Specification

Film thickness: gauge, mil and micron

Three unit systems describe the same dimension, buyers and suppliers routinely use different ones, and the resulting error is not a rounding problem — it is usually a factor of four. Worse, the number itself is a poor predictor of how strong the film will be.

The three units and how they relate

One physical dimension, three conventions, and only one of them is metric.

  • Micron (µm) — one thousandth of a millimetre. The metric standard and the one used across Europe and Asia.
  • Mil — one thousandth of an inch. The common North American unit. Not a millimetre, despite how it reads.
  • Gauge — a North American convention where 100 gauge equals 1 mil. So gauge is simply mil multiplied by 100.

The conversions:

  • 1 mil = 25.4 microns = 100 gauge
  • 1 micron = 0.03937 mil = 3.937 gauge
  • microns → mil: divide by 25.4
  • mil → microns: multiply by 25.4
  • gauge → microns: divide by 3.937

Common equivalents, rounded the way the industry actually quotes them:

  • 0.5 mil = 50 gauge = ~12.7 µm
  • 1 mil = 100 gauge = ~25 µm
  • 1.5 mil = 150 gauge = ~38 µm
  • 2 mil = 200 gauge = ~51 µm
  • 3 mil = 300 gauge = ~76 µm
  • 4 mil = 400 gauge = ~102 µm
  • 6 mil = 600 gauge = ~152 µm

The error we see most often is a specification written in mil and read as microns, or the reverse. Asking for "2 thickness" without a unit and getting 2 microns instead of 2 mil is a fiftyfold miss. Put the unit in the specification every time, on every line.

Why the number alone tells you very little

Two films at identical stated thickness can differ by a factor of two in the load they carry.

Thickness is a dimension, not a performance property. What determines whether a bag holds is the resin, the blend, the number of layers and the process:

  • Resin. LLDPE stretches and resists puncture far better than LDPE or HDPE at the same thickness. An HDPE film feels crisp and thin and can outperform a thicker LDPE one on tensile while losing to it on tear.
  • Layers. A three- or five-layer co-extruded film places the tough resin where the stress is and the sealing resin at the surface. A monolayer film of the same total thickness compromises on both.
  • Orientation. Oriented films are dramatically stronger in the direction of orientation, at the cost of tear resistance across it.
  • Filler. Calcium carbonate filler adds thickness and weight cheaply and reduces strength. A thick film can be a weak one, and often the price is the clue.

This is why we ask what the bag has to carry rather than what thickness you want. If a specification arrives as a bare micron figure we will usually come back and ask for the load, the contents and how the pack is handled, because the same number can be right or badly wrong depending on those.

Tolerance: what a stated thickness actually guarantees

Film is produced to a tolerance band, not to a single value, and where you sit in that band is a commercial decision someone is making.

Blown film is normally produced to a percentage tolerance around the nominal figure. That band is legitimate and unavoidable — but it means a supplier can run consistently at the bottom of it, ship film that is technically within specification, and use measurably less material on every roll.

Two ways to protect yourself. First, specify the tolerance explicitly rather than accepting a default. Second, for anything where the material cost matters, specify by yield — bags per kilogram, or square metres per kilogram — instead of by thickness alone. Yield is the number that cannot be quietly optimised against you, because it is the number you are paying for.

Asking for a thickness report against an agreed sampling plan on the first production run is normal, and any supplier who resists it is telling you something.

Writing a thickness specification that cannot be misread

Six lines, and the argument about who was right afterwards does not happen.

  • The value with its unit, written out — "60 micron (2.4 mil)" rather than "60".
  • The tolerance band, as a percentage or an absolute range.
  • Whether the figure is nominal or minimum. These are different commitments.
  • The resin or blend, or the performance the film must meet instead.
  • The layer count if it is a co-extruded structure.
  • The yield, in bags per kilogram or square metres per kilogram.

The last one is the safety net. If everything else is ambiguous, yield still tells you whether you received the material you paid for.

Common questions

How many microns is 2 mil?

About 51 microns. One mil is 25.4 microns exactly, so 2 mil is 50.8, which the industry quotes as 50 or 51 micron depending on convention.

What does 100 gauge mean?

Gauge is one hundredth of a mil, so 100 gauge equals 1 mil, which is about 25 microns. Gauge is used mainly in North America and mainly for thin films such as stretch film.

Is a thicker bag always stronger?

No. Resin choice, layer structure, orientation and filler content all change strength at a fixed thickness. A well-built 40 micron co-extruded film routinely outperforms a filled 60 micron monolayer one.

Why do suppliers quote different thicknesses for the same job?

Usually because they are assuming different resins or different filler levels. Compare on yield — square metres or bags per kilogram — and on a performance requirement, and the quotations become comparable.

Where to go next

We answer every enquiry within 12 hours and issue a detailed quotation within 24 hours. Send us a specification.