GarageCalc 
What the four checks mean
Trailer rating: the caravan's ATM must not exceed the vehicle's braked capacity. Ball limit: the loaded ball weight must not exceed the towbar and vehicle limit. GVM: vehicle + occupants + cargo + ball weight must stay under GVM. GCM: the loaded vehicle and loaded caravan together must stay under GCM. Fail any one and the combination is unroadworthy, no matter how comfortably the other three pass.
How it's calculated
Frequently asked questions
Where do I find my vehicle's numbers?
Braked rating, GVM and GCM are on the compliance/tyre placard (door jamb) and in the owner's manual; the ball limit is in the towing section of the manual or on the towbar plate. Kerb (tare) weight is on the placard or registration papers. When the towbar plate and the manual disagree, the lower number wins.
Where do I find the caravan's ATM and ball weight?
ATM is on the caravan's compliance plate. Ball weight is not fixed: it changes with how you load the van. The plate lists the unladen ball weight; weigh yours loaded (ball scale or weighbridge) because that is the number that counts.
What ball weight should I aim for?
Australian vans tow most stably with roughly 8-12 percent of the loaded van's mass on the ball. Too little causes sway; too much eats your payload and can overload the rear axle. Stay inside the vehicle's ball limit either way.
My combination fails one check. What are my options?
Load less into the van (lowers ATM), shift weight in the van (changes ball weight), carry less in the car (helps GVM and GCM), or pick a different tow vehicle. GVM upgrades exist for many utes and wagons, but they rarely raise GCM, so they fix payload problems, not towing problems.