If you're shipping orders through Royal Mail, the size of your packaging isn't just a practical consideration — it directly determines how much you pay. The difference between a large letter and a small parcel can be well over £2 per item, and across hundreds or thousands of orders a month, that adds up fast.
This guide covers every Royal Mail size format, the exact dimensions you need to stay within, and the packaging choices that let you ship at the lowest possible rate for your product.
What does PiP mean?
PiP stands for Parcels in Packets — Royal Mail's term for the size-based pricing system that determines which postage rate applies to your item. The format your parcel falls into is determined by its dimensions and weight, not by which service you're using (first class, second class, tracked, etc.).
Getting your packaging right means more of your orders qualify for cheaper formats. Getting it wrong — even by a few millimetres — means paying a higher rate for every single one.
Royal Mail size formats: the full breakdown
Royal Mail uses four main size categories for domestic parcels:
Letter
- Max dimensions: 240 × 165 × 5mm
- Max weight: 100g
Standard letters. Not relevant for most ecommerce products, but worth knowing if you send documents, vouchers, or very flat items.
Large Letter
- Max dimensions: 353 × 250 × 25mm
- Max weight: 750g
This is the sweet spot for many ecommerce sellers. If your product fits within these dimensions — particularly that 25mm depth limit — you can ship it as a large letter, which is significantly cheaper than a small parcel. Clothing, jewellery, accessories, flat books, card games, prints, and many health and beauty products can all be shipped as large letters with the right packaging.
Small Parcel
- Max dimensions: 450 × 350 × 160mm
- Max weight: 2kg
The most common format for ecommerce orders. Products that are too bulky for a large letter but don't exceed these dimensions and 2kg fall into small parcel pricing.
Medium Parcel
- Max dimensions: 610 × 460 × 460mm
- Max weight: 20kg
For heavier or larger orders. Medium parcel rates are noticeably higher, so it's worth checking whether your packaging can be made more compact if orders are frequently landing here.
The dimension that trips people up most: depth
Most sellers think about length and width when choosing packaging, but it's the depth (or height) that most often pushes a parcel into a higher price bracket.
The 25mm depth limit for large letters is strict — and that's the external measurement. If your box has 3–4mm of board thickness on each side, you're working with roughly 17–18mm of internal space. That's tight, but workable for flat products like clothing, accessories, or thin books if you choose the right box.
Similarly, the 160mm depth limit for small parcels catches out sellers using unnecessarily tall boxes. A box that's 250mm deep when 100mm would do isn't just costing you void fill — it could be bumping you into medium parcel pricing.
How to use PiP boxes to minimise postage costs
PiP postal boxes are boxes designed specifically to maximise the usable space within each Royal Mail size format, right up to — but not exceeding — the dimensional limits.
Our PiP postal boxes are sized to fit cleanly within Royal Mail's format boundaries, so you get the most out of each size category without accidentally tipping over into the next one.
The key benefits of using purpose-made PiP boxes:
- Confidence in your format. You know exactly which size bracket you're in, with no guesswork.
- Less void fill. A well-fitting box means less movement and less material needed to protect the product.
- Faster packing. When your box size is matched to your product, packing is quicker and more consistent.
If you're currently using general-purpose boxes and paying small parcel rates, it's worth checking whether a proper large letter box could bring some or all of your products into the cheaper format.
What about cardboard envelopes?
For completely flat items — prints, documents, card-based products, thin books — a rigid cardboard envelope is often a better choice than a box. They're lighter, which helps with the weight limit, and they have a very low profile, making it easier to stay within the 25mm large letter depth.
Our cardboard envelopes are rigid enough to protect contents in transit while keeping the overall package as slim as possible. They're a staple for anyone shipping artwork, greetings cards, documents, or anything else that needs to arrive flat and undamaged.
A practical example: clothing sellers
One of the most common scenarios we see is clothing sellers defaulting to small parcel boxes when many of their products could ship as large letters.
A folded t-shirt, for instance, typically sits well within 25mm depth if it's packed neatly. The same goes for socks, underwear, scarves, and many lightweight accessories. If you're selling these products and currently shipping them as small parcels, switching to large letter packaging could save you £1.50–£2.50 per order in postage — before you even account for Click & Drop or franking discounts.
The maths is simple: 500 orders a month at £2 saved per order is £1,000 back in your pocket every month.
A note on external vs internal dimensions
Royal Mail's size limits are based on external measurements. Box dimensions listed by packaging suppliers — including us — are almost always internal measurements.
This matters because the board adds thickness. A standard single wall box typically adds around 3–4mm per side. So if you need a parcel that measures exactly 450mm externally to hit the small parcel limit, you need a box with an internal length of around 442–444mm.
Always measure your packed box before committing to a size, especially if you're right on the boundary between formats.
Not sure which format your products fall into?
We're happy to help you work it out. Bring us your product dimensions and we can suggest packaging that fits the right Royal Mail format and keeps your postage as low as possible.
Call us on 02476 611234 or get in touch online — we've been doing this for over 20 years and there are very few packing challenges we haven't seen before.
Related products: PiP Postal Boxes | Cardboard Envelopes | eCommerce Boxes | All eCommerce Packaging