**Symbol**: rad / **Base data**: Angle. SI derived units.

Size of central angle for an arc equal to the radius of the circle

**Called “circular measure” in Japan**

The angle formed at the center of a circle and arc equal in length to the radius of the circle is “1 radian”. For example, 360 degrees is 2 π radian. The “°” (degree) according to the familiar sexagesimal system is something divided by humans, but this could be called more logical as it is expressed constantly regardless of how you define a circle.

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