Caravan weights explained: tare, ATM, GTM and ball weight

Weight is the number one thing that goes wrong with caravans. A van that’s over its limits handles badly, wears out tyres and bearings, and can void your insurance. The good news: once you know what five numbers mean, checking is simple.

The numbers on the van

Every new van sold in Australia carries a compliance plate (sometimes called the VIN plate) near the drawbar or door. It lists these figures:

Term	What it means
Tare	What the van weighs empty: no water in the tanks, no gas, no luggage.
ATM (Aggregate Trailer Mass)	The most the van may weigh fully loaded, measured unhitched. This is the limit you must stay under.
GTM (Gross Trailer Mass)	The most the van’s axles may carry when it’s hitched up. It’s lower than ATM because some of the weight sits on the towball.
Ball weight (towball download)	The weight the van’s coupling pushes down onto the towball.
Payload	ATM minus tare. Everything you add has to fit in here: water, gas, food, clothes, bikes, the annexe, the spare wheel if it wasn’t counted.

A common surprise: the plate’s tare may have been measured before the dealer fitted extras such as solar panels, a second battery, a bike rack or a toolbox. Each of those comes out of your payload. Ask for the weighbridge ticket from delivery, or weigh it yourself.

How much payload is enough?

Full water tanks alone weigh a kilo per litre, so two 95-litre tanks are 190 kg. Two gas bottles, a week of food, clothes, camp chairs, an annexe and some tools add up fast. Many couples carry 350–450 kg; families more. A van with a 300 kg payload is easy to overload.

The numbers on the tow vehicle

Your car or ute has its own limits, listed in the owner’s manual and sometimes on a plate in the door frame:

Term	What it means
Braked towing capacity	The heaviest trailer (with brakes) the vehicle may tow. Compare it with the van’s actual loaded weight, not its tare.
Maximum towball download	The most weight the towbar may carry. Your van’s ball weight must be under this, and so must the towbar’s own rating.
GVM (Gross Vehicle Mass)	The most the tow vehicle may weigh, including passengers, cargo, accessories and the ball weight pressing on it.
GCM (Gross Combination Mass)	The most the car and van together may weigh.

You need to be under every one of these at the same time. GCM is the one people miss. A ute might be rated to tow 3,500 kg and also have a GCM of around 6,000 kg. If the loaded ute weighs 3,000 kg, the van can only weigh 3,000 kg, not 3,500.

Ball weight: the stability number

For a stable caravan, aim for a ball weight of around 10% of the van’s loaded weight. Too little and the van is prone to sway; too much and the rear of the tow vehicle squats, lightening the front wheels and the steering.

Where you pack things changes ball weight. Heavy items low and over the axles; nothing heavy in the front boot or on the rear bumper. A full front water tank can add a lot of ball weight on its own.

Brakes

In Australia, a trailer with a loaded weight over 750 kg must have brakes. Over 2,000 kg, the brakes must be operable from the driver’s seat (in practice, electric or electronic brakes with a controller in the tow vehicle) and the van needs a breakaway system that applies the brakes automatically if it comes off the towball. Check your state’s transport department for the details that apply to you.

How to check your rig

1. Load up as you’d travel. Full water, gas, food, gear, passengers.
2. Weigh the van on its own on a public weighbridge, unhitched with the jockey wheel on the plate. That figure must be under the ATM.
3. Weigh the car and van together, then each axle group while still hitched: the car’s axles must be under its GVM, the van’s axles under its GTM, and the whole rig under GCM.
4. Measure ball weight with a towball scale, or work it out from the weighbridge figures.
5. Write the numbers down and compare each one with its limit.

Mobile weighing services visit caravan parks and give you every figure at once, if you’d rather not do the arithmetic.

US terms

If you’re reading American sources: UVW (unloaded vehicle weight) is tare, GVWR (gross vehicle weight rating) is the trailer’s ATM, CCC (cargo carrying capacity) is payload, and tongue weight is ball weight. US guidance for travel trailers is a tongue weight of 10–15% of the trailer’s weight.

The build estimator gives a rough tare, ATM and ball weight for a new build. For a van you already own, the compliance plate and a weighbridge are the only numbers that count.