Quick answer: Specify plastic bag thickness in micrometers (microns) or mils, identify whether the figure means one wall or both walls, and agree measurement locations and tolerances. One mil equals 25.4 micrometers. Thickness alone does not establish puncture resistance, seal performance, food-contact suitability or shelf life.

Two quotations can both say 100 microns and describe substantially different bags. One may refer to a single wall; the other may describe two walls measured together. For laminated packaging, the number may describe the total structure rather than its individual layers. Clear measurement language prevents a price comparison from becoming a comparison of different products.

Microns, millimeters and mils: the conversion table

A micron is one micrometer, written as µm. There are 1,000 micrometers in one millimeter. A mil is one thousandth of an inch, not one millimeter. NIST Handbook 44, Appendix C gives the thickness conversion of 1 mil = 0.0254 mm = 25.4 µm.

Micrometers (µm)	Millimeters (mm)	Mils, rounded
25	0.025	0.98
50	0.050	1.97
75	0.075	2.95
100	0.100	3.94
125	0.125	4.92
150	0.150	5.91

Conversion formulas: micrometers = mils x 25.4; mils = micrometers / 25.4. For example, a 2 mil film is 50.8 µm. Do not quietly round an ordered 2 mil specification to 50 µm: agree the nominal value and tolerance in writing.

Gauge can be used as a trade description with different conventions. If a quotation uses gauge, require its definition and the equivalent value in µm or mils. Do not rely on an unstated conversion.

Single-wall vs double-wall thickness

For an ordinary flat bag, a measurement through the unopened body can include two layers of film. If each wall were exactly 50 µm, the two-wall stack would be 100 µm. That arithmetic is an illustration, not a method for inspecting every finished bag.

A gusset, folded edge, zipper or heat seal can add layers or distort the measurement. Specify a representative flat body area away from those features. State the thickness basis as one wall, complete film structure when that is what the order requires.

For multilayer film, also distinguish total wall thickness from individual layer thicknesses. A laminate with the same total thickness as another structure can have a different sealant, barrier layer or mechanical behavior. Read the packaging material comparison alongside the thickness requirement.

How to agree a useful thickness inspection plan

Define the nominal value and basis. Use a named unit and identify one wall or a stack.
Mark measurement locations. Cover relevant positions across representative bags while excluding seals, closures and folds unless they are being evaluated separately.
Agree the instrument and procedure. Record resolution, contact arrangement and measurement pressure where relevant; flexible film can deform under contact.
Define sampling. State how many bags and which cartons or production positions are checked.
Set acceptance rules. Say whether limits apply to each reading, the average, or both, and define what happens after a failure.
Keep the record. Report the actual readings, lot identification and specification revision.

ASTM D6988’s published guide scope addresses film thickness used in property testing and explicitly excludes commercial quality-control sampling. It is useful background on measurement practice, but citing it does not replace a buyer-supplier lot inspection agreement.

Why thicker film is not automatically better packaging

Within a comparable structure, thickness changes can affect handling and performance, but resin choice, orientation, layer design, seams and closures also matter. A thicker bag may still fail at a sharp corner or poorly made seal. Validate the actual filled package instead of choosing a thickness from product weight alone.

For pouches, use OTR and WVTR data to evaluate the required barrier under stated conditions. Use the seal integrity testing checklist for sealing requirements. Neither property follows automatically from a thickness number.

For shipping bags, choose the fit using our poly mailer size guide and assess film, closure and product protection together. Oversizing or overstretching a bag can cause problems that extra thickness alone does not solve.

A thickness specification buyers can copy

Thickness: [nominal value and unit], measured on [one wall / defined stack] of the complete structure. Locations: [marked body areas]. Method and instrument: [agreed procedure]. Sampling: [bags and production lots]. Acceptance: [individual and average limits]. Exclude folds, seals and closures unless separately specified. Report readings and obtain written approval before changing the structure or nominal thickness.

Place this wording in your custom packaging RFQ and approve a reference sample using our sample approval checklist. Bowen New Material (Dongguan) Co., Ltd. supplies custom packaging from Dongguan, China. Send your bag type, product and current specification to discuss a suitable structure and quotation.

Frequently asked questions

Is 1 mil the same as 1 mm?

No. One mil is 0.0254 mm, or 25.4 micrometers. One millimeter is 1,000 micrometers.

Does 100 micron mean one wall or two walls?

The number alone does not tell you. Specify the measurement basis on the drawing and quotation before comparing offers.

What thickness should a shipping bag have?

There is no universal answer based only on order weight. Consider the product shape, film structure, closure, handling and complete-package trial results.

Can a thickness reading confirm recyclability or food safety?

No. Those require a separate assessment of the complete structure, intended use, documentation and applicable collection or compliance requirements.