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The Ultimate Guide to Postal Packaging

Written by William McAllister

Whether you’re locking down the most efficient Royal Mail postage sizes for your business or switching to more environmentally friendly materials, the decisions you make about postal packaging matter. It’s not just a question of cost-effectiveness or convenience, either – although those are certainly serious considerations. Just as importantly, the packaging you choose for your posted items speaks volumes about your business.

A Simple Guide to Royal Mail Postal Sizes

While you’ve got a range of options for handling postage, the chances are you’re still going to be dealing with Royal Mail a lot of the time. Understanding and picking the right Royal Mail postal sizes for your business needs can be crucial, so you’ll need to make sure your packaging hits the mark.

The basic Royal Mail postal sizing rules look like this:


Maximum of 100g, 24cm x 16.5cm x 5mm.
Pretty basic stuff. The bread-and-butter of UK postage.

Large Letter

Maximum of 750G, 35.3cm x 25cm x 2.5cm.
At this band, you might be looking at larger documents, heavier items like greeting cards or even non-document items like DVDs. Depending on what you’re sending, you might be in need of more durable or protective packaging.

Small Parcel

Maximum of 2kg, 45cm x 35cm x 16cm.
At the Small Parcel level, you’re probably posting things like larger books, electronic gadgets or other similarly sized products. You need to think about protection here, whether that means padded envelopes, Jiffy bags or boxes.

Medium Parcel

Maximum of 20kg, 61cm x 46cm x 46cm.
The packaging you opt for here needs to be carefully matched to the items being sent. Standard mailing bags certainly remain an option, but it’s worth considering more durable packaging materials. Also, give some thought to how you’re packing your items. Filling out a box with “void fill” materials like packing chips or bubble wrap will go a long way toward ensuring a safe journey.

Large Parcel

Maximum of 30kg, 1.5m x a combined length and depth of 3m.
With items of this size, you’d be wise to think about double-walled, reinforced boxes and specialised void fill options. Air cushion bags and “suspension” packaging (where the item is held clear of the walls of the box in between layers of tough, non-slip film) can all be solid options, depending on your needs.


Maximum greatest dimension under 90cm, length plus twice diameter under 104cm.
Postal tubes can be a great choice for longer or “rollable” items. Tubes are typically sturdy and durable, both due to the materials used and the natural resilience of their cylindrical shape.

How to Make Greener Packaging Choices

Eco-friendly packaging materials and design have come a long way in recent years – and it makes good business sense to push forward in a more environmentally sound direction. A healthier environment benefits everyone, naturally, and embracing recyclables in your packaging decisions is a strong message to send out with your items. We’ve also suggested 6 easy swaps for eco packaging harmony.

Jiffy Green Eco Padded Envelopes

One important thing to take onboard is that there’s more to ecologically friendly packaging than switching materials away from plastics. There’s also the sheer quantity of materials we’re using to consider. When it comes to waste, controlling the amount we’re producing is every bit as important as changing the type.

That doesn’t even necessarily start and end with matching the packaging to the finished product, either. In many cases, managing your packaging needs can be considered as far back as the original design phase. Optimised packaging uses less space, requires fewer packing materials and typically weighs less. Optimally packaged items obviously tend to cost less to post – but they also tend to require less storage space, both in your own premises and your postal carrier’s.

Optimal Size Cardboard Boxes

Looking at the actual materials on offer, there’s a lot more to environmentally friendly packaging than biodegradable cardboard – and it’s important to realise that choosing recyclable and biodegradable materials doesn’t mean compromising on protection. Depending on your business and specific needs, you’ve got a surprising range of sustainable packaging options available. Here are a few examples:

Padded Envelopes & Bags

Traditional bubble-lined envelopes can often be tricky to recycle (although some can certainly be reused instead of discarded), but there are other options available that offer just as much durability and protection. Envelopes using paper-fibre padding are a strong contender in terms of sturdiness.

Plastic Free Padded Envelopes

Obviously, if you’re posting something extremely brittle or susceptible to crushing damage, you’re better off looking for some less compressible packaging. However, for a wide range of posted items, a padded bag can be an extremely effective, highly sustainable selection. It’s worth noting that paper may not be the only game in town, either.

Certain types of bubble-padded “poly” envelopes are actually fully recyclable, depending on the materials used. Check the manufacturer’s listings to be sure you know what you’re buying and how easily it can be reused or recycled.

Postal Boxes & Book Packs

Book mailing boxes have become a very popular postal choice, and it’s not difficult to understand why. They offer great protection, add little to the weight of your items and are easily recycled after use. They also typically come in flat-pack form, so they’re easy to store until needed.

Eco Book Packaging

Even larger cardboard postal boxes are simple to assemble, with pre-creased folds to guide you and often featuring cleaver design features like “locking” style lids. Again, many of these boxes are sturdy enough to survive postage in good enough condition to be used more than once. Failing that, recycling generally poses no problems.

