Royal Mail PiP Sizes Explained, and Which Stock Boxes Fit
Why PiP matters to your postage bill
Royal Mail prices parcels by format, not just weight. The format is decided by physical size, and the jump between bands is the single biggest lever you have on postage cost. Get an item into Large Letter instead of Small Parcel and you can more than halve the price on that shipment. Across thousands of orders that difference funds a lot of other things.
PiP stands for Pricing in Proportion, the system Royal Mail uses to sort mail by dimensions. In practice sellers mostly care about two thresholds: the Large Letter gauge and the Small Parcel limits.
The size limits that decide your band
These are the current Royal Mail maximums by format. Measure the outside of your packed item, including the box and any bulge.
Large Letter
- Length: up to 353mm
- Width: up to 250mm
- Thickness: up to 25mm
- Weight: up to 750g
Everything has to be under all four limits at once. Thickness is the one that catches people out. 25mm is not much, so the item has to be genuinely flat.
Small Parcel
- Length: up to 450mm
- Width: up to 350mm
- Depth: up to 160mm
- Weight: up to 2kg (or up to 20mm thick and heavier for some services; check the current tariff)
Medium Parcel
- Length up to 610mm, width up to 460mm, depth up to 460mm, weight up to 20kg.
Royal Mail updates its tariff periodically. Treat the numbers above as the working limits and confirm against the current price guide before you commit a large box order.
Boxes that fit the Large Letter gauge
A Large Letter box has to clear a 25mm slot, so you want a shallow box, sometimes called a PiP box or letterbox mailer. Typical internal sizes that stay under the gauge once packed:
- 220 x 160 x 20mm
- 250 x 175 x 22mm
- 350 x 245 x 20mm
The trick is the 25mm ceiling is external. If your box wall and any tape add a couple of millimetres, a 22mm internal depth is safer than 25mm. Test a packed sample through a PiP gauge, which is a slotted template Royal Mail supplies, before you rely on it.
Good Large Letter candidates: phone cases, cosmetics, jewellery, greeting cards, thin apparel, print media, small spare parts.
Boxes that fit Small Parcel
For Small Parcel you have far more room, so most stock ecommerce boxes qualify. Useful internal sizes that stay well inside the 450 x 350 x 160mm limit:
- 250 x 175 x 100mm
- 305 x 220 x 100mm
- 350 x 250 x 150mm
- 400 x 300 x 150mm
Keep the depth under 160mm and you stay in Small Parcel. It is depth, again, that tends to push a box up into Medium.
The packing detail that costs sellers money
The gauge measures the packed item, not the flat box. A box rated at 20mm internal depth will exceed 25mm external once you add board thickness, product and tape. Always gauge-test the finished parcel, not the empty box.
The same applies at the Small Parcel to Medium boundary. A 150mm deep box packed with something that domes the lid can measure 165mm and get charged as Medium. Either pack flatter or drop to a shallower box.
A simple way to work it out
1. Weigh and measure your product as it will sit packed, with any padding.
2. Find the smallest format it fits under on every dimension at once.
3. Choose a stock box whose external packed size stays inside that band with a few millimetres to spare.
4. Gauge-test a real, sealed sample.
Comparing box prices
Once you know the internal size you need, the remaining question is price, and it varies a lot between suppliers for the same dimensions. You can compare live prices by size across UK suppliers on Packaging Marketplace, including shallow PiP and Large Letter boxes, so you can lock in the right format at the best rate.