  We present the one-loop correction to the $W$ boson mass in U(1)$_z$ type
extensions of the standard model. We compare it to an approximation, often used
in high energy physics tools. We point out that if the $Z'$ boson -- predicted
in U(1)$_z$ type extensions -- is much heavier than the $Z$ boson, then the use
of the complete set of one-loop corrections is necessary.
