Poly Bag Buying Guide: Mil Thickness, Reclosable vs. Layflat, and How to Size Them

Poly bags look like the simplest thing in the packaging catalog, which is exactly why most people over-specify them. A half-mil of unnecessary thickness across a hundred thousand bags a year is real money, and so is a bag two inches too big on every unit you ship.

Here is what actually drives the decision.

Mil thickness — what it means and what it buys

A mil is one thousandth of an inch. It is not millimetres, and the confusion is common enough to be worth saying out loud.

• 1 to 1.5 mil — light parts, literature, retail polybagging, garment bags. Tears if you look at it wrong, which is fine for what it is.
• 2 mil — the workhorse. Small parts, hardware, assemblies, most general handling. If you do not have a reason to go heavier, this is the default.
• 3 to 4 mil — parts with edges or corners, heavier weight, bags that get handled repeatedly or stored long-term.
• 4 to 6 mil — sharp or abrasive contents, heavy castings, long-term storage, anything that will be dragged or stacked.

The mistake: buying thickness to solve a puncture problem that is really a geometry problem. If a corner is going through a 2 mil bag, a 4 mil bag often just delays it. Cushion the corner instead and stay at 2.

Reclosable vs. layflat

Reclosable (zipper-top) costs more per bag and earns it when the bag gets opened more than once — kitting, service parts, bin storage, anything a technician opens, takes one from, and closes again. It also means a part number stays with the part through the whole line.

Layflat is a plain open-mouth tube sealed on one end. Cheaper, and correct when the bag is opened once and discarded, or when you are heat-sealing it anyway. For high-volume outbound packaging where the customer opens it and throws it away, paying for a zipper is paying for nothing.

A practical test: if the bag's job ends the moment it is opened, use layflat.

How to size a bag

Bags are listed width by length, and width is always the open end. Two things go wrong here regularly.

Flat items: add roughly half an inch to the width and one to two inches to the length so the bag closes without straining the seal.

Three-dimensional items: width needs to accommodate half the girth, not just the widest face. A 4" × 4" × 4" box does not go in a 4"-wide bag. Add the depth into your width calculation, then add clearance on top.

Length matters more than people think if you are sealing or folding the top. Too short and the seal sits under tension; too long and you are buying and discarding film on every unit. On high volume, an inch of excess length is a line item.

Static — when a clear bag is not enough

Clear poly is an insulator, and pulling a part out of it generates a charge. For anything with a circuit board in it, that matters. Anti-static bags and anti-static foam exist for this reason, and bare boards want a metallized shielding bag rather than either.

We covered the distinction in the anti-static foam guide — the short version is that anti-static stops the packaging being the source of the charge, while shielding protects against charges from outside.

Things worth checking before you commit to a size

• FDA compliance if the contents are food or anything ingested or medical. Virgin resin, not recycled.
• Case quantity and how it stores. A bag you use forty of a week does not want to arrive a thousand to the case.
• Whether you are standardising or specifying. Three bag sizes covering ninety percent of your parts usually beats eleven sizes fitting each one perfectly — fewer part numbers, better price breaks, less picking error.
• Vented or perforated if the contents need airflow, or if trapped air is making the bag awkward to pack.

Where Supply Tie fits

Flexible plastics is one of our deepest private-label lines — over 570 items in reclosable and layflat poly bags, across the full size range from 4" × 6" up to 48" × 54", in 2-4 mil and 4-6 mil.

Because it is our own line rather than a catalog we resell, the pricing is set rather than marked up through a middle tier.

If you are not sure which size or gauge the job wants, tell us the part dimensions, the weight and roughly how many a year, and we will tell you what to run.

Browse flexible plastics

Narrower selections: poly bags and anti-static bags.

If the size or gauge the job wants is not in the line, the custom bag request-a-quote page takes the spec directly.