Understanding your weights
Six terms cover almost everything. Here they are, on one diagram.
GVM, GCM, ATM, GTM, tow ball mass and axle loads all describe different parts of the same setup. Once you can see where each one sits, the terminology stops being confusing.
The numbers on the diagram match the numbers in the list below.
The six terms, in plain English
The most your vehicle is allowed to weigh once it is loaded.
That includes you, your passengers, fuel, luggage, accessories, tools, the canopy, the fridge, and the tow ball load once a trailer is connected. GVM applies to the loaded vehicle, not the empty one.
The most your caravan or trailer is allowed to weigh, on its own.
Think of it standing in the driveway, unhitched, with everything in it: water, gas, food, bedding, camping gear, batteries. That whole weight is the ATM.
The part of the trailer's weight that sits on its own wheels once it is hitched up.
When you hitch up, some of the trailer's weight lifts off the jockey wheel and presses down on your tow ball instead. What is left, carried by the trailer's own tyres, is the GTM.
The weight the trailer pushes down onto your tow ball.
This is the piece people miss. It does not vanish when you hitch up. It becomes part of what your vehicle is carrying, which means it counts towards your GVM and it lands mostly on the rear axle.
How much of that weight is sitting on each axle.
Each axle has its own rating. A vehicle can be under its GVM overall and still be carrying too much on one axle, which is why the total on its own does not tell you enough.
The most the vehicle and trailer are allowed to weigh together.
GCM is its own separate limit set for your vehicle. It is not simply your GVM added to your ATM.
One more term worth knowing
What you have left to work with.
The gap between what your vehicle or trailer actually weighs right now and the limit it is allowed to reach. Every accessory, every passenger, every litre of water comes out of that gap.
Three things people get wrong
An axle or a tyre can still be over its own rating.
The vehicle, its axles, the tow ball load and the GCM all still need checking.
Once hitched, the tow ball load is already in the vehicle's weight. Adding both counts it twice.
Now find out your numbers
Knowing the terms is useful. Knowing what your own setup actually weighs is where it becomes practical. A PROWEIGHED assessment measures it and explains the results in the same plain language.