The circle calculator is for the common situation where you know one measurement of a complete circle but need another. Choose radius, diameter, circumference, or area as the known value; the page first recovers the radius and then derives the other three basic circle properties.
It is useful for geometry exercises, drawing checks, quick circumference or coverage estimates, and verifying a hand calculation. It is not a general solver for every problem involving a circular shape.
C = 2πr or A = πr² quickly.For an arc, sector, segment, annulus, circle equation, or compound shape, use a specialized geometry calculator instead of forcing that problem into this page.
The four modes are radius r, diameter d, circumference C, and area A. The selected mode tells the page which quantity you already know; you enter one value rather than filling all four fields.
The page supports radius, diameter, circumference, and area. Enter one positive value in the matching mode and it derives the other three measurements.
No. Enter the number only, then keep one consistent length unit when interpreting radius, diameter, and circumference; area uses the corresponding square unit.
Ordinary decimals are supported, but the value must be greater than zero. Zero and negative values are rejected for the complete-circle calculation.
Radius, diameter, and circumference are lengths, while area is two-dimensional. If a length is in cm, the area is in cm².
The result cards show formatted decimal values rather than symbolic forms such as 10π or 100π. Use the formulas with π when an exact form is required.
Enter one positive radius, diameter, circumference, or area value to derive the other circle measurements and keep length and square units clear.