Not all poly bags are created equal!

When you’re fulfilling a government contract with ASTM D3951 packaging requirements, the poly bag you choose can make or break your shipment.

Sure, 2 mil and 4 mil bags may look similar at a glance, but their performance during transit is very different. And if your packaging fails in transit? That’s on you.

Let’s break down what the difference really means and when to use each.

What does “mil” mean?

“Mil” is a unit of measurement equal to one-thousandth of an inch.

  • 2 mil = 0.002 inches thick
  • 4 mil = 0.004 inches thick

Both are approved under ASTM D3951 for commercial packaging. That means either bag technically meets the minimum requirement for many DLA or GSA shipments.

But just because both are allowed doesn't mean they're equally wise choices.

2 mil vs. 4 mil: Which is better?

2 mil: thinner and more flexible

Best for:

  • Lightweight parts
  • Soft, flat items (like washers or gaskets)
  • Items that won’t shift or poke through during shipping

Risk: Thin bags can tear more easily during transit, especially with bulkier or irregularly shaped parts.

4 mil: thicker and more durable

Best for:

  • Bolts, fasteners, tools, and bulkier items
  • Anything with a blunted point or sharp edge
  • Situations where durability matters more than cost

Pro tip: At MeND, we use 4 mil bags for nearly all shipments and we’ve never had a rejection due to bag failure (knock on wood).

What about cost?

Yes, 4 mil bags cost more almost double the price of 2 mil.

But in real terms, we’re talking pennies:

  • ~$0.003 for a 2 mil bag
  • ~$0.006 for a 4 mil bag

That’s $1 for 300 bags vs. 150 bags. The cost difference is marginal compared to the risk of repackaging, rejections, or shipment delays.

Final recommendation

  • Use 2 mil when the part is small, flat, and unlikely to damage the bag.
  • Use 4 mil when: The item is heavy, has bulk, or could tear the packaging during transit.

Bottom line: Both bags meet ASTM standards, but 4 mil offers peace of mind and is the safer choice for most contracts.

Need packaging materials that meet MIL-STD and ASTM specs?
You can buy 4 mil poly bags, get quotes, and order MIL-STD-129 labels at GovPacking.com