Plastic sheeting, tubing and bags are measured by their thickness and this is expressed in "gauge" (imperial) or "microns" (metric).

Gauge relates to thickness in "thou" (thousandths of an inch), and each 100 gauge is 1 thousandth of an inch. Thus 500 gauge is 5 thou thick.

Microns are thousandths of a millimeter. Thus 100 microns are 1/10th of a millimetre. Gauge can also be related to microns by dividing the gauge by 4. Thus 500 gauge polythene film is 125 microns.