Choosing OPP Bag Thickness: Which Micron for Which Product?

"How many microns should it be?" is the first question on almost every OPP bag order we receive. Film thickness decides how the bag feels in the hand, how clearly it shows the product, how well it survives handling and shipping, and of course what it costs. In this guide we walk through how we, as the manufacturer, match thickness to product, how to specify size including the flap, and how to work out the weight of a bag yourself.

What micron means and why it matters

A micron (µm) is one thousandth of a millimetre. OPP bags are made from a single layer of film, so the micron figure you choose is the thickness of each face of the bag. Changing it moves four things at once:

• Stiffness: thicker film stands up straighter and looks crisper on a peg or a shelf. Thinner film is softer and drapes around the product.
• Clarity: OPP is exceptionally clear. At low gauges it looks almost like glass; thicker film stays glossy but folded areas can show a little more depth.
• Strength: tear resistance and puncture resistance rise with thickness, which matters most for heavy or sharp-cornered items.
• Weight and cost: film is bought by the kilogram. Add 20% to the micron and the film weight of every bag goes up by 20% too.

Matching thickness to the product

The right gauge depends on the weight of the item, the shape of its edges and how the bag will be used. Treat the table below as a starting point; testing your product in a sample bag is always the surest way to decide.

Cards and stationery. Flat, light items such as postcards and invitations rarely stress the film, so 20–25 µm usually does the job. The priorities here are sparkle and a low unit cost.

Textiles. For a folded T-shirt or a pack of socks, 30–35 µm is where most customers land: the bag holds its shape without carrying extra plastic. Bulky knitwear or sweatshirts benefit from 40 µm, which gives the side seals and the strip area more margin.

Accessories and hanging packs. On a display peg the bag needs to stand straight rather than sag, so thickness is about presentation as much as strength. We generally suggest 35–40 µm here.

Heavy items. Books, boxes, metal parts and toys have corners that work against the film during packing and transport. Moving to 50 µm or more cuts the risk of punctures noticeably.

Measuring the bag: width, length and flap

Getting the bag dimensions right is just as important as the gauge. An OPP bag is specified with three numbers:

• Width: the inside horizontal size. Add a few millimetres either side so the product slides in easily.
• Length: from the bottom fold to the mouth of the bag. The flap is not included.
• Flap: the lip above the mouth that carries the adhesive strip. A bag flap is typically around 30 mm on small bags and 40 mm on larger ones.

Remember the product's depth. A 2 cm thick item eats into both the width and the length of the bag, so a sensible rule of thumb is to add roughly half the product's thickness to each dimension.

Working out bag weight: a worked example

An OPP bag is formed by folding a strip of film and sealing the sides, so the film length used is twice the bag length plus the flap. Header bags add the hanging header twice as well. The formula is:

Weight (g) = Width (mm) × (2 × Length + Flap [+ 2 × Header]) (mm) × Micron × 0.91 / 1,000,000

The 0.91 is the density of OPP film in g/cm³. Take a 20 × 30 cm self-seal bag with a 40 mm flap at 35 µm:

• Film length: 2 × 300 + 40 = 640 mm
• Width × length × micron: 200 × 640 × 35 = 4,480,000
• Apply density: 4,480,000 × 0.91 / 1,000,000 ≈ 4.08 g

So 1,000 bags weigh about 4.1 kg and a 20,000-piece run uses roughly 82 kg of film. Drop to 30 µm and each bag comes down to about 3.49 g; go up to 50 µm and it rises to 5.82 g. That one calculation shows exactly why gauge has such a direct effect on price. The same logic sits behind any grammage figure.

Rather than doing the sums by hand, enter width, length, flap and micron into our OPP bag calculator and it shows the bag weight alongside an estimated price.

Balancing protection against cost

A bag that is thicker than necessary means paying for extra film on every order; one that is too thin means split packs, returns and a poor first impression. A few questions help find the balance:

• Is the product heavy, sharp-edged or does it have protruding parts?
• Will it hang on a peg or sit inside a carton?
• Will it be packed by hand or on a machine?
• Does the bag travel on its own, or inside an outer shipping box?

Light garments that ship inside a carton are usually fine at 30 µm, while the same garment sent out in the bag alone is safer at 40 µm. The calculator also accepts thicknesses that are not on the preset list, anywhere from 20 to 100 µm.

Minimum orders and film on rolls

Our current minimum for bags is 20,000 pieces, and film on reels starts from 200 kg. The up-to-date minimum order quantity is always shown in the calculator. If you run your own packing line, OPP film on rolls is slit to the width and gauge you need; choosing the micron follows the same thinking as for bags, with machine tension and sealing settings added to the picture.

Where to start

For a pack the customer can open and close again, the self-seal OPP bag is our most requested format. If you will heat seal or label the mouth, a plain OPP bag is the economical route. Not sure which gauge suits your product? Send us its dimensions and weight through the contact page and we will come back with a recommended size and thickness.