What Size Mailing Bag Do I Need?

Getting the size right is one of those things that sounds obvious until you're standing with a coat that won't fit in the bag you ordered, or a tiny t-shirt swimming around in something three times too large. If you're selling clothes on Vinted, Depop or eBay, or just shipping the odd parcel, this guide will help you match the bag to what you're sending every time.

Why bag size actually matters

A bag that's too small won't seal properly, which risks the contents falling out or getting damaged in transit. A bag that's far too large looks unprofessional, gives the item room to shift around, and can actually attract more handling damage because postal workers tend to fold or scrunch oversized packaging to fit it in sorting systems.

Beyond the practical side, getting the size right keeps your postage costs down. Royal Mail and other UK couriers calculate pricing partly on package dimensions, so an unnecessarily bulky parcel can push you into a higher price bracket for no reason.

How mailing bag sizes work

Mailing bags are sized by their internal dimensions, listed as width x length. The internal measurement is what matters because that's the space your item actually has. Some suppliers list external dimensions, so it's worth double-checking which measurement you're looking at before you buy. At Mr Bags, all sizing on our mailing bags is listed as the internal usable dimension.

A good rule of thumb is to add roughly 50mm to each dimension of the folded item you're shipping. That gives you enough room to get the item in easily and seal the bag flat without the contents pushing against the seal.

What size mailing bag for clothes?

This is the question we hear most often, especially from sellers on Vinted and Depop who are new to shipping. The answer depends on the garment, how it's folded, and whether you're sending it alone or with other items.

Here's a practical guide to the most common clothing sizes:

T-shirts and light tops

A standard adult t-shirt folded flat typically measures around 200mm x 280mm. Add your margin and a small mailing bag at around 250mm x 350mm covers this comfortably. If you're asking what size mailing bags for t-shirts specifically, this is the size to go for. It also works for vest tops, light blouses, polo shirts and similar single lightweight garments.

Children's t-shirts and babywear will fit smaller still, and you can often use a bag in the 170mm x 230mm to 200mm x 300mm range for those.

Shirts, light knitwear and mid-weight tops

A folded shirt or lightweight knit sits somewhere between 230mm x 320mm and 260mm x 360mm depending on the size of the garment. A mailing bag at 305mm x 406mm or 350mm x 450mm covers this well and leaves enough room to seal flat.

Jeans and trousers

Folded jeans are denser than they look. A typical pair folds to around 300mm x 400mm. Use a bag at 350mm x 500mm to give yourself room to work with. Wider-leg trousers and cargo styles may need slightly more width, so if in doubt go to 400mm x 500mm.

Hoodies, sweatshirts and heavier knitwear

These are bulkier and need more space. A folded hoodie typically measures around 330mm x 420mm once you've pressed the air out, but the extra thickness means a tighter bag will struggle to seal. A 400mm x 500mm or 430mm x 560mm bag is the safe choice for single heavier garments.

Dresses and jumpsuits

These vary a lot by style. A short casual dress folds down to a similar size as a t-shirt. A maxi dress or jumpsuit is closer to the hoodie range. If there's any doubt, fold the garment first and measure it before choosing a bag. Most sellers find a 350mm x 500mm bag handles the majority of dresses without issue.

Coats and jackets

This is where you move into large shipping bags territory. A folded coat or padded jacket rarely gets below 400mm x 550mm, and a larger winter coat can easily sit at 450mm x 600mm or beyond. These are the items where buying the right size upfront matters most, because squeezing a coat into an undersized bag almost always ends with a broken seal or a split seam.

For large and heavy outerwear, our large mailing bags range covers the sizes most clothing sellers need.

Multiple items in one parcel

If you're sending more than one garment in a single bag, fold each item individually first, then measure the combined stack before choosing a size. It's usually better to go one size up than to cram items in and risk the seal failing.

What size postage bags for clothes sent via Royal Mail?

Royal Mail's large letter and small parcel formats have specific dimension limits that are worth knowing if you're trying to keep postage costs low. A large letter must not exceed 353mm x 250mm x 25mm. For anything thicker or larger, you're into small parcel pricing.

For clothing specifically, most single garments sent via Royal Mail will fall into the small parcel category once bagged, as the thickness of folded fabric typically pushes past the 25mm large letter limit. A mailing bag does not add meaningful dimensions to this, so the sizing of the garment itself is the deciding factor.

If you're sending on Vinted, the platform generates prepaid labels based on the weight and size you declare. Getting the sizing right when listing means you won't be caught out at the post office with a bag that doesn't match the label you've already printed.

How to measure poly bags for shipping

If you're buying poly bags or plastic shipping bags for clothing and want to measure them accurately, here's how to do it properly.

Lay the bag flat on a clean surface. Measure the width from the inner edge of one side seam to the inner edge of the other. Measure the length from the top of the bag opening down to the base, excluding the seal flap. These are your internal dimensions, and they're what tells you how much space the bag actually gives you for the item inside.

Some bags are also sold with a gusset, which adds depth for bulkier items. If the bag has a gusset, measure its depth too, as this affects how much the bag can expand when packed.

A common mistake is measuring the outside of the bag and then finding the item is a tighter fit than expected. Always measure inside, or buy from a supplier that clearly states internal dimensions.

What are shipping bags called?

Worth covering briefly because it causes confusion when people are shopping around. Shipping bags, mailing bags, postage bags, poly mailers and parcel bags are all names for the same product. The terminology varies depending on who you're buying from and where. On our site you'll find everything listed under mailing bags, which is the most commonly used UK trade term. Whatever you call them, the sizing principles in this guide apply across all of them.

A quick reference guide by item type

To make this easy to refer back to, here's a condensed size guide for common clothing items:

Tips for getting it right consistently

If you're selling clothes regularly, a few habits will save you a lot of time and wasted bags. Keep a small stock of two or three sizes that cover most of what you send, rather than trying to have a perfect size for every scenario. Most clothing sellers find that a small, medium and large option covers about 90 per cent of what they ship.

Fold items consistently before bagging them. The more consistently you fold, the more predictably items fit. It sounds like a small thing but it makes a real difference when you're packing five or ten parcels at a time.

If you're buying bags in bulk for the first time, order a sample pack or a smaller quantity across a few sizes before committing to large volumes. This lets you test fit before you're sitting on 500 bags that are slightly too narrow.

You can browse our full range of sizes and find the right bags for your most common items on our mailing bags page. If you're unsure which size works best for what you're shipping, get in touch and we can point you in the right direction.