radius, diameter, circumference, or area — and the Circle Calculator computes all remaining circle measurements instantly. Input your known dimension and get back the complete set of circle properties using standard geometric formulas (A = πr², C = 2πr, d = 2r). Also try the Arc Length Calculator.
The area of a circle is calculated using the formula A = πr², where r is the radius. If you know the diameter instead, use A = π(d/2)². Simply enter your known value above and the calculator will compute the area automatically. See also our Triangle Calculator.
The circumference (perimeter) of a circle is found using C = 2πr, which is equivalent to C = πd. It represents the total distance around the circle. Enter any known measurement and the calculator resolves the circumference for you.
The diameter is twice the radius: d = 2r. If you know the circumference, d = C/π. If you know the area, d = 2√(A/π). All these conversions are handled automatically when you select your known value type above.
The radius is half the diameter: r = d/2. From circumference, r = C/(2π). From area, r = √(A/π). The radius is the fundamental measurement of a circle — all other properties derive from it. You might also find our Total Surface Area — Surface Area of a Rectangular Prism useful.
Pi (π) is a mathematical constant representing the ratio of a circle's circumference to its diameter. Its value is approximately 3.14159265358979... It is irrational, meaning its decimal representation never ends or repeats. This calculator uses the full precision value of π available in JavaScript.
Yes. This calculator accepts any single known value — radius, diameter, circumference, or area — and derives all remaining measurements. Simply choose which value you know from the dropdown and enter it.
The core formulas are: Radius r = d/2; Diameter d = 2r; Circumference C = 2πr = πd; Area A = πr² = πd²/4. All four values are interrelated through these equations and the constant π.
The units you select do not change the numerical calculation — the formulas work the same regardless of unit. However, note that area is expressed in square units (e.g. cm²) while radius, diameter, and circumference are in linear units (e.g. cm). The unit selector helps you keep track of the measurement context. Check out our Missing Side — Pythagorean Theorem as well.