Duke - Elastic Waist Underwear and Swimming Togs.
These measurements are the short pulled tight to what they should be measurements in centimeters | Measurements are accurate to within 1 - 2 centimeters* *
The easiest way is to take an existing pair of elastic waist pants that fits you well lay it flat and measure across the waist stretched tight.
This give you the half waist measurement once you have this, check the size chart and order the size closest to what you measured on your existing pant.