Plastic Bag Thickness Guide: Mil vs Micron vs Gauge

Published 16 October 2026 · Maisie Packing (Weifang) Co., Ltd. · 6 min read

If you have ever compared two packaging quotes and wondered why one supplier's “2 mil” bag feels half as thick as another's, the unit on the spec sheet is usually the culprit.

Thickness is one of the three numbers that decides how your order performs. The other two are bag size and material. Get it wrong and you either pay for film your product does not need, or you ship frozen food in a bag that cracks at −18 °C.

This guide covers the units buyers actually receive from suppliers — mil, micron, gauge and the Chinese 丝 — plus a conversion chart you can forward straight to your team, the thickness ranges that work for common food products, and the way to write thickness into an inquiry so nobody quotes you the wrong film. We manufacture flexible packaging in Weifang, China, and these are the numbers we work with every day.

Mil, Micron and Gauge: The Conversion

The short answer: 1 mil = 25.4 microns = 0.0254 mm = 100 gauge.

So a supplier quoting “50 micron” is quoting 2 mil. A supplier quoting “2丝” is quoting 20 microns — less than half of it.

Three units describe the same physical thing, and which one you see depends on where your supplier is based. North America quotes mil. Europe, Asia and most of the rest of the world quote microns (written µm). Film extruders in the United States still use gauge, a legacy unit that only applies to plastic film.

Convert any value in one step:

Mil × 25.4 = Micron Micron ÷ 25.4 = Mil

Mil × 0.0254 = Millimetres Gauge ÷ 100 = Mil Micron ÷ 10 = 丝

Mil is not millimetre, and that mistake is expensive. A mil is one thousandth of an inch (0.001″). It has nothing to do with mm. If your supplier reads “5 mil” as 5 mm and you read it as 5 mm, you will agree to a bag roughly twenty times thicker than either of you intended. Always write the unit out in full on the first mention.

What “丝” Means on a Chinese Supplier's Quote

If you are sourcing from China, Vietnam or India, your quotation probably will not say microns at all. It will say 丝 (pronounced sī).

One 丝 equals 0.01 mm, which equals 10 microns. That is the whole rule. Below is how the numbers Chinese factories actually quote translate into the units your team works in.

Here is how this goes wrong in practice. A US buyer sends an RFQ asking for “2 mil” bags. A Chinese salesperson, working in the unit they use every day, quotes “2丝”. Both numbers are written “2”. One is 50.8 microns; the other is 20 microns. The order arrives, the film is flimsy, the buyer blames the factory, and neither party can point to a document where the unit was ever agreed.

The fix takes one line in your purchase specification. Write thickness as “50 µm (2.0 mil)” — both units, every time, on every document. It costs nothing and removes an entire category of dispute. If a supplier quotes only in 丝, convert it yourself before you compare it to anything else.

Why Thickness Matters — and Where It Does Not

Puncture and tear resistance

Every extra micron adds material to resist a sharp corner or a rough edge. But the relationship is not linear, and it is not the only variable. A 60 µm oriented film can outperform an 80 µm unoriented one, because orientation aligns the polymer chains and multiplies the strength per unit of thickness. This is why an OPP bag and a PE bag at the same micron count behave nothing alike.

Moisture and oxygen barrier

Thickness is a weak lever here, and this is where most buying guides mislead people. Barrier performance comes from the polymer and the structure — metallised layers, EVOH, aluminium foil — not from adding microns of plain polyethylene. An 80 µm single-layer PE bag has a far worse oxygen barrier than a 60 µm PET/foil/PE laminate. If your coffee is going stale or your nuts are going soft, the answer is a better structure, not a thicker bag.

Seal strength and machine compatibility

Thicker film needs more heat, more pressure and longer dwell time to seal. If you run an automatic form-fill-seal line, its sealing window is fixed. Order film above that window and you get weak, leaking seals on a bag that was thick enough on paper. Tell your supplier which machine you run, and at what speed, before they quote a thickness.

Clarity, print and hand-feel

Thinner films are usually clearer. Very thick or heavily laminated film can look slightly hazy, which matters if you print barcodes or ingredient panels directly on the bag. Thicker also means stiffer, which changes how a flat bag drapes and how it packs into a carton.

