Radius is the fingerboard radius of your instrument. This is the measurement of the curvature of your fingerboard from E string to E string.

It is usually measured in inches (7-1/4″…9-1/2″…14″). The larger the number, the flatter the fingerboard radius. The lower the number, the rounder the fingerboard radius. Another type of fingerboard radius is a compound radius, which starts round and gets flatter as you move up the fingerboard.

When measuring your radius, measure at the highest fret.