Feb 24, 2019 · And the thickness in millimeters can be calculated by dividing the micron by 1000. For example: 50mu = 200 gauge = 0.05mm. 100mu = 400 gauge = 0.10mm. 200mu = 800 gauge = 0.20mm. Polythene thickness is sometimes also measured in ‘mil’. Example of nversion from mil to micron (mu) below: 1 mil = 25.4 mu. 3 mil = 76.2 mu.