Cost and freight

Thickness is one of the two biggest drivers of unit cost, alongside material type. Film cost scales roughly with thickness: moving a specification from 40 µm to 50 µm adds about 25% to the film portion of the bag price. The same film travels as extra weight, so you pay for it again in freight. Over-specifying is rarely a rounding error at container scale.

Thickness Chart by Food Product

The ranges below are starting points for single-layer OPP and PE bags, the kind we manufacture. Treat them as a basis for discussion in your RFQ rather than a fixed rule: bag size, fill weight, whether the contents have sharp edges, and your storage temperature all shift the answer.

Two rows say “laminate required” for a reason. Coffee and liquid food need an oxygen barrier that no single layer of OPP or PE can provide, regardless of how thick you make it. If a supplier offers to solve a shelf-life problem by increasing the micron count of a plain bag, that is a sign to get a second opinion. See our comparison of OPP vs PE bags for food packaging for where each material's limits sit.

OPP vs PE: The Same Micron Number, a Different Bag

This is the part buyers most often miss. Thickness tells you how much film there is. It says nothing about what the film does.

OPP (oriented polypropylene) is stretched during production, which aligns its polymer chains. The result is a big jump in stiffness and tensile strength per micron, plus glass-like clarity. A 30 µm OPP bag feels crisp, holds its shape on a shelf and shows the product inside beautifully. It also becomes brittle in the cold and can split quickly from a small nick at the edge.

PE (polyethylene) is not oriented the same way. It is softer, stretches under pressure instead of splitting, and stays flexible down to about −40 °C. To match the puncture performance of a stiff OPP bag, PE generally needs more microns. That is why PE bags for frozen and heavier food start around 50 µm while OPP bakery bags work fine at 25–30 µm.

Practically: if you are comparing a 40 µm OPP quote against a 40 µm PE quote, you are not comparing two versions of the same bag. You are comparing two different products that happen to share a number.

Why a 60 µm Laminate Can Beat an 80 µm Monolayer

Most retail food pouches are not one layer of film. A typical coffee pouch is built from three or four layers laminated together, each doing one job: an outer layer that takes the print, a barrier layer that blocks oxygen and light, and an inner seal layer that touches the food.

That creates a second ambiguity in any quotation. When a supplier writes “60 µm”, do they mean the whole structure or just one layer? A structure written as PET12 / AL7 / PE60 reads as 12 µm PET plus 7 µm aluminium plus a 60 µm PE seal layer — a total of 79 µm. Compare that against another supplier's “60 µm total” and you would conclude the first bag is thicker. You would not know it is also doing an entirely different job.

Ask for the layer structure, not just the number. Then ask about tolerance. Extrusion and lamination run to a tolerance rather than an exact figure: roughly ±5% is normal, and up to ±10% is possible on very thin film. Because the measurement method changes the result, agree on the test standard up front — ISO 4593 covers film thickness by mechanical scanning, and it is the reference we work to. Arguing over a 3 µm difference without a shared method wastes everyone's time.

How to Specify Thickness in Your Inquiry

Most thickness problems are specification problems, not manufacturing problems. Include these ten points in your first email and you will get comparable quotes from every supplier you approach.

1. Thickness in two units. “50 µm (2.0 mil)”. Never a bare number.

2. Total or single layer. State which one you mean, or ask for the full structure.

3. Material and resin. OPP, LDPE, LLDPE — or the laminate structure you need.

4. Bag size and style. Width × length, plus gusset or bottom fold if applicable.

5. Fill weight per bag. Roughly 250 g, 1 kg, 5 kg. This drives thickness more than anything else.

6. Whether the contents have sharp edges. Bone, fins, dried pasta, hardware. It changes the answer immediately.

7. Storage and handling temperature. Ambient, chilled, frozen, or hot fill.

8. Your sealing method. Hand impulse sealer, automatic FFS line, and the line speed.

9. Closure type. Plain heat seal, press-to-close zipper, slider, or hang hole. See our zipper bags for resealable options.

10. Target market and compliance. US, EU or elsewhere, and whether you need FDA or EU 10/2011 documentation.

Then order samples at two thicknesses and test them with your actual product, at your actual filling speed, before you commit to a production run. A sample test costs a few days. A wrong specification costs a container.

