Overview
What Is a Circle Calculator?
This tool calculates a circle's area and circumference from its radius, using the standard geometric formulas involving pi.
Calculate area, circumference, radius, and diameter instantly.
This tool calculates a circle's area and circumference from its radius, using the standard geometric formulas involving pi.
Useful for geometry homework, construction projects, or any task involving circular measurements.
Input the circle's radius.
Area, circumference, and diameter update automatically.
Area = π × radius²
Circumference = 2 × π × radius, or π × diameter.
Diameter is the full width of the circle through its center; radius is half of that, from center to edge.
This uses JavaScript's built-in Math.PI, accurate to about 15 decimal places.
Divide your diameter by 2 first, then enter that as the radius.
No, this calculates 2D circle properties - sphere volume and surface area use different 3D formulas.
Yes, calculating circular areas (like a round patio or pool) is a common practical use.
Yes, decimal values are fully supported for precise measurements.
No, calculations happen entirely in your browser.
Yes, completely free with no sign-up required.