A UK supplier quotes 250 gauge, a US distributor offers 2.5 mil and a Turkish converter specifies 60 micron. Are they offering the same bag? The first two are identical (both 63.5 µm); the 60 µm film is about 5 % thinner, a gap that shows up in cost per thousand and sometimes in legal treatment. Below are the conversions between the three units, how thickness drives bag weight, and why thickness alone does not decide strength. Formats and ordering are covered on the custom printed plastic bags hub.
How do micron, gauge and mil convert?
1 mil equals 25.4 micron (µm), and in the UK system 100 gauge also equals 25.4 µm, so 1 mil = 100 gauge. To estimate quickly: micron × 4 ≈ gauge, and micron ÷ 25.4 = mil.
The exact formulas:
- Micron to mil: µm ÷ 25.4
- Mil to micron: mil × 25.4
- Micron to UK gauge: µm ÷ 0.254 (about µm × 3.94)
- UK gauge to micron: gauge × 0.254 (about gauge ÷ 4)
"Gauge" causes the most errors. In UK polythene trade usage, 100 gauge = 25.4 µm. In US film trade, gauge is also used for some films with the same 100 gauge = 1 mil relationship, but US bag buyers normally use mil. Gauge numbers used for sheet metal or wire are unrelated. If a quote uses gauge, confirm it means the UK film gauge.
Conversion chart
| Micron (µm) | UK gauge (approx.) | US mil (approx.) | Typical use |
|---|---|---|---|
| 10 | 39 | 0.39 | Very thin HDPE produce and vest bags |
| 12 | 47 | 0.47 | Thin HDPE vest bags |
| 15 | 59 | 0.59 | EU "very lightweight" threshold reference point |
| 20 | 79 | 0.79 | Standard HDPE vest bags |
| 25 | 98 | 0.98 | Heavier vest bags, light LDPE bags |
| 30 | 118 | 1.18 | Light die-cut bags |
| 40 | 157 | 1.57 | Die-cut retail bags |
| 50 | 197 | 1.97 | EU "lightweight" threshold reference point; die-cut and patch bags |
| 57 | 224 | 2.24 | Close to 2.25 mil (57.2 µm), an older US "reusable plastic bag" threshold in some laws |
| 60 | 236 | 2.36 | Patch handle and mailing bags |
| 70 | 276 | 2.76 | England carrier bag charge threshold reference point |
| 80 | 315 | 3.15 | Heavy patch and loop handle bags |
| 100 | 394 | 3.94 | Heavy reusable bags |
| 125 | 492 | 4.92 | Very heavy duty bags |
Values are rounded. Legal thresholds are shown only as reference points; what each one means is explained in plastic carrier bag regulations in the EU, UK and US.
How thick should a plastic bag be for its handle and load?
Typical starting points are around 10-25 µm for HDPE vest bags, 35-80 µm for LDPE die-cut bags and 50-100 µm for patch and loop handle bags, rising with bag size and weight carried. Treat these as starting points to test, not guaranteed capacities.
| Bag type | Light load (roughly under 2 kg) | Medium load (roughly 2-5 kg) | Heavy load (roughly 5-10 kg) |
|---|---|---|---|
| HDPE vest bag | ~12-15 µm | ~18-25 µm | Usually not recommended |
| LDPE/LLDPE die-cut | ~35-45 µm | ~50-70 µm | ~70-90 µm with fold-over or patch |
| Patch handle | ~45-55 µm | ~60-75 µm | ~75-100 µm |
| Soft loop handle | ~50-60 µm | ~60-80 µm | ~80-100 µm+ |
| Mailing bag | ~50-60 µm (soft goods) | ~60-80 µm | ~80-90 µm+ |
All figures are indicative for mid-size bags and depend on film recipe, bag dimensions and seal design. Larger bags need more thickness for the same load because the handle area carries a bigger, swinging mass.
Does a thicker bag always carry more?
No. Thickness is only one factor. Film recipe, handle construction and seal quality can each matter as much, and a well-built thinner bag often outperforms a poorly built thicker one.
- Film recipe: LLDPE in the blend raises puncture and tear resistance, allowing lower thickness for the same performance; HDPE is strong in tension but splits once punctured. See LDPE vs HDPE vs recycled PE carrier bags.
