Towing weight terms read like alphabet soup: ATM, GTM, GVM, GCM, tare, payload, ball weight. They’re not intuitive, they overlap in confusing ways, and getting them wrong has real consequences. This guide explains every weight term you’ll encounter as a caravanner, what it means in practice, and which numbers actually matter for your setup.
Caravan Weight Terms
Tare Weight: The weight of your caravan as it leaves the factory, with no water, no gas, no personal belongings, and no aftermarket accessories. Think of it as the empty weight. This number is on the compliance plate. It’s your starting point, not your real-world weight.
ATM (Aggregate Trailer Mass): The maximum your caravan is legally allowed to weigh when fully loaded, including water, gas, food, clothing, and everything else. This is the big one for the caravan. It’s on the compliance plate and it’s a legal limit. Exceed it and you’re breaking the law.
GTM (Gross Trailer Mass): The maximum weight supported by the caravan’s axles when it’s connected to the tow vehicle. The difference between ATM and GTM is the tow ball weight (because when hitched, some weight transfers from the caravan’s axles to the vehicle’s rear axle via the tow ball). GTM is always less than ATM.
Payload: The difference between tare weight and ATM. This is how much stuff you can actually put in the caravan. A van with a 2,000kg tare and 2,500kg ATM has 500kg of payload. That 500kg needs to cover water (a full 200L tank weighs 200kg), gas bottles, food, clothing, tools, and everything else. Payload disappears fast.
Tow Ball Weight (Ball Weight / Tongue Weight): The downward force the caravan exerts on the tow ball. Ideally this should be 8 to 12% of the caravan’s loaded weight. Too light and the van sways; too heavy and you overload the vehicle’s rear axle. This is one of the most important numbers for safe towing.
A caravan with 2,500kg ATM should have a tow ball weight between 200kg and 300kg (8–12%). You can adjust ball weight by moving heavy items forward or backward in the van. Heavy items towards the front of the van increase ball weight; towards the rear decreases it.
Vehicle Weight Terms
Kerb Weight (Tare Weight): The weight of the vehicle as manufactured, with all standard equipment, full fuel tank, and all fluids, but no passengers or cargo. Found in the owner’s manual or on the compliance plate.
GVM (Gross Vehicle Mass): The maximum the vehicle is legally allowed to weigh when fully loaded with passengers, cargo, fuel, accessories, and tow ball weight. The GVM is on the compliance plate and is a legal limit. Note that the tow ball weight counts towards GVM because it’s pressing down on the vehicle’s rear axle.
Payload (Vehicle): The difference between kerb weight and GVM. This is how much weight you can add to the vehicle: passengers, luggage, aftermarket accessories (bull bar, roof rack, long-range fuel tank), and tow ball weight. A vehicle with 2,400kg kerb weight and 3,100kg GVM has 700kg of payload. Four adults (320kg), a full long-range fuel tank (+50kg), aftermarket accessories (+80kg), luggage (+100kg), and a 250kg tow ball weight equals 800kg. Already over.
Maximum Towing Capacity: The maximum weight the vehicle can legally tow. This is separate from GVM and is rated by the vehicle manufacturer. A vehicle with a 3,500kg maximum towing capacity can tow a caravan with an ATM up to 3,500kg (assuming all other weight limits are also met).
GCM (Gross Combined Mass): The maximum combined weight of the vehicle (at GVM) plus the trailer (at ATM). This is the overall limit for the whole rig. GCM is often the limiting factor that people overlook: your vehicle might be rated to tow 3,500kg and the caravan might have a 3,000kg ATM, but if the GCM is 6,000kg and the vehicle weighs 3,200kg loaded, you can only tow 2,800kg before hitting the GCM limit.
How They All Fit Together
Here’s a practical example. Say you have a Toyota LandCruiser 300 Series towing a 21-foot caravan:
Vehicle specs: Kerb weight 2,590kg. GVM 3,300kg. GCM 7,000kg. Max towing 3,500kg. Max tow ball weight 350kg.
Caravan specs: Tare 2,200kg. ATM 2,800kg. Payload 600kg.
With the vehicle loaded (4 adults, gear, full fuel): approximately 3,100kg. Tow ball weight from the loaded caravan: 280kg. Vehicle loaded weight including ball weight: 3,380kg. That’s over the 3,300kg GVM.
The towing capacity (3,500kg) and ATM (2,800kg) are fine. The GCM (3,100 + 2,800 = 5,900kg against 7,000kg limit) is fine. But the GVM is over. This is exactly how experienced caravanners with big, capable vehicles still end up exceeding their limits. The GVM is often the bottleneck.
Solutions: reduce vehicle load, get a GVM upgrade, or accept that full water tanks in the caravan (which increase ball weight) need to be managed carefully.
How To Weigh Your Setup
The only way to know your actual weights is to weigh the rig at a weighbridge. Most regional towns have a public weighbridge (often at the local council depot or grain handler). It costs $10 to $30 per weigh.
Weigh in this order:
First, weigh the whole rig connected (vehicle + caravan). This gives you your combined mass to check against GCM.
Second, unhitch the caravan and weigh the vehicle alone (with passengers and luggage as you’d travel). This gives you your vehicle’s actual loaded weight to check against GVM.
Third, weigh the caravan alone. This gives you the caravan’s actual loaded weight to check against ATM.
For tow ball weight, you can use a ball weight scale ($50–$100) at home, or some weighbridges can measure axle weights that let you calculate it.
- ATM is the caravan’s maximum loaded weight. GVM is the vehicle’s maximum loaded weight. GCM is the combined maximum. All three are legal limits.
- Payload disappears fast. Water alone can account for 200kg of your caravan’s payload.
- Tow ball weight should be 8–12% of the caravan’s loaded weight. Adjust by moving heavy items forward or backward in the van.
- GVM is often the limiting factor, not towing capacity. The tow ball weight counts towards your vehicle’s GVM.
- Weigh your rig at a public weighbridge, loaded as you’d travel. It’s the only way to know your real numbers.
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