GarageCalc 
Why caravan payload disappears so fast
A typical 2,200 kg tare van with a 2,800 kg ATM has 600 kg of payload. Fill two 95 L water tanks (190 kg), two 9 kg gas bottles (about 18 kg full), a fridge full of food, bedding, clothes, chairs, tools and a couple of jerry cans, and 600 kg is gone before the kids' bikes go on the back. The number that matters is the loaded mass, not the tare.
Once you know your loaded van, check the whole combination against your car's ratings in the caravan towing calculator: braked rating, ball limit, GVM and GCM all have to hold at once.
How it's calculated
Frequently asked questions
What is the difference between tare, ATM and GTM?
Tare is the caravan empty, as built (no water, no gas, no gear). ATM (Aggregate Trailer Mass) is the maximum it may weigh fully loaded and unhitched. GTM (Gross Trailer Mass) is the maximum weight on the caravan's own axles when hitched, which is ATM minus the ball weight resting on the car. Tare and ATM are on the compliance plate; payload is simply ATM minus tare.
Does water count toward my caravan payload?
Yes, every litre. Water weighs 1 kg per litre, so full 2 x 95 L tanks add 190 kg before you have packed anything. Most Australian vans have far less payload than owners assume once water, gas, batteries and a full fridge are counted.
How much ball weight should my caravan have?
Most Australian vans tow best with roughly 8 to 12 percent of the loaded mass on the ball. Too little invites sway, too much overloads the car. Ball weight changes as you load: heavy items ahead of the axle add to it, behind the axle subtract. Weigh it loaded with a ball scale, the plate figure is unladen.
My loaded van is over ATM. What now?
Remove weight or reduce water, that is the only legal fix short of an engineering upgrade. An ATM upgrade may be possible through an approved engineer if the chassis, axles and tyres are rated for it, but it is a certified modification, not a sticker. Towing over ATM is unroadworthy and can void insurance.