The idle capacity variance is the amount by which actual production usage declines below the normal or expected production level, multiplied by the overhead application rate. For example, a machine has a normal, long-term usage level of 400 hours per month (essentially two shifts of work per business day). In May, the actual usage level was 320 hours. The factory overhead application rate is $30 per hour. Based on this information, the idle capacity variance is the 80-hour difference between the normal and actual usage, multiplied by the $30 overhead rate, which is $2,400.
The idle capacity variance may not be a useful measurement, since it creates an incentive to keep using production facilities even when there is no need to build excess inventory levels. Thus, it can be better to absorb the negative variance than to avoid the variance by investing in more output.