How to Accurately Measure Your Packaging in CM and Inches

Whether you need to take accurate cardboard box measurements, make sure you’re ordering the correct width steel banding, or you’re checking that your letterbox packaging conforms to Royal Mail’s Parcel sizing, this short guide will help you understand the conversions between both Metric and Imperial measurements.


Common Measurement Conversions:


Inches to Cm, and Cm to Inches:

1 inch is equal to 2.54cm.

0.39 inches are the same as 1cm.

To calculate inches to cm, multiply the

For example, 75mm in cm is 29.53″.


Inches to Mm, and Mm to Inches:

1 inch is equal to 25.4 mm

0.039 inches are the same as 1mm.

For example, 75mm to inches is 2.95″.


Mm to Cm, and Cm to Mm:

1cm is equal to 10mm.

1mm is the same as 0.1cm.

For example, 75mm in cm is 7.5cm.


Which way around do measurements go?

The industry standard for a set of measurements is length x width x height. When written, this often looks like this 24x18x18″. This set of measurements shows a length of 24″, followed by a width of 18″, and a height of 18″.


This is especially useful to keep in mind if you are packing boxes to dispatch with carriers such as Royal Mail who have specific sized and weight-based pricing.


Royal Mail Parcel Sizes in Inches and CM:

Letter: 0.94×0.65×0.2″ or 24×16.5x5mm, max weight of 100g.

Large Letter: 13.9x9x0.98″ or 35.3x25x2.5cm, max weight of 750g.

Small Parcel: 17.7×13.8×6.3″ or 45x35x16cm Max weight of 2KGs.

Medium Parcel: 24×18.1×18.1″ or 61x46x46cm, max weight 20kg.

Measurements are accurate as of May 2023, for up-to-date information, please see the Royal Mail website. 


Key things to remember:

There are 25.4 millimetres in an inch, one inch is 2.54 cm, and 1cm equals 10mm. Measurements are always ordered as length, width, height.

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