You’ve bought a stack of poly mailers for your online store, and now you’re staring at a 10×13 bag wondering if it’s actually the right size, or if the numbers printed on the packet even mean what you think they mean.
Guess wrong and you end up in one of two spots. Either your product is swimming around inside a bag that tears in transit, or you’ve left the height field blank on your courier’s booking form and get hit with a surcharge once they remeasure it at the depot.
Measuring a poly mailer correctly comes down to three numbers, width, length and filled thickness, and knowing exactly where on the bag to take each one.
This guide walks through how to measure the mailer itself, how to work out the right size for your product, and what to enter into your shipping carrier’s system so your labels come out accurate every time.
Here’s Everything You Need to Know in Under a Minute
- A mailer’s width is measured straight across the open end, and its length runs from the bottom fold to the base of the opening flap
- Once your item is packed and sealed, measure the filled thickness too, that’s the number most carriers actually use to price the parcel
- To size a mailer for your product, add roughly 1cm to the width and 2cm to the length, then round up to the next standard size
- The printed size on a poly mailer is its flat, exterior measurement, the seal flap and bottom seam eat into the usable space inside
- Leaving the height blank on a courier’s booking form is one of the most common reasons Australian sellers get hit with a surcharge
Table of Contents
- What Is a Poly Mailer and Why Measuring It Matters
- Poly Mailer Size Calculator
- How to Calculate the Right Poly Mailer Size for Your Product
- How to Measure a Poly Mailer for Shipping
- Usable Space vs the Stated Mailer Size
- Common Poly Mailer Sizes and What They Fit
- Why Accurate Measurements Matter for Australian Shipping Costs
- Understanding Poly Mailer Thickness (Gauge and Mil)
- When a Poly Mailer Isn’t the Right Fit
- Why Australian Sellers Choose The Boxman for Packaging
- Frequently Asked Questions
- Measure Once, Ship With Confidence
What Is a Poly Mailer and Why Measuring It Matters
A poly mailer is a lightweight plastic shipping bag made from polyethylene, with a self-sealing adhesive strip along the opening. They’re the go-to choice for clothing, soft goods and anything that doesn’t need rigid protection.
The catch is that a mailer’s advertised size, say 10×13 inches or 25 x 33cm, isn’t necessarily the space you actually have to work with once your product is inside. Get the measurement wrong at either end, the bag or the product, and you’ll end up with a parcel that’s too tight, too loose, or priced incorrectly at the counter.
Whether you’re comparing stock sizes or weighing up bags against boxes in our ecommerce packaging guide, measuring correctly from the start saves you money on freight and returns.
Poly Mailer Size Calculator
Skip the maths, enter your product’s folded dimensions below and this tool works out the minimum poly mailer size you’ll need.
Enter your product’s folded dimensions and this tool works out the minimum poly mailer size you’ll need, using the same formula as our poly mailer measuring guide.
How to Calculate the Right Poly Mailer Size for Your Product
The Sizing Formula
Measure your product’s length, width and depth as it will actually sit once folded or packed. Then apply this formula to find the minimum mailer size you’ll need:
- Width needed = product width + product depth + 1cm
- Length needed = product length + product depth + 2cm
This gives you the smallest mailer your product will comfortably fit into. Round up to the next standard size rather than down. An item that only just fits is an item that strains and tears at the seams in transit.
Worked Example
Say you’re shipping a folded jumper measuring 30cm long, 25cm wide and 5cm deep once folded. Run those numbers through the formula above:
- Width needed: 25cm + 5cm + 1cm = 31cm
- Length needed: 30cm + 5cm + 2cm = 37cm
You’d need a mailer of at least 31 x 37cm. Since that falls between standard sizes, round up to the next one on the chart below, a 38 x 48cm bag, which also leaves room for a packing slip or a thank-you card without straining the seal.
How to Measure a Poly Mailer for Shipping
To measure a poly mailer for shipping, lay the empty bag flat on a table and take a ruler or tape measure to two spots: straight across the open end, and from the bottom seam up to where the opening flap begins.
Measuring the Physical Mailer
- Width: Measure straight across the open end of the bag, from the left edge to the right edge.
- Length: Measure from the folded bottom seam up to the opening, stopping at the base of the flap rather than its tip. The sticky lip where the adhesive strip sits isn’t part of the usable bag.
- Thickness (Height): Once your item is packed and the bag is sealed, measure how thick or bulging the finished parcel is. This is the figure carriers use to calculate freight.
Entering Dimensions for Shipping Carriers
- Length and Width: Enter the flat dimensions printed on the bag or measured above, for example 25 x 33cm.
- Height/Thickness: Enter the real filled thickness, even if it’s only 2 to 3cm, rather than leaving the field blank or entering zero. Carriers use this figure to calculate cubic weight, and an inaccurate entry is one of the most common reasons sellers get hit with a surcharge after the parcel’s already been collected.
Usable Space vs the Stated Mailer Size
Most poly mailers list their size as the flat, exterior dimension, the measurement you get from an empty, unsealed bag lying open. Your product doesn’t sit flush against those exact edges once it’s inside.
The seal flap takes up part of the printed length, and a couple of centimetres get lost in the bottom seam. If a supplier doesn’t list “usable space” separately from the printed size, assume the real internal space is roughly 1 to 2cm smaller on each dimension than what’s on the packet.
