Failure to abide by the towing regulations, including maximum loads, may result in a fine, or in the case of an accident, refusal of the insurance claim, and the possibility of further legal action.

In order to tow safely and legally, you must tow within your car's towing capacity, which is the maximum amount of weight your vehicle is able to tow as recommended by the manufacturer. Unfortunately some of the ​abbreviations, acronyms and definitions regarding weights, can be confusing. And to make matters even more confusing, 3500kg towing capacity as advertised by the manufacturer, does not mean you can tow 3500kg legally.

The information and examples below, will hopefully help you understand weights and what it means for your towing capacity. If you need some help or advice, do not hesitate to contact us with your vehicle details.

Caravan & Trailer weight terminology explained

Aggregate Trailer Mass (ATM)

Unhitched from Tow Vehicle

ATM is the combined weight of the trailer and its full load when it is not coupled to a tow vehicle.
The ATM includes the Tow Ball Mass as it is unhitched from the vehicle, where as the GTM (see below), the tow ball mass is transfered to the vehicle.
ATM = Caravan Tare Mass + Max Payload

Gross Trailer Mass (GTM)

Hitched to Tow Vehicle

GTM is the weight of the fully loaded trailer imposed on the trailer’s axle when it is coupled to the tow vehicle.  Note, this is when the Caravan or Trailer is carrying its maximum load.

GTM will always be less than ATM as some of the trailer weight is transferred to the tow vehicle when the trailer is hitch to it.


Tare Mass or Tare Weight

Tare Mass is the weight of the Caravan or Trailer straight off the manufacturer line. This is the total weight of an empty caravan.  It does not include people, luggage, water or gas.

Where this gets tricky, is the Tare Mass recorded on your plate, may not be your actual weight.  Additional aftermarket accessories are not included in this measurement from the manufacturer, as they are not original components.  This includes items like toolboxes, solar panels added and even additional batteries.

Trailer Ball Download or Load (TBD)

TBD should also be around 10-15 percent of the Gross Trailer Mass (GTM).  Other terms refering to TBD are tow ball mass or tow ball weight

Ball Load is the amount of weight the fully laden trailer imposes (vertically) on the tow bar of the tow vehicle.  Trailer Ball Load is not a specification defined by the trailer manufacturer – it is the actual weight imposed on the rear of the tow vehicle and as such is a function of the trailer’s axle position and the manner in which it is loaded.

While there is no requirement to list a trailer’s Ball Load, it can be measured at a weighbridge by disconnecting the fully laden trailer from the tow vehicle and resting only the trailer’s draw bar (via the jockey wheel) on the scales. Alternatively, some caravan dealers have special ball mass scales for this purpose. Some caravan plates show Ball Load at Tare.  This is the ball load when the caravan is empty.  It is not helpful in determining the ball load of the laden caravan. Ball Load can be measured at a weighbridge by disconnecting the fully laden trailer from the tow vehicle and resting only the trailer’s draw bar (via the jockey wheel) on the scales. Alternatively, low cost ball mass scales are available for this purpose.


Payload is the trailer’s carry capacity.  It’s the difference between its Tare Weight (un-laden weight) and its ATM. Payload is important for all trailers, however it is critical to campers and caravans, many of which have quite limited carrying capacity to start with.  Modifications and additions to the trailer can very quickly eat into its carrying capacity and result in an overloaded trailer, or the inability to legally carry necessary items. 

Max. Payload = ATM – Tare Mass – TBD