How to Measure and Convert Plastic Film Thickness

The thickness of thin sheet material such as plastic sheets needs to be known as it will influence properties such as the sheet stiffness – i.e. how easily the sheet can flex.

Sheet thickness is often stated by the manufacturer or it can be measured using a micrometre, given that conventional rulers are not capable of capturing such small measurements.

In the UK's metric system, the millimetre (mm) is the preferred unit for such measurements. For instance, a sheet's thickness might be denoted as 1 mm. For measurements that fall below 1 mm, it's standard practice to further divide the millimetre into 1000 parts, known as microns. To put it into perspective, 0.5 mm is equivalent to 500 microns.

Conversely, the term 'mil' is used in the USA, representing 1/1000 of an inch. This is analogous to the old UK imperial unit known as 'Thou'. In the US, the plural is expressed as 'mills'. This nomenclature can be a source of confusion for those familiar with the metric system. When spoken, "mil" or "mils" can be mistaken for the casual reference to millimetres, even though it's consistently written as 'mm'.

Now, introducing the concept of 'gauge': it's another method used to denote thickness, in plastics. The gauge number inversely correlates with the material's thickness; a higher gauge number indicates a thinner material. It's crucial to note that the gauge system can vary based on the material in question, so always ensure you're referencing the correct gauge chart for the material at hand.