Every poly bag and mailer spec starts with a number and the word “mil”: a 2 mil bag, a 1.5 mil FBA bag, a 4 mil industrial liner. It's the first thing a buyer asks for and the first thing a supplier quotes. It's also the most misunderstood number in poly packaging — because it describes one property (thickness) and gets treated as if it described another (strength).
Here's what a mil actually is, how to read a gauge chart, and why two bags at the exact same mil can perform nothing alike on your line and in your boxes.
Mil, gauge, micron — one thickness, three units
A mil is one-thousandth of an inch (0.001″). It has nothing to do with millimeters — a common and expensive point of confusion. The same thickness shows up in three units depending on who wrote the spec:
- 1 mil = 0.001 inch = 25.4 microns ≈ 100 gauge.
- 1.5 mil = 0.0015 inch ≈ 38 microns ≈ 150 gauge (the Amazon FBA minimum).
- 2 mil = 0.002 inch ≈ 51 microns ≈ 200 gauge.
- 3 mil = 0.003 inch ≈ 76 microns ≈ 300 gauge.
- If a quote is written in microns (common outside the US) or gauge (common in older industrial specs), convert to mil before you compare — otherwise you're comparing two different numbers as if they were the same.
A quick gauge chart by use case
Thickness is chosen for the load and the failure mode, not for its own sake. A rough starting map:
- 1.5 mil — light apparel mailers and FBA suffocation-warning bags. The Amazon minimum; below it the bag is non-compliant and too thin to carry warning print at scale.
- 2 mil — general-purpose poly bags, soft goods, bundling and protective overwrap. The everyday workhorse gauge.
- 2.5 mil — standard e-commerce poly mailers. Enough body to survive the parcel network and present a clean printed face.
- 3 mil — heavier or lightly-edged products, multi-item shipments, and mailers that take rough handling.
- 4 mil and up — industrial parts, hardware, and anything with a sharp edge or point that will punch through a thinner film.
Why two “2 mil” bags aren't the same bag
The mil is necessary, not sufficient. Two bags can both be labeled 2 mil and behave completely differently, because three things the label doesn't show drive performance more than the headline number:
- Tolerance. A 2 mil bag at ±0.05 mil holds 1.95–2.05 mil across the web. The same 2 mil bag at ±0.10 mil can run as thin as 1.90 mil — and the thinnest point is where it tears or fails the warning-print spec. The label says 2 mil; the bag is only as strong as its thin patch.
- Resin blend. LDPE, LLDPE, and their ratio set whether the film is stiff or stretchy, tear-resistant or puncture-resistant. A well-blended 2 mil can outperform a cheap 2.5 mil. The mil is the same; the formula is the product.
- Layers. A mono-layer film and a three-layer co-extruded film at the same total mil distribute strength differently — co-extrusion lets you put toughness where the bag needs it (the seal, the outer skin) instead of spreading the same resin evenly and hoping.
Thicker is not automatically stronger
The instinct to “just go up a mil” when a bag fails is usually the wrong fix and always the expensive one. Resin is the single biggest cost in the bag, and a mil of extra thickness is a mil of extra resin on every unit you'll ever buy.
Most strength complaints are puncture or seal failures, not the film simply being too thin overall — and those are solved by changing the blend or the layer balance, not by adding gauge. A vertically integrated supplier can rebalance a co-extruded film to add tear or puncture strength while holding the gauge flat, which matters even more when resin prices climb: it recovers strength without recovering cost. A converter who only buys finished film can't touch the formula — their one lever is to sell you a thicker, more expensive bag.
How thickness meets your bagger and your freight
Gauge isn't only a durability decision; it's an operations one. Too thin and the film slumps and double-feeds on an automatic bagger. Too thick and you've added weight and cost to every parcel for protection the product didn't need. The right gauge is the lowest one that survives your drop test, runs clean at your line speed, and carries your print — not the highest one that feels safe.
That's the gauge a supplier who runs their own extrusion can actually dial in, because they can measure variation per roll and adjust the line. A catalog supplier sells you the die they happen to stock.
Questions to ask before you accept a “mil” spec
- What's the tolerance on that gauge — ±0.05 mil or ±0.10? The thin point is the real spec.
- What's the resin blend, and is it documented per lot so it doesn't change silently between shipments?
- Is it mono-layer or co-extruded — and if co-extruded, where is the strength placed?
- Is gauge monitored inline during extrusion, or only spot-checked after?
- Can you run a sample lot at the gauge you'd recommend, so we can drop-test and bagger-test it before we commit?
The mil is where a poly spec starts, not where it ends. Two suppliers can both say “2 mil” and mean two different bags. The five questions above are how you find out which one you're actually buying.