PE Bag Thickness for Beginners – Microns, mm and "Si" Without the Confusion

"How thick is this bag? What is 0.05 mm in 'si'? Is 40 microns thick or thin?"

Thickness questions are the most common ones we get from customers. Understandably – microns (μm), "si" (丝), and millimeters (mm) mixed together is confusing. Today we skip the formulas and use everyday objects to build your sense of thickness.

First, the units: what is the relationship between microns, "si" and mm?

The unit most used in the PE bag industry is micron (μm). Sometimes people also say "si" (丝) – these actually mean the same thing.

• 1 si = 1 micron (μm)
• 1 millimeter (mm) = 1000 microns = 1000 si

So a 0.05 mm bag can also be called a 50-micron or 50-si bag. In daily production, our factory talks in microns.

Build your sense of thickness with everyday objects

Numbers alone mean little, so here is a comparison using things you already know:

Thickness	Feels like	Hand feel	Typical uses
0.01–0.02 mm (10–20 microns)	Household cling film	Extremely thin, soft, slightly sticky to the touch; stretches with light pull-back	Single-use covers, food wrapping, light surface protection
0.03–0.04 mm (30–40 microns)	Thin supermarket shopping bag	Thin but with some toughness; a crisp plastic sound when shaken	Everyday packaging, garment bags, bin liners
0.05–0.06 mm (50–60 microns)	Ordinary document folder plastic	Clear thickness; holds its shape when pinched, does not crumple into a ball	Electronics inner bags, stationery packaging, multi-item packing
0.08–0.10 mm (80–100 microns)	Plastic backing of a phone screen protector	Substantial hand feel, hard to tear, keeps its shape when picked up	Hardware packaging, heavy parts, packaging needing structure
0.12–0.15 mm (120–150 microns)	Heavy document pouch (courier inner liner)	Very substantial, very hard to tear by hand, almost like thin card	Heavy parts, industrial consumables, protective packaging for long-haul shipping

How to use this table? Next time you receive a sample, pinch it in your hand, then match it against the descriptions above – you will be able to estimate the thickness range.

Is thicker always better?

Not necessarily. We meet plenty of customers who say "I want the thickest," but choosing a thickness is more like choosing a tool – not heavier is better, but fitting is better.

• Too thin: bags with even slightly sharp-edged products break easily, and the seal is prone to opening.
• Too thick: cost doubles, and the bag becomes stiff and heavy, making the customer's unboxing experience worse.

For example, if you make apparel, one 0.03 mm flat bag per T-shirt is perfectly adequate – the thickness fits and costs stay controlled. If you make machine parts, you should start at 0.08 mm.

Two easy places to trip up

1. Suppliers quoting thickness with "water" in it

Some suppliers quote 0.05 mm but actually produce 0.04 mm. The difference is almost invisible to the eye, but you discover it after a few months – "why do these keep breaking?" Our factory's practice is to gauge every batch after extrusion and write the reading in the shipment report. The thickness you receive is the thickness we made.

2. Different resins feel completely different at the same thickness

At the same 0.05 mm:

• LDPE (low-density PE) – soft, feels like cling film, good transparency
• HDPE (high-density PE) – stiff, feels like paper, has a rustling sound, lower transparency
• LLDPE (linear low-density PE) – between the two, with the best toughness

This is why we always say: look at thickness and material together, never at the number alone.

If you remember only one thing from this page

Next time you hold a bag, take three seconds to do three things:
1. Pinch it – soft or stiff?
2. Shake it – does it sound crisp?
3. Match it against the feel table above – which range is it roughly in?

Three seconds and you no longer need formulas. If you still cannot decide, send us a photo and we will help you judge.