Polythene thickness explained: micron, gauge and mil

What is a micron, and why is polythene measured in microns?

A micron is one thousandth of a millimetre, and it is the standard unit for polythene film thickness in the UK. You will see it written as micron, micrometre or mu, so 100 micron, 100mu and 0.1mm all describe exactly the same film. Polythene is thin enough that millimetres would leave you in decimals all day, which is why the trade settled on a smaller unit.

Microns describe thickness and nothing else. They tell you nothing about the width of a reel, the size of a bag, the length on a roll or how strong the film is. A polythene specification is only complete when it states thickness alongside dimensions, which for a bag means width, length, any gusset depth, and the gauge.

Every polythene product we manufacture in Blackburn is made to a micron figure you specify rather than a fixed stock thickness, from around 15 micron for light shrink film up to 500 micron for heavy duty sheeting.

What is gauge, and why do UK buyers still use it?

Gauge is an older unit of film thickness in which 100 gauge equals 25 micron. Multiply microns by four to get gauge, or divide gauge by four to get microns, so 400 gauge is 100 micron and 1000 gauge is 250 micron. Nothing about the film changes when you switch units. A 250 gauge bag and a 62.5 micron bag are the same bag.

Gauge survives in British packaging because it arrived with imperial measurement and never fully left. Long running specifications, old drawings, machine settings, purchase records and the habits of buyers who have ordered the same sack for years all keep it in circulation. Refuse sacks and building film are still commonly quoted in gauge, while mailing bags and technical films are usually quoted in microns.

Strictly, gauge derives from the US mil, where 100 gauge is one mil, or 25.4 micron. UK practice rounds that to 25 micron so the arithmetic stays simple. The difference is small, but it explains why a film converted strictly from mil comes out at 101.6 micron rather than a round 100. Quote us either unit and we will work in the one you prefer.

How do you convert between micron, gauge, mil and millimetres?

There are four conversions worth knowing for polythene. Microns to gauge, multiply by four. Gauge to microns, divide by four. Microns to millimetres, divide by 1,000. Microns to mil, divide by 25.4. Going the other way, multiply mil by 25.4 to get microns, and multiply millimetres by 1,000.

A worked example. A 500 gauge film is 125 micron, which is 0.125mm, which is a shade under 5 mil. A 50 micron mailing bag is 200 gauge, 0.05mm, and just under 2 mil. If you would rather not do the arithmetic, our polythene thickness converter handles all four units at once and updates as you type.

Mil is the conversion that catches people out. One mil is one thousandth of an inch, or 25.4 micron, and it is not a millimetre. A US supplier offering 4 mil film means roughly 100 micron, not 4mm, which would be a rigid board rather than a film. If an overseas quote looks a thousand times too thick, a mil and millimetre mix up is almost always why.

Is polythene thickness measured on one wall or two?

Polythene thickness is quoted per wall. A 100 micron bag has two walls of 100 micron each, so a micrometer closed across the flat bag will read around 200 micron. Layflat tubing behaves the same way, and so do pallet covers, gusseted sacks and mattress covers. If you measure a sample and get double the figure on the paperwork, the paperwork is probably right.

To measure film properly you need a micrometer or a thickness gauge rather than a rule, and you should take readings at several points across the width instead of one in the middle. Open the bag out and measure a single wall, or measure the folded bag and halve the reading.

There is a second check that needs no micrometer. Polythene has a predictable density, around 0.92 grams per cubic centimetre for standard low density grades, so a given size and gauge should land at a given weight. A 400mm by 600mm bag at 100 micron works out at roughly 44 grams. If a bag comes in well under the weight its specification implies, the gauge is worth questioning.

What thickness of polythene do I need for my product?

Thickness follows the job, and four things drive it: the weight of the contents, how sharp or awkward they are, how many times the pack is handled, and whether it sits in storage or travels through a transit network. Light, clean, low weight items such as garments, paperwork and small components are usually fine at 25 to 50 micron. General packing that gets handled a few times sits around 50 to 100 micron. Above 100 micron the film starts to resist puncture and tearing properly, which is what heavy, sharp or repeatedly handled goods need.

