Two facts decide most of what you pay to post a parcel: the size band it falls in, and for couriers, its volumetric weight. This guide explains both. The figures are the published limits at the time of writing, and carriers change them, so check the current tariff before you rely on them.
Royal Mail size bands
| Band | Maximum size | Maximum weight |
|---|---|---|
| Large letter | 353 by 250 by 25 mm | 750 g |
| Small parcel | 45 by 35 by 16 cm | 2 kg |
| Medium parcel | 61 by 46 by 46 cm | 20 kg |
The large letter band is set by the 25 mm gauge. A packed mailer that is 26 mm thick is a small parcel, and costs several times as much to post. That is why every mailer we sell states whether it goes through the large letter gauge when packed flat.
Volumetric weight
Most couriers charge on whichever is greater: the actual weight, or the volumetric weight. Volumetric weight is length by width by height in centimetres, divided by 5000. A 40 by 30 by 30 cm box has a volumetric weight of 7.2 kg, even if the contents weigh 1 kg. Dropping to a 30 by 23 by 23 cm box brings that down to 3.2 kg. Some carriers use a different divisor, so check yours.
Choosing a mailer or a box
- Flat items under 25 mm packed: a padded mailer in the large letter band.
- Soft goods such as clothing: a padded or gusseted mailer, sized to the folded item.
- Fragile or heavy items: a box with 30 to 50 mm of void fill all round, double wall above about 10 kg.
- Books and flat goods: a rigid board mailer or a low depth postal box.
Not sure? Use the parcel sizer, or email us your product sizes and we will suggest a size.
Postroom is a trading name of Mohammed Saber Uddin, a sole trader. Address: 185 Plashet Road, London, E13 0QZ, United Kingdom. Email info.msutrade@gmail.com, telephone 07477 110459. Not registered for VAT.