Circle Calculator

How to use

1. 1Pick which value you already know. r is radius, d is diameter, C is circumference, A is area.
2. 2Type a positive number into the input field. Comma or period both work as the decimal separator.
3. 3Choose the length unit. The same unit labels every length output; areas use the unit squared.
4. 4Optional: check Add a sector or arc and enter a central angle in degrees to compute arc length, sector area, chord length, and segment area.
5. 5Read the four circle quantities in the Results panel. Copy any value individually or use Copy summary for the full report.

About this tool

Circle Calculator solves every standard quantity of a circle from a single known input. Pick whether you have the radius, diameter, circumference, or area, type one positive value, choose a length unit, and the tool derives the other three instantly using the canonical formulas (d = 2r, C = 2πr, A = πr², r = √(A / π)). Enable the sector option and enter a central angle in degrees to unlock arc length (s = rθ), sector area (½ r² θ), chord length (2r sin(θ / 2)), and circular-segment area (½ r² (θ − sin θ)), where θ is the angle in radians. The page shows what fraction of the full circle your central angle represents and the matching reflex angle, so the geometry is easy to read at a glance. Every result has its own copy button, and a single Copy summary button puts the full report (input, all four circle quantities, sector breakdown, and units) on your clipboard. Inputs accept comma or period as the decimal separator and any positive value, including scientific notation, so the tool works as well for engineering drawings as it does for homework. Length units (millimeters, centimeters, meters, kilometers, inches, feet, yards) are applied uniformly to every length output, with areas presented in the unit squared. Everything runs in your browser; nothing about your geometry is uploaded and no signup is required. Useful for math homework, geometry refreshers, CNC and laser-cutting layout, gardening and landscape design, pipe and tubing sizing, fitness disc and weight plate sizing, telescope and lens optics, baking pan sizing, woodworking pattern layout, and any time a single circle parameter is known and the rest need to be derived.

Free to use. Works in your browser. No signup, no login.