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). Chinese factories often say "si" (丝) – these are different units: 1 丝 = 10 microns = 0.01 mm.
- 1 丝 (si) = 10 microns = 0.01 mm (i.e. 1 micron ≈ 0.1 si)
- 1 millimeter (mm) = 1000 microns = 100 si
So a 0.05 mm bag can also be called a 50-micron or 5-si bag. In daily production, our factory talks in microns.
One-line summary: 1 丝 = 10 microns = 0.01 mm, so the number changes with the unit.
0.05 mm = 50 microns = 5 si. Next time a customer says "5-si bags," you know it is 0.05 mm.
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.
💡 Our advice: if you are not sure what thickness to use, send us a sample of your current bag, or tell us what you pack and how heavy it is. We review the specification and recommend the right thickness. Adjustments are confirmed against your written order.
Two easy places to trip up
1. Suppliers quoting thickness with "water" in it
Visual inspection alone cannot verify film thickness. State whether the requirement is single-wall or total thickness, define the tolerance, agree the sampling locations and measurement method, and require the agreed inspection record before shipment.
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.
Not sure what thickness to use? Let us help
Send the intended application, packed product, dimensions and load requirements. We will review the specification and recommend suitable material and thickness options.
📩 Get a Thickness Recommendation →