How to Measure a Poly Mailer for Shipping the Right Way

Have you ever ordered poly mailers that felt just slightly too small or way too big? You're not alone. Getting the measurement right matters more than you'd think. It affects shipping costs, how well your product fits, and whether you're wasting material.

This article breaks down exactly how to measure a poly mailer correctly, so you can choose the right size every time. Note: all measurements are in inches.

How to Measure a Poly Mailer: Step-by-Step

1. Lay It Flat

Place the mailer on a flat surface and smooth it out completely, and don't stretch the material. Poly mailers have some give, but stretching creates inaccurate measurements. You want the mailer's natural, relaxed shape.

2. Measure the Width

Measure straight across the opening from left edge to right edge. This is your interior width (the space your product has to fit horizontally).

Example: 10 inches wide

3. Measure the Height (Stop Before the Flap)

Measure from the bottom seam up to where the adhesive flap begins. That crease line is your stopping point.

Do not include the flap length in this number.

Example: 13 inches tall

4. Record It as Width x Height

Your mailer size is now 10x13. Simple.

What You're Actually Measuring

The most common confusion? Figuring out which parts of the mailer count toward its size.

Poly mailers are measured flat and empty, not filled with product. The size on the label reflects usable interior space.

Here's what matters:

• Width: Measured across the opening

• Height: Measured from bottom seam to where the flap starts

• Flap: Not included in the stated size (it's just for sealing)

The flap doesn't count in the measurement, but here's what does matter: product thickness. When you put something bulky inside, it pushes the mailer outward and takes up space. A thick item won't fit the same as a flat one, even if the length and width seem right.

Matching Your Product to the Right Mailer Size

Measure your product as it will actually be packed:

• Fold clothing the way it'll ship

• Include tissue paper or inserts if you use them

• Account for thickness, not just length and width

Your mailer should be slightly larger than your packed product dimensions, but not dramatically larger.

Example: A folded shirt that measures 9x12x1 inch typically fits well in a 10x13 mailer. This gives you room to insert the product easily and seal properly without excess material flopping around.

Why Thickness Still Counts

Poly mailers are flexible, but bulky items push outward and take up space. Overstuffing strains the seams and can compromise the adhesive seal.

When you're on the fence between two sizes, size up. A properly sealed mailer protects better and reduces damage-related returns.

Common Measuring Mistakes to Avoid

Even experienced shippers make these errors:

• Measuring the mailer after it's already filled

• Including the flap in the height

• Measuring the exterior instead of interior space

• Stretching the material while measuring

• Ignoring product thickness and only considering length and width

Avoid these, and you'll have much more consistent results.

Why Poly Mailer Measurement Actually Matters

Undersized mailers cause packing headaches, tears, and seal failures. Oversized mailers waste material and can trigger dimensional weight charges that can add unnecessary shipping costs.

Right-sized packaging uses only what's needed to protect your product and get it safely to your customer. Better for your budget and less waste overall.

Make It a System

If you ship the same products regularly, document your ideal mailer sizes. This keeps your team consistent and prevents "size creep" where mailers gradually get bigger without anyone noticing.

When your product line changes—new fabric weights, different inserts, updated folding methods—revisit your measurements. Small changes can affect the best mailer size.

The Bottom Line

Measuring a poly mailer comes down to three things: measure flat, exclude the flap, and be realistic about your product dimensions.

Do that, and you'll save money on shipping, waste less material, and avoid the frustration of mailers that don't fit.

Need help choosing the right poly mailers? EcoPackables offers sustainable options built for real-world shipping. Reach out and we'll help you find what works.