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 Pythagorean Theorem Calculator.
Area (A)
--
Radius (r)
--
Diameter (d)
--
Circumference (C)
--
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 find Arc Length with Arc Length Calculator 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 find Congruence Result with Congruent Triangles Calculator as well.