- Handle construction: a patch handle doubles film where stress concentrates; a plain die-cut handle in thin film tears from the corners of the cut-out. See plastic bag handle types.
- Seal quality: side and bottom seals that are too cold, too narrow or burned through become the failure point regardless of film thickness.
- Film orientation and blow-up ratio: blown film is stronger in some directions than others; extrusion settings change how the bag tears.
How bag strength is tested
Agree a test that reflects real use rather than relying on thickness. Common approaches:
- Hang or load test: fill the bag with a defined weight and suspend it by the handles for a set time, sometimes with repeated lifts.
- Drop test: drop a filled bag from a defined height and inspect seals and handles.
- Dart impact (for example ASTM D1709): measures the impact resistance of the film.
- Tear (Elmendorf, for example ISO 6383-2 or ASTM D1922) and tensile tests (for example ISO 527-3 or ASTM D882) on film samples.
- Thickness measurement across the web (for example ISO 4593) to confirm the nominal value and spread.
How does thickness affect bag weight and cost?
Bag weight rises in direct proportion to thickness, and film weight is the biggest cost driver in a plastic bag. Doubling the thickness roughly doubles resin use, freight weight and, in the UK, the tonnage that counts for Plastic Packaging Tax.
The formula for a flat bag:
Bag weight (g) = width (m) × length (m) × 2 × thickness (µm) × density (g/cm³)
Add the gusset depth to the length for bottom-gusseted bags, and to the width for side-gusseted bags. Subtract handle cut-outs for die-cut bags, and add patch or loop film for those constructions.
Worked example: a 380 × 450 mm LDPE bag at 50 µm with density 0.92 g/cm³.
- Film grammage: 50 × 0.92 = 46 g/m²
- Film area: 0.38 × 0.45 × 2 = 0.342 m²
- Bag weight: 0.342 × 46 ≈ 15.7 g, before subtracting the handle cut-out
- 10,000 bags ≈ 157 kg of film
The same bag at 70 µm weighs about 22 g, a 40 % increase in film for each bag. Down-gauging with a better blend, or adding a patch only at the handle rather than thickening the whole bag, can save meaningful material. Other cost factors such as print colours and handle type are covered in packaging cost drivers.
Using weight for compliance reporting
Bag weight per unit is the figure you need for EPR reporting, the UK Plastic Packaging Tax 10-tonne threshold and German single-use plastic fund reporting, all of which are calculated by weight. Ask your supplier for the nominal weight per bag in the specification, and keep it updated if thickness or size changes.
Why tolerance matters near legal thresholds
Blown film is never perfectly even. Thickness varies across the web and along the roll, and specifications commonly allow a tolerance around the nominal figure, often in the order of ±10 %, agreed between buyer and supplier. If a law draws a line at 50 µm or 70 µm, a bag specified exactly on that line could measure on either side of it.
Practical rules:
- Do not set the nominal thickness on a legal threshold. Choose a value clearly above or below it.
- State the measurement method and how many points are averaged.
- Write the agreed tolerance into the specification and purchase order.
- Note the market of use so the supplier can flag a threshold issue at technical review.
The EU uses 15 µm and 50 µm thresholds under Directive (EU) 2015/720, England's carrier bag charge applies to single-use plastic bags of 70 µm or less, and some US laws have used mil-based thresholds for "reusable" plastic bags. Which of these apply, and how, is covered in plastic carrier bag regulations.
How to state thickness in a specification
- Give thickness in micron as the primary unit, with gauge or mil in brackets if your team uses them.
- Specify whether the figure is per single film wall (standard) or total for both walls (rare, but it happens).
- Add film type and blend, since 50 µm HDPE and 50 µm LDPE are very different bags.
- State the load and test method the bag must pass.
- Add the tolerance and the measurement standard.
- For patch or loop handle bags, state the patch or loop thickness separately.
With those points defined, quotes become comparable. Send them with your bag size, handle type and artwork through the custom printed plastic bags hub or our contact page.
To convert a quoted thickness between microns, gauge and mil, use the film thickness converter.