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How to Use the Royal Mail Size Guide Ruler Correctly

If you’re franking mail or posting parcels regularly, you’ve probably heard of and/or got a Royal Mail postage size guide ruler – but are you actually using it correctly? At Mailcoms, we supply a Royal Mail size guide ruler with every franking machine starter pack and with every first time online order, and for good reason: Using it properly ensures you’re choosing the right postage category, avoiding rejected mail, and saving money on every item you send.

In this guide, we’ll explain how to use the Royal Mail size guide ruler properly, step by step – and why it’s a must-have tool in any modern mailroom.

What Is a Royal Mail Size Guide Ruler?

The size guide ruler is a plastic measuring template that helps you determine which size category your letter or parcel falls into. It includes slots for:

  • Letter
  • Large Letter
  • Small Parcel

Each slot is based on Royal Mail’s official dimensions, so you can quickly and confidently select the correct postage rate before franking your item.

Why It Matters

Using the wrong size category when using your franking machine can lead to:

  • Overpaying (franking as a small parcel when it’s only a large letter).
  • Returned items due to underpayment.
  • Confusion for staff, especially in busier mailrooms.

A size guide removes guesswork and helps you avoid costly mistakes.

Royal Mail’s Basic Size Categories (as of 2025)

UK Mail Formats
Format
Examples
Max. Weight
Max. Length
Max. Width
Max. Thickness
Letter
Greeting Cards, Bills, Invoices & Personal Letters
100g
24cm
16.5cm
0.5cm
Large Letter
A4 Documents, Certificates, DVD case & cards with badges
750g
35.3cm
25cm
2.5cm
Small Parcel
Boots, Portable devices & desk lamp
2 Kg
45cm
35cm
16cm
Medium Parcel
Greeting Cards, Bills, Invoices & Personal Letters
20 Kg
61cm
46cm
46cm

Always double-check with Royal Mail for any annual tariff updates.

How to Use the Size Guide (Step-by-Step)

Slide the item through the corresponding slot

Once packaged, start with the Letter slot. If it doesn’t fit, try Large Letter, and so on.

If it passes through easily without resistance that is your size category

Don’t force it through - if it catches or is tight, move up to the next size.

Weigh the item using your franking machine

Size + weight = correct rate.

Frank your item knowning you are franking the right amount

Use this info to frank your item with full confidence that the postage matches Royal Mail’s size guide rules.

Tips for Accurate Sizing Every Time

  • Use the guide after the item is packed – not before.
  • For padded or uneven items (like padded envelopes), check the thickness carefully.
  • Keep your size guide somewhere visible, like next to your franking machine.
  • Train all staff to use it consistently – especially new starters.

Final Thoughts

The Royal Mail size guide is one of the simplest tools to reduce wasted postage and avoid rejected items – yet many businesses don’t use it correctly or even have one. With every Mailcoms franking starter pack, or your first online order, we will include an official Royal Mail Size Guide Ruler so you can confidently size, weigh, and frank every item accurately.

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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May 20, 2025 2:31 pm