Online Circle Measurement: Radius & Diameter
Match the on-screen circle to a round object or shape and read its radius and diameter in your chosen unit.
Quick Usage
Place the center
Move the center handle until it sits at the visual center of the circle you are measuring.
Match the outer edge
Adjust the circle until its edge follows the target's edge as closely as practical. Use the fine controls when the fit is close.
Read the result
The tool shows the radius and diameter and updates the values as you adjust the circle.
Radius vs. Diameter
The radius is the distance from the center to the edge of a circle. The diameter goes from one side to the other through the center, so the diameter is twice the radius.
Measuring a Real Circular Object
For a coin, lid, round opening, watch face, or similar object, calibrate the display before using cm, mm, or inches as physical dimensions. Keep the object as flat and centered as practical.
A Visual Circle Fit
The tool does not find the circle center automatically. You place the center and adjust the size until the on-screen circle matches the target. The quality of the result depends on how well the center and edge are aligned.
Perspective Can Change the Shape You See
A round object viewed at an angle can appear oval because of perspective. For physical measurements, view the object as directly as the setup allows and keep it parallel to the screen when practical.
Frequently Asked Questions
半径、直径、実物の丸い物体、円測定に関するよくある質問。