Mailing bag sizes: choose the right one with our interactive tool

Many issues with despatch start by choosing the wrong size mailing bag. Get your mailing bag sizes right and you cut damage claims, postage costs and the time your team spends repacking orders.

Use the tool below to easily match a size and format to what you’re sending.

What are the standard mailing bag sizes in the UK?

There’s no single standard size for mailing bags in the UK, but the formats settle into predictable ranges. RAJAPACK plastic mailing bags run from 216 x 356mm for single items up to 711 x 589mm for bulky, multi-item orders.

Sizes are quoted width first, then length, in millimetres. Postage bag sizes, parcel bag sizes and courier bag sizes refer to the same measurement, so the terms are interchangeable when you’re comparing suppliers.

Padded formats sit in tighter ranges because they’re built for smaller, more valuable items. RAJAPACK jiffy bags start at 90 x 145mm for jewellery and small electronics and reach 442 x 661mm in the kraft-lined Jiffy Green format.

How do you measure for the right mailing bag size?

Measure the item as it will be packed, not as it sits flat. Take the width of the packed item, add its depth, then add roughly 20mm for the seal lip.

That depth allowance is the part most people skip. A folded hoodie might measure 300mm across, but it’s also 80mm thick, so a 300mm bag won’t close over it without straining the seams.

The most common sizing mistake is getting the length right and the opening too narrow. Forcing a folded item through a tight opening puts the side seam under sideways tension, and that seam will split in transit even at the correct micron grade.

Check the internal opening width on the listing before you order. If you despatch garments, our guide to mailing bags for clothes breaks down the sizing by garment type.

How do mailing bag sizes affect your postage cost?

Royal Mail prices by dimensions as well as weight, so an oversized bag can cost you money on every order. A bag that adds unnecessary depth can move a Large Letter up into a parcel band.

Polythene and paper bags help here because they take the shape of the contents rather than adding a rigid frame. If you’re packing close to a threshold, check the packed item against the small parcel size limits before you commit to a bag size.

When do you need padding instead of a bigger bag?

Padding protects against impact and compression. It isn’t a size upgrade, and it costs more per unit while adding weight to every parcel.

Choose a padded format when a drop or compression could damage the item itself. That covers electronics, cosmetics, jewellery, books and mixed orders of soft and hard goods. Padded envelopes cover most of these and range from 90 x 145mm to 340 x 445mm.

For clothing, bedding and other soft goods, padding adds cost without adding protection. A correctly sized polythene or paper bag does the job for less.

Before you order

Sizing comes down to two numbers: the packed width plus depth of your item, and the internal opening width of the bag. Get those to match and you’ll have far fewer issues.

Order a sample before you commit to a pallet quantity. Pack a real order into it, seal it and check the seams aren’t under tension.

Find your mailing bag size

Not sure which size to commit to? Browse the full range of mailing bags and request a sample before you order in volume.

FAQ: Mailing bag sizing

How big is a standard shipping bag?

The most widely used size for single-garment despatch is around 250 x 350mm. Across the wider range, UK polythene mailing bags run from roughly 216 x 356mm up to 711 x 589mm.

What size mailing bag fits a shoe box?

A 350 x 450mm bag takes most adult shoe boxes, with 400 x 500mm needed for boot boxes. The bag adds waterproofing and tamper evidence; the box itself provides the rigidity.

What size mailing bag do you need for jeans?

Around 350 x 450mm, because folded denim is considerably thicker than a t-shirt or a lightweight top. Two pairs in one order need 400 x 500mm or larger.

Are courier bag sizes different from mailing bag sizes?

No, they’re the same product measured the same way. Courier bag, postage bag, parcel bag, poly mailer and mailing bag all describe a flexible despatch bag sized by width and length.

Should you size up if you’re between two sizes?

Size up, but only by one step. Excess material folds and catches in courier sorting equipment, and it can push the parcel into a higher postage band.

Are mailing bags recyclable?

Kraft paper mailing bags are kerbside recyclable with household paper and cardboard, with nothing to remove first. Polythene bags aren’t accepted in standard UK council kerbside collections and need a soft-plastic collection point.

For a fuller breakdown by format, see our guide to recycling envelopes and mailing bags.

How many mailing bag sizes should you stock?

Three sizes cover most e-commerce despatch profiles: one for single flat items, one for bulkier single items and one for multi-item orders. Add a fourth only when a product group sits well outside those three.

About the author

Mike Jeavons: Mike is an author, copywriter and content creator, with over 15 years of experience telling stories and helping to improve the customer experience.

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