Trailer weights explained

Published 10 October 2026. Every figure was checked at its source that day.

The short version

Maximum authorised mass (MAM) is the most a vehicle or trailer may weigh when loaded. It is on the plate.
Gross train weight is the most the loaded car and loaded trailer may weigh together. It is on the car's VIN plate.
Unladen weight is the weight with nothing and nobody in it.
The tow ball needs a downward load, and the manufacturer's figure for it should not be exceeded.

Towing rules are written in four or five different weights. This is what each one means and where to find it.

Maximum authorised mass

GOV.UK defines maximum authorised mass (MAM) as the weight of a vehicle or trailer including the maximum load that can be carried safely when it is used on the road. It is also called gross vehicle weight or permissible maximum weight.

It is listed in the owner's manual and is normally shown on a plate or sticker. On a trailer the plate is usually on the drawbar or the chassis. The driving licence limit of 3,500kg is a limit on the trailer's MAM, so it counts what the trailer is allowed to weigh, not what it weighs today.

Gross train weight

The gross train weight is the weight of the fully loaded car plus the fully loaded trailer. It is also called gross combination weight. It is on the car's VIN plate and must not be exceeded.

GOV.UK says that if the VIN plate lists no train weight, you should not use the vehicle for towing.

The four weights on a car's plate

The most the car can weigh on its own.
The most the car and trailer can weigh together.
The limit for the front axle.
The limit for the rear axle.

Each maximum includes passengers, other loads and fuel.

A worked example

The numbers here are made up to show the sum. They are not from any real car.

First weight on the plate: 2,100kg.
Second weight on the plate: 3,900kg.

3,900kg less 2,100kg leaves 1,800kg. With the car loaded to its own maximum, the loaded trailer can weigh up to 1,800kg. If the handbook gives a lower towing limit than that, the lower figure applies.

Unladen weight

GOV.UK defines unladen weight as the weight of a vehicle or trailer when it is not carrying any passengers, goods or other items. It includes the body and all parts normally used on the road. It does not include fuel.

It matters for a trailer with no brakes, which should weigh no more than half the weight of the car, and for the operator's licence thresholds.

Tow ball load

This is the weight the front of the trailer puts down on the tow ball. It is often called nose weight. The Highway Code says to make sure there is a downward load on the tow ball, and that the manufacturer's recommended weight and tow ball load should not be exceeded. The figure for your car is in its handbook.

Which limit applies?

All of them at once. The trailer has to be inside your licence, your car's towing limit, the gross train weight and the tow ball load. The tow checker compares your figures with the licence, brake and size limits.