How to use the parcel size checker

Measure the parcel after it has been packed, not the item before packaging. Use the longest side as length, the next side as width, and the smallest side as height or thickness. Enter the packed weight in kilograms.

The calculator sorts the dimensions for you, so it does not matter which way round you hold the parcel. The result shows the first size band that the parcel is likely to fit.

Royal Mail size and weight limits

All three dimensions and the weight must fit the same band. These are the UK 1st Class limits checked on 5 September 2026, not a guide to every courier or Royal Mail service.

Source: Royal Mail 1st Class: sizes and what you can send. This checker compares maximum limits only. It does not check minimum mailing dimensions, contents restrictions, compensation or service availability.

When a few millimetres change the band

A packed item measuring 35.3 × 25 × 2.5 cm and weighing 750 g reaches the large-letter limits exactly. Increase its thickness to 2.6 cm, or its weight to 751 g, and it moves to small parcel in this checker.

A 45 × 35 × 16 cm parcel at 2 kg reaches the small-parcel limits. At 2.01 kg it needs the medium-parcel band, even though its dimensions have not changed. The tool sorts the three dimensions, so rotating the same box does not change its category.

Measure bulging corners and the thickest point without compressing the packaging. Include the label, padding and any enclosure when weighing it.

Tubes, other services and buying a label

Tubes and rolls have separate length-and-diameter rules and are not modelled here. International mail, Tracked services, business contracts and other couriers may have different limits. Check the service you will actually buy.

This is an estimate only, not acceptance of a parcel. After checking its size, use the Postage Comparison Calculator to compare the prices and cover you have been quoted. The PDF Label Mover can position a purchased label on a part-used sheet.