How is the thickness of plastic bags measured?

How thick are resealable bags and what are the units of measurement used?The accepted UK standard for smaller domestic usage ziplock/grip seal polythene bags is 40 microns.30, 45 and 50 micron versions are also available.Science and industry often require 50 micron as a minimum and the heavy bags that you find displaying screws and bolts in hardware stores could be 100 microns or more.The US industry standard for ziplock bags is 2 mil or 50.8 microns. Heavier 4 mil (101.6 microns) are also available.