Postage is usually the second biggest cost in an online order after the product itself, and the single thing that decides it is which Royal Mail size band your parcel falls into. Get it right and you pay the cheapest rate available for that item. Get it wrong by a few millimetres and you pay more on every order you send, indefinitely.
PiP stands for Pricing in Proportion. It is the system Royal Mail use to price mail by size as well as weight, and it is why a light but bulky parcel can cost more to send than a heavier, flatter one. Here is how the bands work and how to work out where your product actually sits.
The Royal Mail size bands
Royal Mail split UK mail into five formats. Every figure below is a maximum, and your item has to satisfy all of them to qualify for that band.
- Letter: 240 x 165 x 5mm, up to 100g. It also has to be flexible.
- Large Letter: 353 x 250 x 25mm, up to 750g.
- Small Parcel: 450 x 350 x 160mm, up to 2kg.
- Medium Parcel: 610 x 460 x 460mm, up to 20kg.
- Large Parcel: handled by Parcelforce, up to 1500mm long and 30kg.
If one single measurement goes over, the whole item moves up a band. There is no partial credit and no published tolerance, so treat every number as a ceiling rather than a target. Royal Mail update their specifications from time to time, so it is worth confirming the current figures on royalmail.com before you commit to a packaging size for the long term.
Depth is the one that catches people out
Length and width are easy to picture. Depth is where most sellers quietly lose money. The Large Letter band allows 25mm, which is not very much once you have added a product, a layer of tissue and a bit of trapped air. A box that measures 20mm flat but sits at 28mm when packed is a Small Parcel, and it will be priced as one.
That is exactly why Large Letter is such a valuable band to design for. A slim box that fits through a standard letterbox also removes the failed delivery problem, because nobody needs to be in to receive it. We have written more about that in our guide to letterbox-friendly packaging.
Measure the packed item, not the product
Royal Mail measure and weigh the parcel as it is presented to them: box, product, void fill, tape and all. So weigh and measure a finished, sealed example of what you actually send, rather than the item on its own. If you are consistently within a few grams or millimetres of a threshold, you are one busy packing day away from being tipped into a more expensive band.
Two practical checks before you order in quantity. Pack a real order into a candidate box, seal it, then measure all three dimensions at their widest points and weigh it. Then repeat the exercise with your bulkiest variant, not your smallest.
How our range maps to the bands
Our boxes and mailers are categorised by PiP size, which saves you doing the maths from scratch. The bands are set out across our Royal Mail PiP postal boxes, and there is a much wider choice of sizes and depths in the full postal cardboard boxes range. For books, prints, magazines and anything else flat and rigid, book and magazine wrap mailers fold down to the depth of the contents, which will often keep an item inside Large Letter that a fixed-depth box would push into Small Parcel.
Allow for manufacturing tolerance
Stock cardboard packaging is made to a tolerance of roughly 10 to 20mm either way. That is normal across the industry, but it matters a great deal if you are designing right up against a band limit. If your product only just fits inside Large Letter on paper, the honest advice is not to build your postage model on that assumption. Order a blank sample for a couple of pence, pack it, measure it, and find out for certain before you buy a pallet of the wrong thing.
Printing does not change your band
Worth saying plainly, because we get asked. Adding your branding does not alter the size, weight or PiP category of a box. We print full colour onto pre-manufactured stock packaging in pre-defined print areas, so the printed product is the same size as the plain one. What changes is what the parcel says about you on the doorstep.
It does mean your artwork has to suit the print areas available on that particular product rather than the whole outside of the box. Those areas are shown on each product page, and our guide to supplying artwork covers how to set files up so nothing gets held up at proofing. Because we print in-house on our own equipment and hold stock of our own blank products, we can turn orders around same day locally subject to availability, next day, or on a standard 3 to 5 day service.
A simple working method
Start from the product, not the box. Measure a typical order packed the way you would genuinely send it. Find the smallest band it fits inside with room for the tolerance and for a slightly fuller order than usual. Then choose the box that hits that band with the least wasted space, because empty air costs you twice: once in postage, and again in the void fill you need to stop things rattling around.
If you are not certain which band your product falls into, or you want to compare two sizes before committing, just ask. We would far rather spend ten minutes on that than have you buy 500 boxes that cost more to post than they needed to. There is more on sizing, samples, proofs and lead times on our frequently asked questions page.
