Almost every price in this record is a band rather than a rate, and two measurements decide which band a parcel lands in: what it weighs and how big it is. Getting either wrong is the most common reason a booked price is not the price charged, because every carrier here reserves the right to re-measure in its own depot and bill the difference. Both are measurable at home in about a minute with no special equipment, and doing so before you book is worth more than any amount of comparing.
What girth is and how to work it out
Girth is the distance around the parcel at its thickest point, and the standard way to calculate it is length plus twice the width plus twice the depth. Carriers use it because a long thin parcel and a short fat one can have the same volume and behave completely differently on a conveyor. A girth calculator is doing nothing more than that sum, so a tape measure and a piece of paper give the same answer. Measure the packed box, not the item inside it, because the box is what the carrier will measure.
How to weigh a parcel at home without parcel scales
Stand on bathroom scales holding the parcel, note the figure, stand on them without it and subtract. That is accurate to a few hundred grams, which is enough for every band in this record except the very lightest. If your parcel is close to a band edge, round up: Royal Mail's Small Parcel stops at 2kg and its Medium at 20kg, and being fifty grams over either is the same as being a kilogram over.
The size limits the published grids actually use
Royal Mail publishes its limits beside every price: a Letter at 24cm x 16.5cm x 0.5cm and 100g, a Large Letter at 35.3cm x 25cm x 2.5cm, a Small Parcel at 45cm x 35cm x 16cm and 2kg, and a Medium Parcel at 61cm x 46cm x 46cm and 20kg. Evri's published table splits Small from Standard below 2kg and then prices on weight alone. Those two grids together are most of what a UK sender needs to know about size.
Volumetric weight, which is the same idea one step up
Above the flat bands, carriers charge on the greater of actual weight and volumetric weight, which is volume divided by a divisor they set. It is why a large box of something light can be billed as though it were heavy. If your parcel is bulky rather than dense, the number that will decide its price is the one you get from measuring, not the one from the scales.
Questions people ask about how to measure girth
How do I measure girth?
Length plus twice the width plus twice the depth, measured on the packed box. That is all a girth calculator does.
How can I weigh a parcel at home?
Stand on bathroom scales holding it and then without it, and subtract. That is accurate enough for every published band except the lightest, and if you are near a band edge, round up.
What size parcel can I send?
On Royal Mail's published grid, up to 61cm x 46cm x 46cm and 20kg as a Medium Parcel. Parcelforce publishes a weight ladder to 30kg. Above that the trade prices by pallet or by freight.
Why was I charged more than the price I booked?
Almost always a re-measure. Every carrier here can re-measure and re-weigh in its own depot and bill the difference, and the measurement that counts is the packed box.