This is also why the same product can fit comfortably in one supplier’s 25 x 33cm mailer and feel tight in another’s. Always measure the mailer itself before assuming the printed label tells the whole story.
Common Poly Mailer Sizes and What They Fit
If you’d rather skip the maths, this chart covers the sizes most Australian sellers reach for and what they typically suit. Treat it as a starting point, then confirm the fit against your own product using the formula above.
| Size (cm) | Size (inches) | Typical Use |
|---|---|---|
| 17 x 23cm | ~7 x 9″ | Small accessories, jewellery, phone cases |
| 25 x 33cm | ~10 x 13″ | T-shirts, a single folded garment, small books |
| 30 x 39cm | ~12 x 15.5″ | Jumpers, hoodies, multiple folded items |
| 38 x 48cm | ~15 x 19″ | Jackets, bulkier apparel, small bundles |
| 48 x 61cm | ~19 x 24″ | Bedding, bulky soft goods, larger bundles |
If your product needs rigid protection rather than a flexible bag, our shipping box sizing guide runs through the same process for cardboard cartons instead.
Why Accurate Measurements Matter for Australian Shipping Costs
Australian carriers don’t just charge by weight. They use cubic weight, which is based on volume. Get the height field wrong on a poly mailer booking and the carrier’s system either undercharges you upfront, then corrects it with a fee later, or overcharges you on a parcel that’s actually flat and light.
Australia Post explains the mechanics in Australia Post’s cubic weight formula: multiply the parcel’s length, width and height in metres to get the volume, then multiply that figure by 250. If the result is higher than the parcel’s actual weight, you’re charged the cubic weight instead.
For poly mailers, this means the filled thickness you measured earlier isn’t optional information. It’s the number that decides whether an $8 mailer costs $8 or $14 to send. Check Australia Post’s parcel size guidelines before booking anything oversized or irregular.
Understanding Poly Mailer Thickness (Gauge and Mil)
Poly mailer thickness is usually described in mil, a US measurement equal to one thousandth of an inch, or in microns, the metric equivalent. A standard mailer sits around 2.5 to 3 mil, roughly 60 to 75 microns.
Thicker isn’t always better. It just means more puncture resistance for heavier or sharper items. Our packaging thickness guide breaks down how mil, gauge and microns relate if you’re comparing suppliers.
If you’re shipping anything with corners, buckles or hard edges, size up in thickness rather than just in dimensions. A bigger bag in a thin gauge will still tear.
When a Poly Mailer Isn’t the Right Fit
Poly mailers are built for soft, flexible items. Once a product has hard edges, sharp corners or needs to stay perfectly flat, a bag stops being the safer option.
For anything rigid, shipping boxes or purpose-built packing boxes give your product structural protection a flexible bag can’t. Flat, fragile items like screens and artwork need even more specific protection, our TV boxes and art boxes are built to stop exactly that kind of damage.
If you’re shipping something in between, not quite soft enough for a bag but too small for a full carton, our range of cardboard wraps and mailers covers books, records and other awkward shapes.
And once you’ve settled on packaging, don’t skip the finishing touches. Packing tape and labels keep a parcel sealed properly, and bubble wrap adds a layer of cushioning inside a mailer or box for anything with a bit more fragility.
Why Australian Sellers Choose The Boxman for Packaging
The Boxman manufactures and stocks packaging locally out of Perth, and ships custom cardboard boxes and stock cartons to sellers right across Australia, not just WA.
If your product has outgrown poly mailers altogether, or you’re building out a full packaging range, our self-locking mailing boxes are made to measure and give your parcels a sturdier, more presentable finish without the lead times of importing.
We also offer bulk importing alongside local manufacturing, so whether you need a handful of custom cartons or a full container of stock boxes, we can help you land on the option that suits your volume and budget.
Frequently Asked Questions
A few questions come up often enough once sellers start measuring their own mailers, so they’re worth answering directly.
How Big Is a 10×13 Poly Mailer?
A 10×13 inch poly mailer measures roughly 25 x 33cm as a flat, exterior size. Once you account for the seal flap and bottom seam, the usable space you’re actually packing into is closer to 24 x 31cm, enough for a single folded t-shirt or a small stack of documents.
What Can Fit in a 10×13 Poly Mailer?
A 10×13 mailer suits a single folded garment like a t-shirt or light jumper, small books, and bundled accessories. Anything bulkier, like a folded hoodie or multiple garments together, is better suited to the next size up to avoid straining the seams.
Is It Cheaper to Ship in a Poly Mailer?
Generally yes. Poly mailers are lighter than cardboard mailing boxes, which usually keeps them under a lower cubic weight and a cheaper postage bracket. For bulkier or higher-value items, the trade-off is presentation and protection, our poly mailers vs cardboard boxes comparison breaks down when the saving is worth it.
Does a Poly Mailer Count as a Large Envelope?
It depends on the carrier and how full the bag is. Most Australian carriers classify a parcel by thickness as much as size. Once a poly mailer is packed with anything more than a few millimetres of bulk, it’s generally treated as a parcel rather than a large letter, which changes the pricing bracket.
Can I Measure My Bag Online?
Some suppliers offer online sizing calculators where you enter your product’s dimensions and get a recommended mailer size back. They’re a useful starting point, but they can’t account for how your specific product folds or compresses, so it’s still worth confirming the fit against a physical mailer before ordering in bulk.
