Royal Mail Size Guide: When to Use a Large Letter vs. Small Parcel

When you’re sending mail using a franking machine, understanding Royal Mail’s size categories is key to avoiding overpaying – or worse, having your mail rejected. Two of the most commonly confused categories are Large Letter and Small Parcel. In this guide, we’ll break down the differences, explain when to use each, and show how to easily measure your mail using the Royal Mail Size Guide.

Why Size Matters for Your Postage

Every millimetre counts. Sending something as a Small Parcel when it qualifies as a Large Letter could cost your business up to 50% more in postage. With Royal Mail, pricing is based on format and weight, and the line between categories can be surprisingly slim.

What Is Considered a Large Letter?

A Large Letter is bigger than a standard letter but still flat enough to fit through a letterbox. According to Royal Mail’s guidelines, to qualify as a Large Letter, your mail must:

  • Be no larger than (Length x Width x Depth): 35.3cm x 25cm x 2.5cm.
  • Weigh no more than: 750g.

Common examples:

  • Documents.
  • Certificates.
  • Padded envelopes.
  • Thin books or brochures.

✅ Tip: If your item is uniform and doesn’t bulge beyond 2.5cm thick, it likely qualifies as a Large Letter.

Cardboard Envelopes
Royal Mail Postal Cardboard Boxes

What Is Considered a Small Parcel?

If your item is bulkier or exceeds the dimensions above, it falls into the Small Parcel category. These parcels can be:

  • Be no larger than (Length x Width x Depth): 45cm x 35cm x 16cm.
  • Weigh no more than: 2kg.

Common examples:

  • T-shirts or clothing in mailing bags.
  • Small boxed products.
  • Gift items.
  • Bulkier padded packages.

How to Tell the Difference Quickly

The best way to check is with a Royal Mail Size Guide, like the one included in our Mailcoms Starter Packs or the one we send out to you with your first online order with us. Simply try sliding your item through the slots prior to franking your mail item.

  • ✔ If it slides through the Large Letter slot without resistance – you’re good!
  • ❌ If it doesn’t fit – try the Small Parcel slot to confirm it is a Small Parcel (or even larger – a Medium Parcel).

Why This Matters to Franking Users?

Franking machines make postage more cost-effective – but only when used correctly. Accidentally franking at the wrong rate can lead to:

  • ❗ Returned or delayed mail.
  • 💸 Overpaying on postage.
  • ⚠️ Non-compliance with Royal Mail.

Using the right size category helps you stay accurate, efficient, and compliant.

Our Top Tips for Correctly Categorising Mail

  • Use cardboard envelopes for flat items to help them stay under 2.5cm.
  • Avoid overstuffing padded mailers – even 0.5cm can bump it into Small Parcel territory.
  • Have a Royal Mail Size Guide at every packing station.
  • Regularly train staff or colleagues on format and weight guidelines.
  • Utilise your franking machine – it has a weighing scale which allows you to pay the correct postage every time once you know the size.

Final Thoughts

Sending your mail at the correct size and format isn’t just about compliance – it’s about saving money and avoiding hassle. With a franked item, you already benefit from Mailmark discounts – now make sure you’re not overspending due to misclassification.

At Mailcoms, we offer a range of franking machines. If you are interested in getting a franking machine or wish to upgrade to a Mailmark franking machine with even lower prices available, then please view our franking machine page here or call us today on 01543 572 776.

We are a Royal Mail Authorised Independent Inspector & Maintainer of Franking Machines with over 20,000 customers nationwide and we can supply you with our very own range of models.

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