Stationery & office
Poly mailing bag size guide: which bag fits which parcel
Buying mailing bags is guesswork the first time. The sizes look close together on screen, the measurements do not obviously match anything you own, and ordering the wrong one means either a parcel that will not close or a bag with half of it flapping loose.
This guide translates the numbers into things you can picture, and explains the one dimension that quietly decides what your postage costs.
How mailing bag sizes are measured
A poly mailer is measured flat, width first, then height, and the height is measured to the bottom of the seal strip rather than the top of the flap. So a bag sold as 25 x 35cm gives you about 25cm of usable width and about 35cm of usable height once the flap is folded over.
That matters because your item needs a little slack, not an exact fit. As a rule of thumb, allow 2 to 3cm more than the flat size of what you are posting. A bag that is exactly the size of the contents will strain at the seams the moment the item has any thickness at all.
Thickness is the part people forget. A bag has no gusset, so anything bulky eats into both the width and the height as the plastic wraps around it. A thin item can fill a bag corner to corner; a thick one needs a size up.
What fits in each size
17 x 30cm
The small everyday bag. Phone cases, jewellery, cosmetics, single books, socks and underwear, small electrical accessories, anything that would otherwise go in a padded envelope.
20 x 30cm
The same height with more width, which suits paperbacks, A5 items, small boxed goods and anything squarish rather than long.
25 x 35cm
The most useful general size. A folded t-shirt, a pair of leggings, a small toy, an A4 document wallet, most single clothing items.
28 x 42cm
A step up for bulkier clothing: a jumper, a pair of jeans, a couple of t-shirts together, or a small soft homeware item like a cushion cover.
32 x 45cm
Multi-item clothing orders, a hoodie, bedding for a single bed, curtains folded down, or several smaller items shipped together.
35 x 45cm
Bulky soft goods: winter coats, thick knitwear, larger bedding, or a bundle of several garments.
40 x 55cm
The large bag. Duvet covers, heavy coats, multiple-garment orders, soft furnishings and anything where the volume matters more than the weight.
The dimension that decides your postage
For UK sellers, the difference between a Large Letter and a Small Parcel is mostly about depth, not length or width. Large Letter allows only a couple of centimetres of thickness, which is why a flat-packed item can go one way and the same item loosely folded goes the other, at a noticeably higher price.
So the habit worth building is not choosing a smaller bag, it is packing flatter. Fold clothing to the width of the bag rather than stuffing it, press the air out before sealing, and keep the seal flat instead of rolling it. Bags are flexible, and a well-folded item in a slightly larger bag is often thinner than a crammed item in a small one.
Royal Mail publishes the current maximum sizes and weights for each format, and they do change from time to time, so check their size guide rather than working from a number you remember. If you post regularly, a cheap size gauge on the packing bench pays for itself quickly.
Choosing the bag itself
Beyond size, three things separate a bag that arrives intact from one that does not.
Opacity. A properly opaque bag hides what is inside. That is a privacy point for the customer and a security point for you.
The seal. A good peel-and-seal strip closes in one pass and does not need tape over it. If you find yourself taping every bag shut, the bags are the problem.
Waterproofing. Polythene keeps rain out, which is the main reason to choose it over a paper envelope for anything fabric. Just make sure the address label is either under the plastic or covered with clear tape.
For small items that need to stay visible or sorted, such as parts, samples or components, a resealable clear bag inside the mailer is often more useful than a second mailer.
What to buy
- 17 x 30cm poly mail bags - small items, accessories, single books.
- 25 x 35cm poly mail bags - the general-purpose size for single clothing items.
- 32 x 45cm poly mail bags - multi-item orders and bulkier clothing.
- 40 x 55cm poly mail bags - duvet covers, coats and soft furnishings.
- Resealable clear zip bags - for sorting small parts inside the mailer.
The full range is in postal bags. If you also seal boxes, our packing tape guide covers which tape to use for what.
Frequently asked questions
Should I size up or size down if I am between sizes?
Size up. A bag that is slightly too big can be folded and sealed flat; a bag that is slightly too small will split at the seam or force you to tape it, and taped-over seals are the ones that come open in transit.
Do mailing bags need an outer label pouch?
Not for most sellers. A printed label stuck straight onto the bag is fine as long as the surface is smooth and dry. A strip of clear tape over the label protects the ink from rain.
Can I reuse a mailing bag?
Only if it has a second seal strip, which some bags include for returns. Otherwise the original seal is single-use, and taping a used bag shut is not reliable enough for a paid delivery.