Caravan Towing Weights Explained

Caravan Weights explainedThere are some very important weights that you need to be aware of and understand when towing a caravan or trailer. The following definitions need to be considered carefully:

Tare Weight or Tare Mass

Tare weight or mass this is the net weight of your caravan as delivered from the manufacturer (with no occupants, payload or after-market accessories), usually without water in the tanks or the gas bottles filled. The Tare Weight is normally listed in the owner’s manual or is printed on the caravan’s compliance plate.

Gross Vehicle Mass (GVM)

GVM is the maximum permissible weight of the vehicle (Tug) in operation. GVM is the tare weight plus the payload (water, fuel in tank, clothing, tools, etc). Operating a vehicle greater than its GVM is illegal and will void the manufacturer’s warranty and likely the insurance as well.


Payload is the trailer or vehicle’s maximum load carrying capacity. It’s the difference between its Tare Weight (un-laden weight) and its ATM. The figure remaining is the maximum allowable weight of the cargo which can be carried in or on the vehicle.

Modifications and additions to the trailer or vehicle can very quickly eat into its carrying capacity and result in an overloaded vehicle. Additional water tanks, batteries and gas cylinders are commonly fitted to campers and caravans and can add significant extra weight. As a guide, each additional litre of water carried adds 1kg (excluding the weight of the tank and fittings).

ATM (Aggregate Trailer Mass)

ATM is the total laden weight of a trailer, which includes the tow ball mass and whatever you add as payload. This weight includes the tow ball or pin mass imposed on the tow hitch plus whatever you add as a payload (e.g. water, luggage, gas being the sum of the GTM plus the weight on the tow ball).

ATM can be measured by placing the fully loaded trailer or caravan on a vehicle scale. Often the local tip is a good location to find a licenced weighbridge.  If using the local tip mid-week would be less busy.

GTM (Gross Trailer Mass)

The total permissible mass which includes whatever you add as payload (eg. water, gas and luggage) that can be supported by the wheels of a trailer. This does not include the mass supported by the tow ball. The GTM is specified by the manufacturer and must not be exceeded.

GCM (Gross Combined Mass)

GCM is the total mass of the tow vehicle and the caravan or trailer, with everything loaded in the vehicle. The GCM is specified by the vehicle manufacturer.

Ball Weight or Tow Ball Mass (TBM)

TBM is the proportion of the trailer weight that is applied to the rear of the tow vehicle. A vehicle’s Ball Load specification can be found in the vehicle’s handbook or compliance plate and will be listed as a weight in kilograms or as a percentage of the trailer’s ATM.

Ball Load specifications are often around 10% of the maximum towing mass specification however, this isn’t always the case. One way to calculate the ball load/tongue weight is the difference between the ATM and the GTM.

Weighbridge weighing procedure…

If you are on a weighbridge with your tow vehicle hitched-up to the caravan/trailer and are fully loaded up this will give you the Gross Combined Mass (GCM).
If you have the fully loaded caravan/trailer unhitched (on its jockey-wheel) with its wheels on the weighbridge this will give you the Aggregate Trailer Mass (ATM).
With the van/trailer hitched to the tow vehicle with only the van/trailer’s wheels on the weighbridge this will give you the Gross Trailer Mass (GTM).
The difference between the ATM and GTM will give you the Tow Ball Mass (TBM).

Updated: February 1, 2017 by admin