The formula for the circumference \( C \) of a circle is given by:
\[ C = \pi d \]
where \( d \) is the diameter of the circle. Given that the circumference is \( 20\pi \), we can set up the equation:
\[ 20\pi = \pi d \]
To solve for the diameter \( d \), divide both sides by \( \pi \):
\[ 20 = d \]
Thus, the diameter of the circle is \( 20 \) units.