Thickness is the first thing on any polyester film specification, and the easiest to get wrong. Suppliers quote it in microns, older drawings in gauge, North American data sheets in mil and engineering drawings in millimetres or inches. This guide explains each unit, gives a conversion table for the thicknesses most often specified, and shows how thickness changes the weight, the area per kilogram and the way the film performs.
Microns, millimetres, gauge and mil
A micron (micrometre, µm) is a thousandth of a millimetre, and it is the unit most film is sold in. A 50 micron film is 0.050 mm thick.
Gauge is a traditional film unit in which 100 gauge equals 25.4 microns, so one micron is roughly 4 gauge. Trade names round the figure: a 23 micron film is usually called 92 gauge, and a 12 micron film 48 gauge.
A mil is a thousandth of an inch, or 25.4 microns. It is common on North American data sheets and in conservation supplies: a 3 mil film is about 75 microns, and a 5 mil film about 125 microns. Do not confuse a mil with a millimetre, which is almost 40 times thicker.
To convert: divide microns by 25.4 for mil, or by 0.254 for gauge, and divide by 1,000 for millimetres. Our film calculator does the conversion for you.
Polyester film thickness conversion chart
The chart covers the thicknesses most often specified. The weight and the area per kilogram are worked out from the relative density of 1.40 published for Polymex® polyester; other polyester films are very close to this.
| Microns | Millimetres | Gauge (approx.) | Mil | Weight (g/m²) | Area per kg (m²) |
|---|---|---|---|---|---|
| 12 | 0.012 | 47 | 0.47 | 16.8 | 59.5 |
| 19 | 0.019 | 75 | 0.75 | 26.6 | 37.6 |
| 23 | 0.023 | 91 | 0.91 | 32.2 | 31.1 |
| 36 | 0.036 | 142 | 1.42 | 50.4 | 19.8 |
| 50 | 0.050 | 197 | 1.97 | 70.0 | 14.3 |
| 75 | 0.075 | 295 | 2.95 | 105.0 | 9.5 |
| 100 | 0.100 | 394 | 3.94 | 140.0 | 7.1 |
| 125 | 0.125 | 492 | 4.92 | 175.0 | 5.7 |
| 175 | 0.175 | 689 | 6.89 | 245.0 | 4.1 |
| 250 | 0.250 | 984 | 9.84 | 350.0 | 2.9 |
| 350 | 0.350 | 1,378 | 13.78 | 490.0 | 2.0 |
Working out weight and yield
Film is usually bought by weight and used by area, so it helps to convert between the two. For polyester with a relative density of 1.40:
- Weight in g/m² = thickness in microns x 1.40
- Area per kilogram in m² = 1,000 ÷ (thickness in microns x 1.40)
So a 1,000 m long reel of 23 micron film, 500 mm wide, covers 500 m² and weighs about 16 kg, before the core. Halve the thickness and you double the area for the same weight, which is why packaging films are specified as thin as the job allows.
Work it out for your own film: the PSG film calculator gives the weight, area and yield for any thickness and size, the approximate length of film on a reel from its outside and core diameters, and the number of sheets per kilogram, for polyester, polypropylene and other films.
Polypropylene is much lighter than polyester for the same thickness, which is one of the differences covered in our article on polyester vs polypropylene film.
How thickness changes performance
Thickness does more than set the price. As polyester film gets thicker:
- Stiffness rises quickly. A thicker sheet resists bending far more, which matters for self-supporting windows, sleeves, overlays and die-cut parts.
- Electrical strength rises. Polymex® polyester is published at 6.4 kV at 23 microns, 16 kV at 125 microns and 23 kV at 250 microns (ASTM D 149), so insulation thickness is chosen from the voltage it must withstand.
- Flexibility falls. Thin films wrap, fold, seal and conform; thick films stay flat and hold their shape.
- Area per kilogram falls. Every extra micron adds weight and cost per square metre.
Typical thicknesses by application
Every job is different, but these ranges are a sensible starting point:
- Capacitors and ribbons: very thin grades, from about 3.5 microns.
- Flexible packaging, lidding and metallised barrier films: usually 12 to 36 microns, with heat-sealable grades typically 12 to 40 microns.
- Labels, overlays and graphics: thinner films for flexible labels, and heavier films, often 100 microns or more, for overlays and panels that must stay flat.
- Archival encapsulation and sleeves: conservators commonly use 75 to 125 microns (about 3 to 5 mil). See our guide to archival polyester film for encapsulation.
- Electrical insulation: chosen from the voltage, the insulation system and the mechanical demands of assembly. Our guide to polyester insulation film gives the published electrical data.
- Shims and spacers: set by the gap to be filled, from 12 microns upwards.
For the thickness ranges in which each type of polyester film is made, see our guide to polyester film grades and typical thickness ranges.
Thickness you can see: Polymex® colour coding
When thickness has to be checked on the shop floor, measuring every piece is slow and easy to get wrong. That is why PSG introduced colour-coded polyester in 1969. In Polymex® shimstock each colour identifies one thickness, so the right shim can be picked by sight without a micrometer: clear for 0.012 mm, purple for 0.025 mm, blue for 0.050 mm, amber for 0.100 mm and white for 0.250 mm, for example. Polyester covers the thinner gauges, from 0.012 mm to 0.350 mm, and polypropylene continues the range up to 3.000 mm. To make up a measured gap from these thicknesses, the shim stack calculator works out the fewest shims, and which colours, to use.
Thickness tolerance
Film is made to a nominal thickness, and the actual thickness varies slightly across and along the reel. If your process is sensitive to thickness, for example in precision shimming, layered insulation or close-fitting laminates, say so in your specification, and ask for the manufacturer's data sheet for the grade, which states its tolerance.
Thin films are hard to measure accurately with a hand micrometer, because the anvils compress the film and a single reading only covers a tiny spot. A more reliable check is by weight: cut an accurately measured area, weigh it, and divide the weight in g/m² by 1.40 to get the average thickness in microns. Measure several samples across the width of the reel, not just one.
Specifying thickness
When you ask for a quotation, give the thickness in microns or millimetres, with the unit, and include the gauge or mil figure from your original specification if that is how it was written. Tell us what the film does in your process, and we will confirm the right thickness and grade before you order. We can then supply it in the format you need, from master rolls to narrow reels, sheets and die-cut parts.
Not sure which thickness you need? Ask our technical team for a recommendation and a quote, with the application and the quantity, and we will come back to you with the right film for the job.
