What does 10 × 13 mean on a poly mailer?

A width of 10 in across the opening and a length of 13 in. Whether that is the usable interior or the outside edge depends on the brand.

How big an item fits in a 12 × 15.5 mailer?

At 1 in thick, with half an inch of ease and an inch for the closure, a flat item up to about 10.5 × 13 in. A thicker item has to be smaller.

Why is my mailer smaller inside than its listed size?

The seams take up some width, the closure takes up length, and some brands list the outside size or include the flap.

Are poly mailer sizes the same between brands?

The common sizes are similar, but how they are measured isn’t. Compare interior dimensions rather than the headline size.

What is a gusseted poly mailer?

One with folded sides or a folded bottom that open out, so it holds bulkier items than a flat mailer of the same face size.

Illustration of three poly mailers of increasing size side by side with their flaps folded open

Shipping & packaging

Poly Mailer Sizes and Usable Dimensions Explained

“10 × 13” on a pack of poly mailers is a width and a length, but the usable space inside depends on where the brand measured from and what the item does to the bag. Knowing how sizes are stated, and how much a product’s thickness eats into them, makes size charts much easier to read.

Ready to try the tool this guide describes?

Open Poly Mailer Size Calculator →

Width comes first, then length

Mailer sizes are normally written width × length, with the opening along the width. A 10 × 13 in mailer is 10 in across the opening and 13 in deep. The length is the direction the closure eats into.

Listed, usable, and outside sizes

Usable or interior size: the space inside the seams, below the opening. This is what matters for fit.
Outside size: edge to edge, including the welded seams, which can be a quarter inch or more on each side.
With or without the flap: some brands add the flap to the length, which can make a mailer look several inches longer than its usable space.

When a listing doesn’t say, assume the smaller usable figure and check a sample.

What each common size holds

The largest flat footprint for a product 1 in thick, with 0.5 in of ease and 1 in for the closure, is the mailer width less 1.5 in by the mailer length less 2.5 in:

What each common size holds comparison
Mailer (W × L)	In centimeters	Largest 1 in thick item
6 × 9 in	15.2 × 22.9 cm	4.5 × 6.5 in
7.5 × 10.5 in	19.1 × 26.7 cm	6 × 8 in
9 × 12 in	22.9 × 30.5 cm	7.5 × 9.5 in
10 × 13 in	25.4 × 33 cm	8.5 × 10.5 in
12 × 15.5 in	30.5 × 39.4 cm	10.5 × 13 in
14.5 × 19 in	36.8 × 48.3 cm	13 × 16.5 in
19 × 24 in	48.3 × 61 cm	17.5 × 21.5 in
24 × 24 in	61 × 61 cm	22.5 × 21.5 in

These are a reference set of widely sold sizes. Brands also make sizes in between and gusseted versions that expand.

How thickness shrinks the chart

Every extra inch of thickness takes an inch off both the usable width and the usable length. A 10 × 13 in mailer that holds an 8.5 × 10.5 in item at 1 in thick holds only a 6.5 × 8.5 in item at 3 in thick.

That is why a thick, small item can need a bigger mailer than a thin, larger one.

Reading a supplier listing

Find whether the size is interior or exterior.
Check whether the flap is included in the length.
Note the film thickness if the product has corners that could puncture it.
Look for “gusseted” or “expandable” if the item is bulky.

Poly Mailer Sizes Explained FAQ

What does 10 × 13 mean on a poly mailer?: A width of 10 in across the opening and a length of 13 in. Whether that is the usable interior or the outside edge depends on the brand.
How big an item fits in a 12 × 15.5 mailer?: At 1 in thick, with half an inch of ease and an inch for the closure, a flat item up to about 10.5 × 13 in. A thicker item has to be smaller.
Why is my mailer smaller inside than its listed size?: The seams take up some width, the closure takes up length, and some brands list the outside size or include the flap.
Are poly mailer sizes the same between brands?: The common sizes are similar, but how they are measured isn’t. Compare interior dimensions rather than the headline size.
What is a gusseted poly mailer?: One with folded sides or a folded bottom that open out, so it holds bulkier items than a flat mailer of the same face size.