Both bags say 80 microns. Why can their thickness and feel differ?

Two suppliers quote an “80-micron plastic bag”, yet their samples feel noticeably different. Has one supplied less material, or are they describing different thickness measurements?

Before judging by feel, confirm what 80 μm refers to: one bag wall, an individual film layer, the finished laminate, or a reading taken through both walls together.

This guide covers thickness communication for flat film and finished bags. It does not directly address measurement of bubble film, foamed materials or embossed structures.

Separate bag walls, material layers and stacked measurements

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“One wall” counts bag walls. “One layer” describes the material structure. A single bag wall can contain several layers.

A combined reading is approximately twice a one-wall reading only under suitable conditions: identical front and back walls, no additional folds, and an appropriate measurement method. Bottom or side gussets, seals and folded openings can contain additional material. Dividing a reading at those positions by two may give the wrong result.

Align the units as well. 80 μm equals 0.08 mm. One mil is 0.001 inch, or 25.4 μm, so 80 μm is approximately 3.15 mil. A mil is not a millimetre. Convert informal descriptions such as “double-thickness” and locally used units into an agreed written value in μm, together with the measurement object.

Teaching example: the thickness basis can double the theoretical material mass

All inputs below are teaching assumptions, not an order, quotation or factory measurement. Consider a flat, open-top bag made from two rectangular pieces of film, each 300 × 400 mm. It has no gussets, zipper, handles or cut-away sections. The seals overlap the two pieces and do not add film area.

The total film area is:

2 × 0.30 × 0.40 = 0.24 m².

Assume uniform material with no print or added components, and use a density of 0.92 g/cm³. This is an input for this example, not a fixed density for every PE film.

After unit conversion, theoretical mass for a uniform single material can be expressed as:

Mass (g) = area (m²) × thickness (μm) × density (g/cm³).

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This does not establish that a supplier has cut corners. It shows why an undefined thickness basis can leave the material quantity itself unaligned.

For a multilayer structure, account separately for each layer's area, thickness and density, and include relevant adhesive, ink or accessories. Inferring thickness from finished-bag mass also requires composition and complete dimensional information. Weighing does not replace thickness measurement.

Equal nominal thickness does not mean equal thickness everywhere

These two sets are also teaching data, in μm, rather than inspection results:

A: 78, 79, 80, 81, 82.

B: 60, 70, 80, 90, 100.

Both have a mean of 80, but their distributions differ. A contract that specifies only “average 80” does not answer whether the thinnest local readings are acceptable.

Define the target, permitted variation and the object to which that requirement applies: the mean, individual readings, batch statistics, or a combination. Also agree how samples are selected, where readings are taken and how unusual results are rechecked.

This does not mean every order needs the strictest available requirement. It prevents two parties from judging the same batch using different conventions. This guide sets no universal thickness tolerance or sampling quantity.

Match the measurement method to the material

ISO 4593 covers mechanical thickness measurement of plastic film and sheeting. Its public abstract excludes embossed film and sheeting. A familiar standard title does not make the method suitable for every surface structure.^[1]

Record the instrument, measurement configuration, sample condition, number of stacked layers and measurement positions. The measuring foot and pressure configuration should match the selected method. Thickness-instrument suppliers offer different configurations; the word “micrometer” alone does not make readings comparable.^[2]

For ordinary flat bag walls, agree positions that avoid extra folds. If seals or embossed areas need evaluation, define their measurement object and method separately instead of combining unlike locations into a single mean.

After defining thickness, return to the intended use

Thickness is one specification, not a substitute for every performance property. At the same 80 μm, resin, orientation, layers and sealing design can still differ. Manufacturers' PE and BOPP products are designed for different performance priorities.^[3][4]

When proposing a thinner alternative, describe the change, verify its performance in the agreed use, and then decide whether to approve it. A reduction in theoretical material mass alone does not establish that the complete alternative is better.

A useful enquiry can say:

Define thickness in μm for one complete bag wall. State the material layers, nominal value, proposed variation limits and sampling and measurement method. Identify combined-wall, gusset and seal readings separately; do not mix them with flat-wall readings.

Agree which layer, wall and location you are measuring before deciding whether the material is sufficiently thick or whether the price is comparable.

This guide uses the listed standards’ official public scopes and summaries to explain purchasing checks. It does not reproduce complete test procedures or establish product compliance. The factory or testing provider must confirm the method, edition, sampling and acceptance conditions for the material and use.

Film thickness confirmation checklist
Packaging Supply
https://globewhim.com/guides/plastic-film-thickness-basis/

[ ] Bag type / wall and layer structure:

[ ] Thickness object: single wall / layer / stacked walls:

[ ] Unit in μm / nominal value / measured value:

[ ] Sampling location / folds / gussets / seams separated:

[ ] Instrument / measuring head / pressure configuration:

[ ] Sample state / layer count / measuring method and edition:

[ ] Individual readings / sample count / mean and minimum:

[ ] Agreed tolerance scope / exceptions and retest:

[ ] Dimensions / composition for theoretical mass estimates:

[ ] Sample revision / use checks / change approval:

Mark unresolved items as “to confirm”. This checklist supports a supplier discussion and does not establish supply, engineering approval or a quotation.