Mylar bags explained: thickness, sizes and what to buy

Buy 7 mil for anything you intend to keep for years. Mylar is measured in mils, one thousandth of an inch, and the number is the single most useful thing on the packet: 3.5 mil punctures, 5 mil is acceptable for stock you rotate, 7 mil is the long-storage standard.

What mylar actually is

Metallised polyester film, usually a laminate of polyester and aluminium with a food-grade inner layer. The aluminium layer is what makes it work: it blocks light completely and is a far better oxygen and moisture barrier than any plain plastic. A clear bag, however thick, lets light through and lets oxygen creep in over years.

Thickness, decoded

Thickness	Best for	Verdict
3.5 mil	Single meals, weeks to months	Punctures too easily for stored food
5 mil	Rotating pantry stock	Acceptable if handled gently
7 mil	20–25 year storage	The standard, and what we recommend
7 mil inside a bucket	Rodent-prone spaces	Best protection available

The reason thickness matters is not that thin mylar is a worse barrier per square inch — it is that thin mylar punctures. A pinhole you cannot see is the same as an open bag, and freeze-dried food is often sharp: dried pasta, brittle fruit slices and anything with a bone fragment will work through 3.5 mil given a few years of being moved around.

Sizes, and how many you need

Size	Holds roughly	Good for
1 quart	200–300 g finished	Single meals, snacks, herbs
1 gallon	700 g–1.2 kg finished	The default. Most batches split into these
2 gallon	1.5–2.5 kg finished	Bulk single ingredients
5 gallon (bucket liner)	8–12 kg finished	Long-term staples, grain, rice

A Blue Alpine Medium batch of 10 lb fresh food yields something like eight to twelve gallon bags. So a 50-pack covers roughly five batches, which at weekly use is a reorder every five or six weeks.

Zipper or plain?

Zipper bags cost more and are worth it only for food you will open and reclose — snacks, herbs, anything in daily use. For genuine long-term storage a plain bag heat-sealed across the top is better, because a zipper is a potential leak path and you are not going to reopen a 25-year bag until you need it.

A common approach is both: zipper bags for the pantry, plain 7 mil for the deep store.

What to buy with them

Never buy bags without matching absorbers, and ideally buy them together so the sizing decision is made for you. A gallon bag needs a 300cc absorber; getting that wrong is the most common storage mistake there is.