In
engineering and
machining, an
allowance is a planned deviation between an actual dimension and a
nominal or
theoretical dimension, or between an intermediate-stage dimension and an intended final dimension. The unifying abstract concept is that a certain amount of difference
allows for some known factor of compensation or interference. For example, an area of excess metal may be left because it is needed to complete subsequent machining. Common cases are listed below. An
allowance, which is a
planned deviation from an ideal, is contrasted with a
tolerance, which accounts for expected but unplanned deviations.