Mailing Bags

When we think about environmentally friendly packaging materials, plastic bags don’t spring readily to mind. However, if you need a serious degree of water protection for your mailed items, there are times when paper or card simply won’t be enough. As mentioned above, certain types of polythene packaging can actually be made biodegradable through use of an additive during manufacture.

Mailing Bags

Depending on what you’re posting, you might pick a clear document bag insert for a traditional envelope or a heavy-duty, fully opaque polythene mailing product with a self-sealing flap and tamper-evident features. The point is that going “green” doesn’t necessarily narrow down your options – nor does it risk compromising your commitment to quality.

The Beauty of Bespoke Packaging

If the endless rise of online “unboxing” videos demonstrates anything, it’s the undeniable fact that the experience of opening a package matters to the recipient. A posted item that’s clumsily packed in one-size (or one-style) fits-all fashion impresses no one. Worse, it can actually damage the way the contents are perceived – if not the contents themselves.

Bespoke, creative packaging puts you in control of the way your customers and mailed items meet. Ideally, you want a love-at-first-sight experience every time. That means no clumsy, difficult fumbling to get the packaging open, no worrying rattling around of the contents in transit and a strongly forged link between the ease and aesthetics of the experience and your own brand or business.

Beverages Bespoke Cardboard Box

Balancing these requirements against the more mundane, pragmatic concerns of security and protection can be tricky – which is all the more reason to go the bespoke route. Depending on your packaging supplier, there may already be easy options available to make your mark on the unboxing experience without compromising the postal practicalities.

Bespoke, made-to-measure options also come with the added benefit of reducing wasted space and materials. Properly used, these options can be powerful marketing tool in their own right. It might sound like magical thinking, but it’s a simple fact that the moment the box is opened is your final opportunity to frame the customer’s encounter with the product they’ve received.

Sportswear Bespoke Postal Box

That moment will matter whether you decide to take advantage of the opportunity it presents or not. Missing out on the chance to engage in this initial encounter with some creative packaging could be a significant waste of potential, though. Even something as simple as a welcoming message on the interior of the package can greatly affect the customer’s experience.

You don’t have to reinvent the wheel here. Simply ensuring that your creative packaging is well fitted, clearly durable enough to protect the contents and nicely presented is enough. If the unboxing process leaves the recipient knee-deep in packing chips and digging through shredded cardboard to find the product itself, a little more thought might be required. Beyond that, though, a few smart decisions made on design and presentation can go a long way toward making the right kind of impression.

Naturally, these decisions will tend to cost money – and exactly how much they’re worth will be a question you’ll have to answer for yourself. Keep in mind that even fully bespoke packaging solutions may not be as expensive in the long run as they seem at first. Once you’ve pinned down a design with your supplier you may well find the costs dropping in subsequent print runs, since the original development work has already been paid for.

Bespoke Marmite Cardboard Box

Choosing the Right Envelope

There’s a lot more to picking the right envelope than understanding the Royal Mail postage sizes available. Pocket or wallet style, expandable gusset or board backed – these can be important decisions to make, whether you’re going the bespoke route or not.

The three most popular sizes of envelope are the C4 (324mmx229mm), the C5 (162mmx229mm) and the DL (110mmx220mm). Pocket styles have their sealing flaps along a short edge, while wallet styles have theirs along the long edge. For the seals themselves, your options range from standard “lick and stick” gummed strips to peel and seal release tape and self sealing adhesive strips.

You’re basically looking at the practicalities and convenience of sealing the envelopes with these options, rather than the customer’s opening experience – which probably won’t be affected much whichever you go for. The more envelopes you’re expecting to seal in a day, the less appealing a lick and stick option is likely to sound.

Printed Do Not Bend Cardboard Envelopes

Another thing you’ll want to look at is whether or not your posted items would benefit from a windowed envelope. If you’re sending out a lot of business mail with recipients’ addresses printed on it, you could stand to save yourself a significant amount of time and cash by opting for windows instead of printing the envelopes individually. Obviously, you’ve got to match the window dimensions and position with the documents inside to make this work.

As for the envelopes themselves, you’ll always need to think hard about the condition your posted items will arrive in. Heavier items like catalogues won’t survive a postal journey nearly so well in a standard envelope than in one with an expandable gusset, for example. Obviously, you’ve got to keep the Royal Mail postage dimensions in mind when making these decisions.

ColomPac, board backed and bubble lined envelopes are also good options for protecting their contents. Board backs are great for documents that need to stay unfolded or free of creases and dog-eared corners. Padded and bubble lined envelopes are surprisingly lightweight for the impact protection they offer, but obviously won’t generally offer as much resistance to creases and bends.

If security’s a serious concern, there’s a range of specialised envelopes that can offer durable protection from tampering or burst and tear damage. These are typically used for critical items and confidential legal documents. Materials like tear-resistant Tyvek and tamper-evident polythene give you a valuable range of security options.

