Understanding Royal Mail Size Guidelines for Postal Boxes
Royal Mail size bands explained: how to choose postal boxes that qualify for the cheapest rates.
Royal Mail size bands
Royal Mail classifies mail into size bands that determine pricing. Understanding these bands lets you choose postal boxes that qualify for the cheapest applicable rate, saving money on every shipment.
Quick answer
Royal Mail small parcel (up to 61cm x 46cm x 46cm, 2kg) is the most cost-effective band for postal boxes. Keeping within large letter dimensions (35.3cm x 25cm x 2.5cm, 750g) is even cheaper for flat items. The biggest price jumps happen when you exceed the 2kg small parcel weight limit or when dimensions push you into medium parcel territory. Always weigh and measure before choosing a box size.
Letter and large letter
At 93p (2nd class), it is dramatically cheaper than small parcel rates for items that fit.
Many items fit the length and width but are too thick (over 25mm) and get bumped to small parcel.
Purpose-designed boxes that maximise the 35.3cm x 25cm x 2.5cm dimensions.
Small and medium parcel
Small parcel is the standard band for most e-commerce postal boxes. The 2kg weight limit is the key constraint. Medium parcel allows up to 20kg but costs significantly more. If your product is under 2kg, staying in the small parcel band saves the most money.
Optimise further: size optimisation guide or complete sizing guide.
Sizing tips
Tips for staying in cheaper bands0 / 4 checked
Royal Mail-friendly postal boxes
Postal boxes designed to fit Royal Mail pricing bands for maximum shipping savings.
Browse postal boxesFrequently asked questions
Letter, large letter, small parcel, and medium parcel - each with specific dimension, weight, and price thresholds.
Large letter (up to 35.3cm x 25cm x 2.5cm, 750g) at 93p 2nd class is cheapest. Small parcel is next.
Up to 61cm x 46cm x 46cm and up to 2kg.
Yes, going over 2kg bumps you from small parcel to medium parcel, a significant price increase.
Use lightweight board, right-size your box, minimise void fill weight, and weigh packed boxes before shipping.
Part of our guide
Product Guide: Postal Boxes
Expert guides on postal boxes, from sizing and materials to branding and cost optimisation.
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