If it's European size or US size?

What is European size and US size, there is no international definition. We have bended at underwear over 8 years and we know that even 2 clients from same city, they may need different size M. Client A needs 28cm, client B needs 36cm.

The right method is to measure the panty in centimeters. If u want certain sizes, pls tell me the waist length and center length.