Circle Equation
Enter the center and radius.
Result
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Enter numbers to see the result.
Area
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Circumference
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Formula: Standard form: (x - h)^2 + (y - k)^2 = r^2. Area = pi r^2, circumference = 2 pi r.
Write the standard equation of a circle from its center and radius, with area and circumference.
Enter the center and radius.
Enter numbers to see the result.
Every point (x, y) on the circle sits exactly radius r from the center (h, k), and Pythagoras turns that into (x - h)^2 + (y - k)^2 = r^2. The tool formats this standard form for you, handling zero and negative center coordinates cleanly.
If your equation looks like x^2 + y^2 + Dx + Ey + F = 0, complete the square on x and y to reach standard form. The center is (-D/2, -E/2) and the radius is the square root of the constant side after completing.