How to pack to fit Royal Mail’s size bands
5 min read 06 July 2026
Your packaging decides which Royal Mail size band an order falls into, and that band is the biggest lever you have on postage. Pack to fit Royal Mail’s size bands and you keep more orders in the cheaper formats.
This guide is for online sellers who want to cut despatch costs without losing protection. It’s about packing choices, not the price table.
In this article, we’ll cover:
- The four Royal Mail size bands and what fits each
- Why your packaging, not just the product, sets the band
- The large letter limits worth packing to
- How to stop an order tipping into the next band
- When a flatter box or mailer saves you money
What are the Royal Mail size bands?
Royal Mail sorts post into four size bands: letter, large letter, small parcel and medium parcel. Your item sits in whichever band its largest dimension and its weight both still fit inside.
What fits a large letter?
A large letter measures up to 35.3 x 25 x 2.5 cm and weighs up to 750 g. That covers most A4 documents, slim paperbacks and thin padded mailers, so long as they stay under 2.5 cm thick.
A standard letter is smaller still, up to 24 x 16.5 x 0.5 cm and 100 g, and has to flex. The large letter band is the one sellers miss most, because a few millimetres of thickness tips it into a parcel.
What fits a small parcel?
A small parcel goes up to 45 x 35 x 16 cm, with a 2 kg weight limit. A medium parcel goes up to 61 x 46 x 46 cm and 20 kg; anything larger needs a parcel service such as Parcelforce.
How does your packaging set the size band?
Your packaging, not just your product, sets the band, because Royal Mail measures the packed item. A slim product in a thick box can price as a parcel when it would post as a large letter in a flat mailer.
The step from large letter to small parcel is the biggest single price jump, so it pays to keep eligible items flat and under 2.5 cm. The right postal box or a flat mailing bag can hold you in the cheaper band.
What do the Royal Mail size bands cost?
Royal Mail charges by band first, then by weight within it, with 1st Class costing more than 2nd Class. Prices usually change each April, so confirm the live rate before you post.
As an example, at the April 2026 prices a 1st Class letter stamp is £1.80 and 2nd Class is 91p, but parcel rates vary by service and weight. For the full table by format and weight, check the official Royal Mail prices page.
Volumetric vs actual weight: which applies?
Royal Mail’s standard parcel bands are based on size and actual weight, but couriers and some services use volumetric (dimensional) weight. Volumetric weight charges you on the space a parcel takes, so a big light box can cost more than its weight suggests.
Our guide to volumetric weight and reducing packaging costs shows how to calculate it. The fix is the same either way: use the smallest packaging that still protects the product.
How to keep an order in a cheaper band
Keep eligible items flat and under the large-letter limits, and right-size everything else so it doesn’t creep up a band. Small packing changes add up fast across hundreds of orders.
Packing to fit Royal Mail’s size bands lets you design despatch around the cheaper formats.
Pack to the bands
Sizing your packaging to Royal Mail’s size bands is the easiest postage saving there is. Browse boxes and mailing bags chosen to keep eligible orders in the cheaper formats.
Key takeaways
- Royal Mail sorts post into four size bands, and an item sits in whichever band its largest dimension and weight both fit.
- A large letter is up to 35.3 x 25 x 2.5 cm and 750 g; a small parcel is up to 45 x 35 x 16 cm and 2 kg.
- The step from large letter to small parcel is the biggest price jump, so keep eligible items under 2.5 cm thick.
- Royal Mail measures the packed item, so your packaging sets the band as much as the product does.
- Prices usually change each April, so check the live rate before you price your despatch.
FAQ: Packing to Royal Mail size bands
Is a padded jiffy bag a large letter or a small parcel?
A thin padded mailer can post as a large letter if the packed item stays within 35.3 x 25 x 2.5 cm and 750 g. Once it’s thicker than 2.5 cm, it becomes a small parcel.
Does Royal Mail measure the box or the item inside?
Royal Mail measures the packed item, including the box or bag, not the product alone. That’s why right-sizing your packaging directly affects the price you pay.
Can I send clothes as a large letter?
A single thin garment in a flat mailer can qualify as a large letter if it stays under 2.5 cm thick and 750 g. Bulkier or multiple items will usually price as a small parcel.
Why did my parcel cost more than I expected?
It most likely crossed into the next band, often on thickness rather than weight. Measuring the packed item against the large-letter limits before posting avoids the surprise.
How often do Royal Mail prices change?
Royal Mail usually updates prices once a year, typically in April, with occasional other adjustments. Always check the Royal Mail website for the live rate rather than relying on a printed table.
