Short answer: a large letter box (often called a PIP box) is a slim cardboard box designed so the packed item stays within Royal Mail's Large Letter format. Royal Mail's size and weight guide sets that format at no more than 35.3cm long, 25cm wide and 2.5cm thick, with a 750g weight limit (UK Tracked bought online has a 1kg limit). If your product, plus any wrapping, fits inside a 20mm-deep box and the finished parcel stays under the weight limit, a large letter box can let you post at the Large Letter rate rather than the Small Parcel rate, with more protection than an envelope.
This guide explains how the format works, what tends to fit and what does not, how to choose between the common sizes, how to judge whether a box is worth it compared with an envelope, and the checks that stop a large letter turning into a parcel at the post office or in a sorting centre.
Choose a large letter box size
Contents
- How the Large Letter format works
- Why use a box rather than an envelope?
- Large letter box sizes compared
- What fits, and what usually does not
- Is a large letter box worth it? A simple cost check
- Pre-posting checks
- How to pack a large letter box
- Working with large letter boxes at volume
- When to choose something else
- Questions sellers ask
How the Large Letter format works
Royal Mail prices UK mail partly by format: the physical size and weight of the item. The main formats for smaller items are Letter, Large Letter and Small Parcel. According to Royal Mail's guide at the time of writing:
| Format | Maximum length | Maximum width | Maximum thickness or depth | Maximum weight |
|---|---|---|---|---|
| Letter | 24cm | 16.5cm | 0.5cm | 100g |
| Large Letter | 35.3cm | 25cm | 2.5cm | 750g (1kg for UK Tracked bought online) |
| Small Parcel | 45cm | 35cm | 16cm | 2kg |
An item that exceeds any one of the Large Letter limits, whether length, width, thickness or weight, is no longer a Large Letter. Thickness is where most items fail. A product that is only slightly thicker than 2.5cm, or that bulges when packed, will be treated as a parcel.
Large letter boxes are often called PIP boxes, after "Pricing in Proportion", the name used for Royal Mail's size-based pricing. You will see both names used for the same type of product.
Rules and prices change, and other services such as tracked options can have their own limits. Before you design your packing around the format, check the current Royal Mail guidance for the specific service you buy.
Why use a box rather than an envelope?
Many items that fit the Large Letter size could go in a padded envelope instead. A box offers a few advantages:
- Rigidity. A cardboard box resists bending better than an envelope, which matters for prints, cards, flat accessories and anything with corners that should not be crushed.
- Consistent thickness. A box made to a fixed depth helps you stay within the 2.5cm limit, while a well-filled envelope can bulge.
- Presentation. A neat brown box often looks more considered than an envelope, which some brands prefer.
- Reuse. Customers can more easily reuse a box for returns.
An envelope has its own advantages: it is usually lighter, takes even less storage space, and suits items that are soft or naturally protected. Our separate guide comparing padded envelopes and postal boxes covers that decision in more detail. As a rule of thumb, choose a box when the item could be bent or crushed, and an envelope when it could not.
Large letter box sizes compared
Our Royal Mail PIP large letter boxes are die cut cardboard boxes, 20mm deep, supplied flat in packs of 50. The size names follow common envelope formats.
| Size | Listed dimensions (L x W x D) | Fits flat (examples) | Typical uses |
|---|---|---|---|
| Mini | 101 x 101 x 20mm | Small square items | Rings, earrings, badges, small cosmetics |
| C6 | 163 x 112 x 20mm | A6 card (105 x 148mm) | Greetings cards, A6 prints, phone accessories |
| DL | 218 x 108 x 20mm | Long, narrow items | Pens, watch straps, slim tools, gift vouchers |
| C5 | 229 x 159 x 20mm | A5 (148 x 210mm) | Slim books, notebooks, DVDs, A5 prints |
| C4 | 325 x 225 x 20mm | A4 (210 x 297mm) | Prints, documents, thin clothing |
All five listed sizes are within the Large Letter length and width limits, and the 20mm depth leaves a margin under the 2.5cm thickness limit. That margin is small, so anything that makes the closed box thicker, such as a bulging item or folded-over labels, can still take it over.
The dimensions above are those listed on our product page. The space inside a box is slightly smaller than its outside measurements because of the board thickness, so measure your item against the inside of a made-up box before committing to a size.
What fits, and what usually does not
The deciding factor is almost always thickness. Ask: does the item, plus any wrapping, comfortably fit inside 20mm without pushing the lid up?
