Measuring Fitting Allowance

Fitting allowance is the distance between the center of a fitting and the thread engagement of a fitting.

Formula: Fitting Allowance = Center_to_End_Fitting_Measurement minus Thread_Engagement
or, F = A - T

Here, the Fitting Allowance is the difference between the Center to End measurement and the Thread Engagement.

Here, lines were drawn in green for the center-to_end measurements and the fitting allowance measurement of a 1 1/2" elbow in Revit.