Form Measurements used to lie somewhere between the macroscopic CMM and the microscopic surface measurement. But there are some specific details that should be treated differently. And one of these is the effect of the probe on the measurement. In the new standards, this appears already as a morphological filter. But there are still some aspects that may be considered also, for instance, the possibility of deformation the contact between the probe and workpiece. In this work, we developed a mathematical model (experimentally proven) to evaluate this effect during a form measurement in a reference laboratory, while performign the usual measurement procedures for roundness, cylindricity, straightness evaluation, among others. It could be considered either in the uncertainty budget or the measurement itself. This model may be extrapolated to other fields, although it was restricted to form measurements at the beginning, where the conditions differ, for example, on the number of points measured, filters and algorithms, etc. Its implementation in other more sophisticated algorithms developed, will not complicate the theoretical model, but may be helpful when comparing the results obtained from different reference laboratories.