product code: 33075 Mask Measuring Kit assists divers in determining the mask size that fits best. The Neptune Series Masks come in two sizes: small/medium medium/large Sizing and measuring system were designed in accordance with the official measuring procedure used in the Los Alamos Scientific Laboratory charts at the SBCCOM Mask Fit Facility- Aberdeen Proving Ground (Maryland, USA) Mask Measuring Kit includes: Measuring tool (caliper) Full Face Mask size recommendation chart Measuring Instructions Only two simple facial measurements determine mask size .