Circle Calculator
Enter the center (h, k) and radius r. The calculator returns the standard equation (x − h)² + (y − k)² = r², the expanded general form, and the diameter, circumference and area of the circle.
Worked examples
Frequently asked questions
What is the standard equation of a circle?
(x − h)² + (y − k)² = r², where (h, k) is the center and r is the radius.
What is the difference between standard and expanded form?
The standard form keeps the center and radius visible; the expanded form x² + y² + Dx + Ey + F = 0 is what you usually start with when the problem hands you the general quadratic.
Can the radius be 0?
Not for a proper circle — r must be positive. r = 0 would collapse the circle to a single point.