Royal Mail and Courier Size Bands for Mailer Boxes

This guide is general information, not legal or compliance advice. Carrier size bands change; check the carrier's current guidance before you finalise a design.

Royal Mail prices UK post by size format, and each format has maximum dimensions and a weight limit. For a mailer box, the band you pay for is decided by its external dimensions and packed weight, so a few millimetres of board or a slightly heavier flute can move every order into a dearer band. Couriers publish their own limits and many charge on volumetric weight, so size the box for the carrier you actually use.

Royal Mail UK size formats

These are the UK formats in Royal Mail's size and weight guide, as published when we checked it on 11 September 2026. Royal Mail gives the figures in centimetres; we show millimetres first, as a box dieline would.

Two notes from Royal Mail's own pages. First, the Large Letter weight limit is 750 g except for UK Tracked bought online, which Royal Mail lists with a 1 kg limit. Second, Royal Mail's maximum weight for UK parcels is 20 kg; for parcels from 20 kg to 30 kg it directs senders to Parcelforce Worldwide.

For heavier or longer items, Royal Mail's guide lists Parcelforce Worldwide limits:

Parcelforce defines girth as the four shorter sides added together, and length as the longest side, whichever way the parcel is turned.

How the bands are measured

Royal Mail says the length of a parcel is always its longest side, the width is the measurement across the shorter side, and the height is from the surface it rests on to the top. Measure in centimetres and weigh in kilograms and grams, and include the weight of the box and any additional packaging.

For packaging buyers, three practical points follow:

• Measure the packed box, not the empty one. A slightly overfilled mailer bows at the lid and can gain several millimetres of depth.
• Measure at the widest points. Parcelforce measures irregular shapes by the smallest box they would fit into, and any bulge or lip counts.
• Weigh the complete parcel. Board, inserts, tissue, printed cards and the label all count towards the limit.

Royal Mail also warns that under-declared or oversized items can be returned or incur an additional charge. A box designed right on the limit leaves no room for this.

Why mailer-box dimensions matter for postage

You specify a custom box by its internal dimensions, because that is the space your product needs. The postage band is decided by its external dimensions. The difference is the board.

From internal to external

Corrugated board has real thickness. As a rough guide, E flute is around 1.5 mm, B flute around 3 mm and C flute around 4 mm, though exact figures vary by supplier and liner. A box gains at least two board thicknesses on each dimension, and a roll-end tuck front mailer gains more on the dimension where the end walls roll over and are double thickness. Our guide to internal vs external dimensions explains the allowance in full.

That is why the same internal size can land in different bands depending on the flute. A mailer designed with internal depth just under 160 mm in B flute will not fit the Small Parcel depth limit; the external depth will be over it.

Flute choice changes weight as well as size

A heavier flute adds strength, but it also adds weight to every parcel. Near the 750 g Large Letter limit or the 2 kg Small Parcel limit, the board and any insert can decide which side of the line you land on. E vs B vs C flute explains how to balance protection against size and weight.

Leave a working margin

Cutting and creasing have small tolerances, contents vary, and labels add a little. Design a few millimetres inside the limit rather than exactly on it, and check the external size shown on your proof before you approve.

Designing a Large Letter mailer

Slim mailers that fit Royal Mail's Large Letter format are popular for small, flat products: jewellery, cosmetics minis, socks, stationery and printed goods. The limiting dimension is depth. With a 25 mm external maximum, an E-flute box leaves a little over 20 mm of internal depth at most, and less once an insert is added.

Practical rules for a slim mailer:

• Start from the depth. Confirm the product's thickest point, including any packaging around it.
• Use a thin board. E flute or a heavy folding boxboard keeps the walls thin; B flute uses too much of the depth.
• Keep inserts shallow. A card cradle or flat divider rather than a deep cut-out.
• Check the weight with everything in. The 750 g limit includes the box.

If the product will not fit comfortably, it is usually better to design an efficient Small Parcel mailer than to squeeze a product into a box that bows past the limit.

Couriers publish their own limits

Royal Mail's formats are not a universal standard. Each courier publishes its own maximum dimensions, weight limits and pricing bands, and these can differ between services from the same carrier. If you ship through a courier, a fulfilment partner or a marketplace programme, get their current size guide before you fix the box.

Dimensional (volumetric) weight

Many courier services charge on dimensional weight, also called volumetric weight, as well as actual weight. The carrier multiplies length × width × height and divides by a set figure (the divisor); if the result is higher than the actual weight, that is the weight you may be charged on.

Parcelforce Worldwide publishes a worked method for its international services: multiply length × height × width in centimetres, divide by 5,000, compare with the actual and pre-advised weights, and the higher figure is used. For UK account customers, Parcelforce sets a maximum length and a maximum litre volume (length × width × height in centimetres divided by 1,000).

Using that international method as an illustration:

The divisor varies by carrier and service, so use the figure in your own carrier's terms. The lesson holds whatever the number: for light products, the size of the box, not the weight of the product, can set the postage cost. Right-sizing is covered in reducing excess packaging.

A sizing checklist before you finalise

1. Confirm which carrier and service you will use for most orders.
2. Get that carrier's current size and weight guide and note the date you checked it.
3. Measure the product as packed, including any wrap or insert, and set the internal dimensions.
4. Choose the flute or board, then check the external dimensions on the dieline.
5. Estimate the packed weight, including the box, insert and any printed extras.
6. Leave a margin below every limit.
7. Check the external size on the proof before approval.

Our corrugated mailer boxes are made to your internal dimensions, and corrugated board explains the flute options. Brands shipping direct to customers will also find the e-commerce and subscription page useful.

What we do, and what we do not

Custom Boxez makes packaging. We are not a carrier, and we do not sell postage or provide Royal Mail or courier accounts; your postage arrangements stay with your chosen carrier. Delivery of your boxes to you is included on every order, and your quotation confirms the delivery address and timing.

Common mistakes

• Designing to internal dimensions only. The band is decided by the outside of the box.
• Designing exactly on the limit. Tolerances, bulges and labels push it over.
• Forgetting the box weight. Board and inserts count towards 750 g and 2 kg.
• Assuming couriers use Royal Mail's formats. Each courier sets its own limits and may charge on volumetric weight.
• Using an old size chart. Check the carrier's current guidance and date your check.
• Choosing B flute for a slim mailer. It uses too much of a 25 mm depth.