Start with radius, diameter, circumference or area. One measurement gives you the whole circle, with live results and an easy-to-read diagram.
01 / ONE MEASUREMENT
Enter one circle measurement. Results and the diagram update automatically — no Calculate button needed.
02 / YOUR CIRCLE
≈ Results are rounded for readability. π is kept at full available precision. Switching units preserves the original measurement and physical circle size.
03 / UNDERSTAND THE CIRCLE
Radius: center to edge. Diameter: edge to edge through the center — twice the radius. Circumference: the distance around the boundary. Area: the space inside, measured in square units.
π ≈ 3.141592653589793. Formulas use unrounded numbers; rounded substitutions may differ in the last shown digit. One inch is exactly 2.54 cm, one foot 0.3048 m, one yard 0.9144 m and one international mile 1609.344 m. Area conversions square the length conversion factor.
This calculator assumes a perfect circle. Software bounds are an equivalent radius of 1e−12 to 1e12 meters; unsupported extremes are rejected rather than producing misleading results. The illustration is proportional but normalized, not a physical-scale drawing. Everything is calculated in your browser.
Enter your values locally; no inputs are uploaded.
Check validation feedback before reviewing the browser-generated result.
Review the explanation, copy supported outputs or reset the workspace.
Choose the measurement you know, select a unit and enter a positive value. Results update automatically. Switch units to preserve the same physical circle, inspect the labeled diagram and expand the formulas to see how the measurements connect.
One primary numeric input; automatic results; eight length units with matching squared area units; an accessible original SVG diagram; optional display precision, a second unit and individual-value copies.
Measurements and calculations stay in your browser. No uploads, accounts or external processing services are required.
Enter just one positive measurement: radius, diameter, circumference or area. The calculator automatically derives the other three for an ideal circle; four editable measurements are not needed.
Circumference C = 2 × π × radius, or π × diameter. Radius is diameter divided by two, circumference divided by 2π, or the square root of area divided by π. Area is π × radius squared. The expandable explanation substitutes your current values.
Yes. Millimeters, centimeters, meters, kilometers, inches, feet, yards and international miles are supported. Unit switching preserves the original measurement and physical circle size. Area uses squared units and the square of the length conversion factor; for example, 1 m² = 10,000 cm².
The circumference is clearly marked as your given input. Radius, diameter and area are emphasized as calculated measurements. The diagram also highlights the circle boundary to identify circumference.
Calculations use Math.PI and unrounded floating-point values. Display precision can be set to 2, 4, 6 or 10 decimal places; very small or large results use scientific notation. The SVG is a proportional normalized illustration, not an actual-scale drawing or a measurement of a real object.
Copy Result includes all four measurements. Advanced options can add another display unit and buttons for individual values. Empty, zero, negative and malformed entries show validation; the supported equivalent radius is 1e−12 to 1e12 meters. Processing remains local in your browser.