Colompac Rigid Envelopes

Speaking of polythene, if moisture and water damage is a worry, you’d be well advised to look into these kinds of materials. They’re lightweight and watertight – and surprisingly often are biodegradable or easily recyclable for an environmentally friendly option.

Buying the Right Book Packaging

When you’re making decisions about book packaging, there’s more to consider beyond the obvious concerns of manageable weight and appropriate durability. A packaging design built for display can be a world apart from one made for heavy-duty protection. Depending on your needs and budget, you could be looking at anything from a humble Jiffy bag to a bespoke book box with personalised designs and features.

Once again, you’ll need to keep one eye on the Royal Mail postage parcel size guide. Being generally quite regular in shape, you may not end up with a lot of excess space to pad out in any individual box, assuming it’s well matched to its contents. However, if you find yourself posting out a lot of books of varying sizes, you might be better off picking a single box type and “void filling” it where necessary.

Self Adhesive Book Wrap

Cardboard book boxes will usually arrive packed flat, which is easy on storage space but naturally means spending some time on assembly. Depending on the design you land on you might have some features to consider, like adhesive strips that avoid the hassle of individually taping boxes closed. Again, it’s worth thinking about the unboxing experience from the customer’s perspective. A tear-off strip that instantly unfolds the package can make for a much happier first encounter with the product than a box that takes a knife to open and risks damaging the contents.

Specialised “bookwrap” (or even “bukwrap”) boxes can be a great option here, with a wide range of creative design options, easy opening and minimal waste in a durable package. Bookwrap packages are arguably faster to pack than other folding box types, are specifically designed to safeguard common sizes of book and can get the very most out of the Royal Mail’s Large Letter size category.

C3 Book Wrap

Beyond cardboard boxes and bookwraps, there’s always the tried-and-tested protective envelope. The range of available Jiffy bag sizes, for example, is pretty impressive. It’s a classic, world-recognised design for a reason, with basic but reliable protection from most types of damage a book could reasonably be expected to suffer. Not as sturdy as a box or specialised bookwrap packaging, but still a solid option.

While we’re talking about damage, books absolutely hate water. Cardboard boxes and padded envelopes can go a lot way toward protecting against drops and scuffs, but tend to be pretty helpless against moisture. If that’s a worry, you don’t necessarily need to ditch the cardboard option altogether. Plastic zip wallets or simple sealable polythene carriers can offer near-total security from water, can be combined with a cardboard box option and won’t add significantly to the weight of the overall parcel.

FEFCO box codes explained

Not every cardboard box is a purpose-built mailing box, so when words like “FEFCO” are being slung around, you know someone’s taking their box packaging design seriously. FEFCO codes come from the European Federation of Corrugated Board Manufacturers, and once you learn their secret language you’ve got a one-glance idea of the type of box each code refers to.

The codes have four digits, with the first two signalling a general category of box and the second two narrowing down to a specific style. FEFCO codes don’t describe the actual size of the boxes they describe, just the type and design. Here are the basics on some of the most popular and practical styles:

0201, 0203.

“Slotted” boxes. Basically a single piece, glued, taped or stitched with flaps for the top and bottom. You’ll generally be taping these together. Shipped flat and ready for use.


These are “telescope” boxes, with a separate outer section that slides over a slightly smaller inner body.

0409, 0411, 0421, 0422, 0426, 0427.

Usually one-piece designs with a hinged bottom that folds to form sides and cover. Depending on the specific design, you might find locking tabs, handles or other features built in. Folder-type boxes and trays usually consist of only one piece of board.

0914, 0933.

These are “interior” designs generally intended to be used as partitions, liners, dividers and so on. They can feature a range of panel and compartment configurations, and may be either separate or part of an overall case design.

General Packaging Advice

  • Cost Effectiveness – Yes, basic postage costs are a huge part of this calculation – but there’s more to it than that. While you’re working on keeping your package weights and dimensions within the Royal Mail small parcel size limits, think about the space you’re giving over to those boxes in storage. Are you getting the best use of your available space? Are you spending a lot of extra money filling out boxes with packing materials because you’re using a sub-optimal size or style?

  • Protection – Getting to point B is no use if you don’t get there safely. Different types of mailed items will have different safety requirements. You can shield your items and documents against most types of hazard, from tearing and impact to deliberate tampering. Even if the actual risks are small, the fact that you safeguarded your items in transit speaks volumes to a customer.

  • Sustainability – We’re all trying to live and work a little greener these days. Choosing environmentally friendly packaging doesn’t mean lowering any of your standards for presentation or protection. You’re not just building your reputation by using recyclables; you’re helping to build a better future.

  • Customer Experience – The arrival and opening of your package is your final chance to frame your customer’s first encounter with your posted item. If you’re not taking charge of that moment, you’re missing out on an enormous opportunity. Bespoke, creative packaging and simplicity of use make a powerful statement, but thoughtlessness and lack of care speak just as loudly. 
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