Items that usually work
- Jewellery laid flat, in a slim pouch or card rather than a deep presentation box.
- Greetings cards, postcards and prints, which benefit from the rigidity of a box.
- Phone cases and screen protectors in slim retail packs.
- Thin paperbacks, notebooks and booklets, if the spine is under the internal depth.
- DVDs and CDs in slim cases. Royal Mail's own guide gives a cased CD or DVD as an example of a Large Letter item.
- Patches, badges, stickers, keyrings and small flat accessories.
- Thin clothing such as a lightweight T-shirt or scarf folded flat in a C4 box, if it compresses to the box depth without forcing the lid.
Items that often do not work
- Thick or chunky items such as knitwear, hardback books and most boxed cosmetics.
- Items in deep retail packaging. A ring in a hinged presentation box is often too deep, even if the ring itself would fit.
- Fragile items that need cushioning. There is little space for protective wrap in a 20mm box. A thin layer may fit; several layers usually will not.
- Heavy items. Dense items can exceed the weight limit even if they fit the size.
- Irregular shapes that create a bump at one point, because thickness is measured at the thickest point.
If your item does not fit, it is not a failure of the box. It simply belongs in a different format, usually a Small Parcel in a small box or padded envelope.
Is a large letter box worth it? A simple cost check
A large letter box usually costs more per unit than a padded envelope or mailing bag, so the question is whether the protection and format advantages pay for that difference. A simple way to check, for any product, is to compare the full cost of each option per order:
Cost per order = packaging unit cost + postage for the resulting format + (breakage or damage rate x cost of each damaged order)
The last part is the one most often left out. A damaged order typically costs you a replacement product, a second postage charge and the time spent dealing with the customer, and sometimes a lost repeat customer as well.
Illustrative example, using assumed figures rather than real prices: suppose a greetings card seller compares an envelope and a C6 large letter box. Both keep the card within the Large Letter format, so postage is the same. The box costs a few pence more per unit than the envelope. If, with envelopes, a small share of cards arrive bent and have to be replaced, multiply that share by the cost of a replacement order. If the result is larger than the extra few pence per box, the box is the cheaper option overall, even though it looks dearer on the price list. If cards almost never arrive damaged in envelopes, the envelope may remain the sensible choice.
The same reasoning works in the other direction. If an item only just fits the box and sometimes pushes the parcel over 2.5cm, the occasional surcharge or returned item may cancel out the saving from the Large Letter rate. In that case a small box or padded envelope posted as a Small Parcel can be more predictable.
Use your own figures: current packaging prices from the product pages, your carrier's current postage prices, and a realistic estimate of damage or bending rates from your order history.
Pre-posting checks
A few quick checks prevent the most common problem: a parcel you paid for as a Large Letter being treated as a Small Parcel because it is slightly too thick or heavy. Depending on how you post, that can mean surcharges, delays or the item being returned.
- Thickness: after closing the box, check it slides through a 25mm gap at its thickest point. Some sellers make a simple slot gauge from card or a strip of wood cut to 25mm, and test every parcel the first time they pack a new product.
- Length and width: confirm the box with its label is within 35.3 x 25cm. All our listed sizes are, but labels or tape hanging over an edge add to the outline.
- Weight: weigh the packed box. The limit is 750g for most Large Letter services, and 1kg for UK Tracked bought online, according to Royal Mail's guide.
- Service: make sure the postage you buy is for the Large Letter format and the service you intend.
- Label position: place the address label flat on the top face, not wrapped around an edge.
Do the check carefully the first time for each product. Once you know a product fits, you can spot-check rather than measuring every parcel, unless the product or its packaging changes.
How to pack a large letter box
- Make up the box. Large letter boxes are supplied flat and fold into shape along pre-creased lines. Fold firmly along each crease so the box sits square.
- Prepare the item. Place it in a slim inner sleeve or bag if needed. Our clear OPP bags start at 6.5 x 9" (165 x 230mm), which suits cards and A5 prints in the C5 and C4 boxes.
- Place the item flat. Lay it in the base so it does not slide. A sheet of card or paper can stop small items moving.
- Close and seal. Close the lid and seal it with a strip of tape along the opening edge. A narrow tape such as our 24mm clear packing tape is often enough for small boxes and keeps the parcel tidy.
- Check thickness. Make sure the closed box is flat and not pushed up by the item.
- Label and weigh. Apply the label to the top face and weigh the parcel.