Five Thickness Mistakes That Cost Buyers Money

1. Quoting in the wrong unit

Mil against millimetre against micron against 丝. Any two of these confused with each other can move your order by a factor of twenty in either direction. Write both units on every document.

2. Comparing total structure against single-layer thickness

A 120 µm total laminate and a 120 µm single-layer PE bag are not comparable products, even though the number matches. Ask for the layer breakdown before you compare prices.

3. Over-specifying “just to be safe”

Adding 20 µm to a specification feels like cheap insurance. In practice it raises unit cost, raises freight cost, makes the film stiffer and harder to seal, and can create sealing failures on a line that was tuned for thinner film. Choose the thinnest film that survives the job.

4. Expecting thickness to extend shelf life

Barrier comes from structure, not from microns. If the problem is oxidation, moisture loss or aroma escape, adding thickness to a plain PE bag will disappoint you. Change the material or the laminate, or add a valve.

5. Deciding thickness before checking your sealing machine

Sealing equipment has an operating window. If your chosen thickness sits outside it, you get leaks, weak seals and downtime. Send your supplier the machine make, model and line speed along with the RFQ.

Frequently Asked Questions

What is 1 mil in microns?

1 mil equals 25.4 microns, or 0.0254 mm. A mil is one thousandth of an inch, not a millimetre, which is the single most common source of confusion on packaging spec sheets.

Is 2 mil thicker than 50 microns?

Yes, but only marginally. 2 mil converts to 50.8 microns, so a 2 mil bag is about 0.8 microns thicker than a 50 micron bag. In practice you can treat the two specifications as identical.

What does 2.5丝 mean in a packaging quote?

2.5丝 equals 25 microns, or roughly 1 mil. 丝 (sī) is a Chinese unit of thickness where 1丝 equals 0.01 mm, or 10 microns. Chinese factories quote film thickness in 丝 by default, so convert it before comparing bids from different countries.

What thickness is best for frozen food packaging?

For PE or LLDPE freezer bags, 50 to 80 microns (2.0 to 3.1 mil) is the usual starting range. Below roughly 40 microns the film becomes brittle at freezer temperatures and punctures easily on sharp edges such as bone or fish fins.

Is a thicker plastic bag always better?

No. Thicker film costs more per bag, adds shipping weight, becomes stiffer and needs more heat and dwell time to seal properly. The right specification is the thinnest film that survives filling, transport and the customer's use.

What is the most common thickness for food packaging bags?

For light food bags such as bakery, biscuit and candy bags, 30 to 40 microns (1.2 to 1.6 mil) OPP is standard. For frozen, heavier or moisture-sensitive food, 50 to 80 microns (2.0 to 3.1 mil) PE is the usual range.

How much thickness tolerance is normal on plastic film?

Around plus or minus 5 percent is normal on standard extrusion, with up to 10 percent possible on very thin films. Because the measurement method changes the result, agree on the test standard — ISO 4593 covers film thickness by mechanical scanning — before disputing a difference of a few microns.

Can you manufacture food packaging bags at a custom thickness?

Yes. We produce OPP, PE and zipper food bags from about 20 microns upward, to custom size, thickness, print and closure. Send your product type, fill weight and target market and we will recommend a specification and quote within 24 hours.

Custom Bags at the Thickness You Actually Need

Not sure whether you need 40 µm or 60 µm? Send us the product, the fill weight and the storage condition, and we will recommend a specification rather than a guess. Maisie Packing (Weifang) Co., Ltd. has manufactured flexible food packaging since 2006, with seven production bases serving wholesale and private-label buyers worldwide.

• Custom thickness: OPP, PE and zipper bags from about 20 µm upward

• Full customisation: size, gusset, hang hole, zipper or slider closure, up to 8-colour printing

• Food-grade materials: 100% virgin resin, FDA and EU-compliant film available on request

• Samples first: free stock samples, test your own product before committing to bulk

Email: wanjiehuang03@gmail.com
WhatsApp: +86 131 8165 0316
Or browse our full range of food packaging bags and request a quote.

Thickness figures in this guide are typical starting ranges for single-layer OPP and PE bags and are intended as a basis for specification discussion. Final thickness should always be confirmed by testing your actual product with your actual filling and sealing equipment.