Different products sit in different bands for good reason. Shrink film and perforated bags on a reel run light because they are used once and supported by the product inside. Mailing bags sit in the middle, tough enough for a courier network without adding postage weight. Sheeting, pallet covers and high strength polybags run heavy because they take real abuse.

If you are not sure where to start, our bag size finder and product selector will get you to a sensible specification, or send us a sample along with the weight of what goes in it. Guessing upwards is the expensive mistake, because you then pay for film you do not need on every order.

Does thicker polythene always mean a stronger bag?

No. Thickness is only one of the variables that decide how a film performs, and two bags at the same micron figure can behave very differently. Resin grade, blend, the number of layers and how the film is extruded all change puncture resistance, tear strength, seal strength and how far the film stretches before it gives way.

That matters commercially, because the instinctive response to a bag that splits is to order it thicker, and thicker film costs more on every order after that. Often the better answer is a different film at the same thickness, or a co-extruded structure that puts the toughness where the load is. We tune strength, elasticity, slip, stiffness, thickness and recycled content independently, so a specification can be made tougher without simply adding microns.

Weight carries a tax consequence too. UK Plastic Packaging Tax is charged by weight on plastic packaging that does not contain at least the recycled content proportion set by government, so the micron figure on a specification feeds into the tonnage you declare. Check gov.uk for current rates and thresholds.

Why does measured thickness vary across a reel of film?

A quoted polythene thickness is a nominal figure, not a promise that every square centimetre measures identically. Blown film is made by extruding molten polymer through a circular die and inflating it into a bubble, and small variations in die gap, air flow, cooling and haul off speed leave the film slightly thicker in some places than others, so measuring across the width of any reel will show variation.

That variation is managed rather than eliminated. Dies and take off equipment are rotated so the slightly thick and thin bands spread around the reel instead of stacking in one place, which keeps the reel winding flat rather than building a ridge. Thickness is checked across the web during production, and the tolerance that applies is agreed as part of the specification.

For a buyer, one micrometer reading proves very little. Take several readings across the width and along the length, then compare the average against what you ordered. If a batch is genuinely outside the agreed specification, tell your supplier and quote the readings and where you took them. We extrude and convert in house in Blackburn, so we can trace a problem back to the film itself.

Frequently asked questions

How many microns is 400 gauge?

400 gauge is 100 micron. UK polythene converts at 100 gauge to 25 micron, so you divide gauge by four to get microns and multiply microns by four to get gauge. In other units, 400 gauge is also 0.1mm and just under 4 mil.

Is 1 mil the same as 1mm?

No. One mil is one thousandth of an inch, which is 25.4 micron or 0.0254mm, so a millimetre is roughly forty times thicker than a mil. The two are easily confused on quotes from US suppliers, where 4 mil film means about 100 micron rather than 4mm. Convert before you order rather than after.

Does a 100 micron bag measure 100 or 200 micron with a micrometer?

Closed across a flat bag, a micrometer reads around 200 micron, because polythene thickness is quoted per wall and you are measuring two walls at once. Open the bag out and measure a single wall to check it against the specification, or measure the folded bag and halve the reading. Take several readings across the width rather than trusting one.

What is the thinnest and thickest polythene you manufacture?

Across our range we manufacture from around 15 micron, used for light shrink film and perforated bags on a reel, up to 500 micron for heavy duty sheeting. The usable range for any single product is narrower than that, because a mailing bag and a building sheet are not made to the same specification. Tell us what the film has to do and we will recommend a gauge rather than ask you to pick one.

Can you match the gauge of a polythene bag I already buy?

Usually, yes. Send us a sample, or the width, length and gauge from your existing specification, and we will quote a like-for-like item. If you also tell us what goes in it and how it fails when it fails, we can adjust the film rather than copy a gauge that is not quite working, because we extrude and convert in house.