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Choosing the right postal box can help protect items while keeping a package within Royal Mail's Large Letter dimensions. PiP postal boxes are available in a range of practical formats, including C7, C6, DL, C5 and C4, with slim depths designed to help packages remain within the relevant postal limits. This guide explains the common sizes and how to choose the most suitable format.
What are Royal Mail PiP Postal Boxes
Royal Mail PiP boxes are slim postal packaging boxes designed to fit within Royal Mail's Pricing in Proportion (PiP) postal formats. They provide more protection and rigidity than a standard paper envelope while keeping the package compact for posting.
Royal Mail PiP Postal Box Sizes
PiP postal boxes are designed for small and flat items that need more protection than a standard paper envelope. The example sizes below are 20mm deep and sit within the standard Royal Mail Large Letter maximum dimensions when the finished packed item remains within the required size and weight limits.
| Box Size | External Dimensions | Depth | Postal Format |
|---|---|---|---|
| C7 | 101 x 101 x 20mm | 20mm | Large Letter |
| C6 | 112 x 163 x 20mm | 20mm | Large Letter |
| DL | 220 x 110 x 20mm | 20mm | Large Letter |
| C5 | 218 x 159 x 20mm | 20mm | Large Letter |
| C4 | 320 x 230 x 20mm | 20mm | Large Letter |
Royal Mail Large Letter maximum:
353 x 250 x 25mm with a maximum weight of 750g for the standard Large Letter format.
Always measure and weigh the finished packed item before posting.
Shop PiP Postal Boxes
Browse our range of Royal Mail PiP postal boxes in C7, C6, DL, C5 and C4 sizes.
Shop PiP Postal Boxes →Which PiP Postal Box Size Should I Use?
The best size is normally the smallest box that comfortably holds the item and any protective packing without forcing the sides or lid to bulge. Choosing a closer fit can reduce empty space and help keep the finished package neat.
C7 PiP Postal Box
101 x 101 x 20mm
A compact square format suited to small products, components, samples and other small flat items.
View C7 PiP Postal Boxes →C6 PiP Postal Box
112 x 163 x 20mm
A compact rectangular format that provides more usable length than C7 while remaining suitable for smaller products and documents.
View C6 PiP Postal Boxes →DL PiP Postal Box
220 x 110 x 20mm
A long, narrow format useful for items that need extra length without the additional width of a C5 or C4 box.
View DL PiP Postal Boxes →C5 PiP Postal Box
218 x 159 x 20mm
A versatile mid-size format suitable for documents, printed items, accessories and other products that need a little more surface area.
View C5 PiP Postal Boxes →C4 PiP Postal Box
320 x 230 x 20mm
A larger format that provides the greatest usable footprint of the sizes shown while remaining below the standard Royal Mail Large Letter maximum dimensions when correctly packed.
View C4 PiP Postal Boxes →Why Use a PiP Postal Box?
A PiP postal box combines the slim format of Large Letter packaging with the added structure of corrugated cardboard. This makes it a practical option when an envelope does not provide enough protection for the item being sent.
- 20mm box depth: below the standard 25mm Large Letter thickness limit.
- More rigid protection: useful for products that should not be easily bent or crushed.
- Five size choices: select a footprint that more closely matches your contents.
- Flat packed storage: boxes can be stored flat until they are needed.
- Recyclable cardboard: many PiP postal boxes are made from recyclable corrugated cardboard.
How to Measure a PiP Box Before Posting
Postal dimensions apply to the finished package, not just the empty box. Once your item has been packed and the box is closed, measure the longest side, the next longest side and the total thickness at the deepest point.
Pay particular attention to the 25mm Large Letter thickness limit. A nominally slim box can still become too thick if it is overfilled and the lid or sides bow outwards. The finished package should also be weighed before postage is purchased.
Royal Mail PiP Postal Box FAQs
What does PiP mean in Royal Mail packaging?
PiP stands for Pricing in Proportion. Royal Mail postage formats take the size and weight of the finished item into account, so the dimensions of your packaging can affect the postal format and price.
What size is a Royal Mail Large Letter?
The standard Royal Mail Large Letter maximum is 353mm long, 250mm wide and 25mm thick, with a maximum weight of 750g. The final packed item must stay within the relevant dimensions and weight limit.
Are PiP postal boxes suitable for Large Letter postage?
Many PiP postal boxes are designed to fit within Royal Mail Large Letter dimensions, but the finished packed item must still stay within the relevant maximum size and weight limits. Always measure and weigh the package after packing.
Can a 20mm PiP box become too thick for Large Letter postage?
Yes. If the contents overfill the box and cause it to bulge, the finished thickness could be greater than the nominal 20mm box depth. Always measure the closed package at its thickest point before posting.
Can I use PiP postal boxes for ecommerce orders?
Yes. PiP postal boxes are commonly used for ecommerce items that fit within the box dimensions and benefit from the added protection of corrugated cardboard.
Which PiP postal box size should I choose?
Choose the smallest box that comfortably fits the item and any protective packing without causing the box to bulge. The finished package should then be measured and weighed to confirm that it remains within the relevant Royal Mail postal limits.
Size information is provided as a guide. Always check the current postal requirements and measure and weigh the finished package before posting.