If you print shipping labels, standard 4 x 6" labels (around 100 x 150mm) fit on the top of C5 and C4 boxes. For the smaller Mini, C6 and DL sizes, a 4 x 6" label may not fit flat; consider a smaller label or an A4 sheet label layout that produces a smaller address label. Our A4 sheet labels come in layouts from 1 to 65 per sheet.
Choose labels for small parcels
Working with large letter boxes at volume
If large letters make up a big share of your orders, a little organisation saves a lot of time.
Pre-fold a batch
Folding boxes one at a time between orders interrupts the packing rhythm. Many sellers fold a batch at the start of the day or during a quiet period, then stack the made-up boxes near the bench. Folded boxes take more space than flat ones, so only pre-fold what you will use in a day or two.
Keep sizes separate and labelled
The Mini, C6, DL, C5 and C4 sizes can look similar in a pile. Store each size in its own labelled shelf or tray, and keep a list of which products go in which size by the bench. That avoids the common mistake of using a larger box than needed, which wastes money, or a smaller one that will not close flat.
Reorder by usage
Our large letter boxes come in packs of 50. Work out how many of each size you use in a typical week and reorder before you get down to a few days' supply. If one size is used far more than the others, keep more of that size rather than equal stock of all five.
Store flat and dry
Store unused boxes flat, somewhere dry and away from direct heat, so the board stays flat and folds cleanly. Damp board is harder to fold neatly and weaker once made up.
When to choose something else
A large letter box is the right tool for slim, light, rigid-friendly items. For other products, these are usually better:
| Situation | Better option | Why |
|---|---|---|
| Item over 20mm thick | Crash lock boxes or small single wall boxes | More depth for the item and cushioning |
| Soft item that will not be damaged by bending | Mailing bags | Lighter and cheaper |
| Small item that needs light cushioning | Padded envelopes | Bubble lining built in |
| Flat paper items that must not bend | Board back envelopes | Card backing with a Do Not Bend warning, thinner than a box |
| Thicker books or boxed items | Capacity mailers | Rigid board with a gusset that expands up to 70mm |
Note that a capacity mailer expanded beyond 2.5cm is no longer a Large Letter, so it will usually be posted as a Small Parcel.
Limitations to keep in mind
- A 20mm box offers limited room for cushioning, so it is not suitable for most fragile items.
- Cardboard weakens when wet. A large letter box left in rain may soften; it is not a waterproof container.
- Large Letter rules are Royal Mail's. Other carriers may not offer an equivalent format or may price differently.
- Royal Mail can change its formats and prices. The figures in this article reflect its published guide at the time of writing.
On disposal, plain cardboard boxes can generally be recycled with household cardboard. Recycle Now's cardboard guidance asks for tape to be removed first.
Questions sellers ask
What is the maximum size for a Royal Mail Large Letter?
Royal Mail's guide gives 35.3cm long, 25cm wide and 2.5cm thick, with a 750g weight limit, or 1kg for UK Tracked bought online. Check the live guide before relying on these figures, as they can change.
Why are PIP boxes 20mm deep if the limit is 25mm?
The lower depth leaves a margin for the board, the closure, labels and any slight bulge, so the finished parcel is more likely to stay within 2.5cm. It does not guarantee it; a thick item can still push the box over.
Will an A4 item fit in a large letter box?
Our C4 size is listed at 325 x 225 x 20mm, which takes an A4 item (210 x 297mm) flat, as long as it is thin enough for the box depth.
Can I send a book in a large letter box?
Slim paperbacks and booklets often fit the C5 or C4 sizes. Most hardbacks and thicker paperbacks are too thick for 20mm and are better in a capacity mailer or small box.
Do large letter boxes need tape?
They are die cut and fold into shape, and a strip of tape across the lid edge keeps them securely closed in the post. Narrow tape is usually enough.
How many large letter boxes come in a pack?
Our PIP large letter boxes are sold in packs of 50, supplied flat.
Can I use a large letter box with couriers other than Royal Mail?
You can use the box with any carrier, but the price advantage depends on that carrier's own size bands. A slim box will often still be among the cheapest formats, but check each carrier's pricing.
Summary
If your product is slim, light and better protected by a rigid box than an envelope, a large letter box can keep it within Royal Mail's Large Letter format. Match the footprint (Mini, C6, DL, C5 or C4), keep the packed thickness well within 2.5cm, check the weight, and test each new product once before you rely on the format. Compare the full cost per order, including damage, against an envelope, and keep anything thicker or fragile in a different format.
