In the research of profile modification based on static characteristics, the selection criteria of profile modification length mostly adopt the following two methods:

The modification length corresponds to the length corresponding to the highest point of engagement of a single tooth; in some modification criteria, the recommended modification length is proportional to the module of the gear, i.e. the modification length is epsilon LM, where epsilon L is the modification length factor and M is the module of the gear pair.The recommended values for the epsilon L vary according to different standards, e.g. 0.6 in British standards and international standards, 0.45 in Soviet Union standards for cylindrical gears.

The profile modification can be divided into long and short according to the length of the profile modification:

In this formula, the LH will be the length of the modification corresponding to the highest point of single tooth engagement. As shown in the figure, when L < 1, it is called short modification and when L > 1, it is called long modification.