Choosing the right mailing bag size comes down to matching your product to Royal Mail's postal formats: keep a packed item within 353mm x 250mm and under 25mm thick to post it as a Large Letter, and step up to a small parcel bag when your item is bulkier. Picking the correct size protects your product, keeps postage costs down, and gives customers a tidy, professional unboxing. This guide breaks down the standard mailing bag sizes and the Royal Mail bands they map to, so you can despatch with confidence.
Why mailing bag size matters
Mailing bag size is not just about whether your product fits. It directly affects how much you pay to post each order and how your packaging looks when it lands on a doorstep. A bag that is too big wastes material and can rattle in transit, while a bag that is too small strains at the seams and risks splitting. Getting the fit right is one of the simplest ways to trim despatch costs across hundreds or thousands of orders.
The biggest saving comes from staying inside the right Royal Mail band. The jump from a Large Letter to a small parcel can roughly double your postage for an item that is only a few millimetres too thick, so the thickness of your packed bag is just as important as its length and width. If you sell flat, lightweight items such as clothing, this single detail can make a meaningful difference to your margins.
Royal Mail postal formats explained
Royal Mail sorts everything you send into a handful of size and weight bands. Knowing these bands is the foundation for choosing a mailing bag, because the packed bag, not the empty one, has to meet the limits. The current formats most e-commerce sellers use are:
- Letter: up to 240mm x 165mm x 5mm and no heavier than 100g. Too thin for most products, but fine for cards and flat documents.
- Large Letter: up to 353mm x 250mm x 25mm and no heavier than 750g. The sweet spot for clothing, accessories and slim items in a mailing bag. The 25mm thickness limit is the one people miss most often.
- Small parcel: up to 450mm x 350mm x 160mm and no heavier than 2kg. Ideal for bulkier garments, boxed items and padded packages.
- Medium parcel: up to 610mm x 460mm x 460mm and up to 20kg. For larger or multi-item orders that will not flatten down.
You can confirm the latest limits on the Royal Mail size and weight guide before you commit to a bag size, as bands and prices are reviewed periodically.
Common mailing bag sizes and what they suit
Mailing bags are usually described by their sealed dimensions, meaning the usable space once the self-seal strip is closed. As a rough guide, here are popular sizes and the products they tend to suit:
- Small (around 165mm x 230mm): jewellery, cosmetics, phone accessories and single small garments such as socks or underwear.
- Medium (around 250mm x 350mm): folded t-shirts, kidswear and A4 items. This size often keeps a lightweight order within the Large Letter band.
- Large (around 350mm x 450mm): hoodies, jumpers, homeware and multi-item clothing orders that push into small parcel territory.
- Extra large (around 425mm x 600mm): bedding, bulky knitwear, coats and larger soft goods.
Because sizing conventions vary between suppliers, always check the stated internal dimensions rather than relying on labels like "medium" alone. When you order custom printed paper mailing bags, the measurements are listed clearly so you can match them to your typical order.
How to measure and choose the right size
The most reliable approach is to measure your product as it will be packed, not as it sits on a shelf. Fold or roll the item the way you would for despatch, then measure its length, width and depth. Add a little tolerance so the bag is easy to fill and seal without straining, but avoid so much slack that the contents shift around.
A quick method that works for most sellers:
- Pack a typical order and measure the finished bundle at its thickest point.
- Check that measurement against the Royal Mail bands above to find your target format.
- Choose the smallest bag that comfortably holds the item while staying inside that band.
- Order a sample pack to test fit and feel before committing to a large run.
If you sell a range of products, it usually pays to standardise on two or three sizes rather than a different bag for every item. This keeps stock simple and helps your branding stay consistent, a point we cover in our design tips for custom printed packaging.
Do not forget the material
Size is only half the decision. The material affects protection, weight and your environmental footprint. Paper mailing bags are widely recyclable at the kerbside and increasingly expected by shoppers, while padded options add protection for fragile goods. If you are weighing up materials and the rules around them, our guide to paper versus plastic carrier bags explains the wider context. Lightweight paper mailing bags also help keep you within a lower weight band, which can support cheaper postage.
Frequently asked questions
What size mailing bag counts as a Large Letter?
A packed mailing bag qualifies as a Royal Mail Large Letter if it is no larger than 353mm x 250mm x 25mm and weighs no more than 750g. The 25mm thickness limit is the one that catches people out, so measure your bag at its fullest point. Anything over these limits moves into the small parcel band, which usually costs more.
How do I stop a mailing bag pushing my order into a higher postage band?
Fold or roll your item so the packed bag stays as flat as possible, and choose a bag only slightly larger than the contents. Excess material adds thickness and can tip a Large Letter into a small parcel. Weighing and measuring a sample order before you buy in bulk is the surest way to avoid surprises.
Are paper mailing bags strong enough for clothing?
Yes. Modern paper mailing bags are designed with tear-resistant layers and strong self-seal strips that hold up well for garments and soft goods. For heavier or fragile items you may want a padded option, but for most clothing orders a quality paper mailing bag offers plenty of protection while remaining easy to recycle.
Can I get mailing bags printed with my branding?
You can. Custom printed mailing bags let you add your logo, colours and messaging so every despatch reinforces your brand. Branded packaging turns a routine delivery into a memorable unboxing moment and helps your business look polished and professional. Fast Printed Bags offers custom printing across a range of mailing bag sizes.
What is the difference between a mailing bag and a carrier bag?
A mailing bag is a sealed postal bag designed to protect goods in transit through the post, with a self-seal closure and often an opaque finish for privacy. A carrier bag is an open-handled bag for carrying purchases from a shop. They serve different jobs, so many retailers use both depending on whether they are shipping online orders or serving customers in person.
Ready to despatch in the right size?
Getting your mailing bag size right saves money, protects your products and leaves a better impression with every order. Explore the full range of custom printed paper mailing bags at Fast Printed Bags, or order a sample pack to check the fit before you commit. Whatever you sell, we will help you post it in packaging that works as hard as you do.