TLDR: A caravan has two numbers that decide what you can carry – its tare (the empty weight, set at the factory) and its payload (everything you pack). Together they make up its ATM. Your tow vehicle has its own set of limits: braked towing capacity, maximum tow ball download, GVM and GCM. Get all of them right and towing is safe, legal and stress-free. Not sure where your rig sits? Our free caravan weight calculator does the maths for you.
Caravan weights are one of the most confusing parts of buying a van, and one of the most important to get right. A string of three-letter codes stamped on a little metal plate decides whether your setup is safe, legal and covered by insurance. The good news is that once someone explains it in plain English, it clicks. Here is the whole picture, from the team at Explorer Caravan Sales in Adelaide.
Why caravan weights actually matter
Weights are not just paperwork. Load your van past its limits and three things happen at once. Your braking distance stretches out, your van is far more likely to sway on the open road, and if the worst happens your insurer can walk away because you were over your legal weight. Police can weigh your rig at a checkpoint too, and the fines are not small.
Get your weights right, on the other hand, and you can stop thinking about them. You pack the kids, the bikes, the camp kitchen and the water, hitch up, and point the car at the horizon knowing everything is where it should be. That is the whole idea of a getaway – the freedom to explore your own backyard without a number in the back of your mind.
The trick is understanding a handful of terms – a few on the van, and a few on the car. We will take them one at a time.
Tare weight: what your van weighs empty
Tare is your caravan’s empty weight – the way it left the factory, before you add a single thing. It is the baseline everything else is measured from.
The first thing to know is that tare is not a number you can change. It is set at the factory: once the van is finished it goes on the scales, and whatever it weighs that day is the figure stamped on the plate. Two vans off the same production line can come out slightly different. So when you are comparing vans, treat tare as a fixed fact about that particular van, not something to negotiate.
Here is the part most buyers miss. Under the national standard, tare is measured with the fuel and fluid tanks essentially empty and only the standard fitted equipment on board. For a caravan that generally means your fresh water tanks and gas bottles are counted as empty. So the moment you fill a water tank or clip on a full gas bottle, you are eating into your carrying capacity. Water alone weighs one kilogram per litre, so a couple of 95-litre tanks is nearly 200 kilograms before you have packed a fork.
There is a second catch. The tare figure on the plate reflects the van as the manufacturer built it. If dealer-fitted extras, an awning, a bike rack or a second spare have gone on since, your real empty weight is higher than the plate says. That is exactly why we always suggest weighing your loaded van rather than trusting the numbers on paper.
Payload: what you are actually allowed to pack
Payload, sometimes called carrying capacity, is the weight you are allowed to add to the van. This is the number you do control. The sum is simple:
Payload = ATM minus Tare.
Everything you load counts toward it: water, gas, food, clothes, tools, the annexe, the generator, the box of who-knows-what in the front boot. This is the number that runs out fastest, and water is the silent culprit. Fill three tanks, add a couple of full gas bottles, load the pantry and the camp gear, and a healthy-looking payload can be gone before the family climbs in.
A quick illustrative example: say a van has a tare of 2,000 kilograms and an ATM of 2,500 kilograms. That leaves 500 kilograms of payload. Sounds generous, until 190 kilograms of water and 20 kilograms of gas take the first slice. Knowing your real payload before you shop, and before every big trip, is what keeps you legal.
ATM: the big number on the plate
ATM stands for Aggregate Trailer Mass. It is the maximum your fully loaded caravan is allowed to weigh, measured when the van is not hitched to the car – standing on its own wheels and jockey wheel with everything packed.
Think of ATM as tare plus payload. It is the ceiling for the van on its own, set by the manufacturer, and you must never load past it. When you are comparing vans, the gap between the tare and the ATM is the payload you have to play with, so it is worth looking at closely.
GTM and ball weight: where the weight sits when you hitch up
Here is where a lot of people tie themselves in knots, so this graphic lays it out.
When you hitch the van to the car, its weight splits across two points. Most of it stays on the van’s own wheels – that portion is the GTM, or Gross Trailer Mass. The rest presses down on the tow ball – that is your ball weight (also called tow ball download).
So the relationship is:
ATM = GTM + Ball weight.
Ball weight matters for how the rig tows. As a rule of thumb you want it around 10 per cent of your van’s ATM. Too little and the van is prone to sway; too much and you overload the back of the car. Because it is set by how you pack – gear loaded ahead of the axles pushes it up, gear behind pulls it down – it is worth checking. Two limits apply here: the ball weight must not exceed what your caravan’s coupling is rated for, or your tow vehicle’s maximum tow ball download (often 350 kilograms, or 10 per cent of towing capacity, whichever is lower – check the car’s plate).
Your tow vehicle: towing capacity, GVM, GCM and the trap
The van is only half the story. Your tow vehicle carries its own set of limits, and this is where the costliest mistakes hide. Four numbers to know:
Kerb weight is the car empty – the car’s equivalent of the van’s tare.
Braked towing capacity is the most your car is rated to tow with a braked trailer behind it. It is the headline number in the brochure, and the one most buyers check first.
Maximum tow ball download is the most weight the car and its tow bar are allowed to carry on the ball. It is often 350 kilograms, or 10 per cent of towing capacity, but it varies by vehicle – and it is easy to exceed on a big van packed nose-heavy.
GVM (Gross Vehicle Mass) is the most your loaded tow vehicle is allowed to weigh – passengers, fuel, cargo and, importantly, the ball weight pushing down from the van all count toward it.
GCM (Gross Combination Mass) is the most the fully loaded car and fully loaded van are allowed to weigh together.
Now the trap that catches even experienced travellers. You would think if the car is under its GVM and the van is under its ATM, you are fine. Not always. GCM is usually less than the GVM and the ATM added together. In plain terms: you often cannot load both the car and the van to their individual maximums at the same time. GCM is the real ceiling for the whole rig, and it is the number most people never check.
One more myth worth killing. A car rated to tow 3,500 kilograms does not mean you can hook up a 3,500-kilogram van and load the car to the roof. Once you add passengers, fuel, gear and the ball weight, the GCM usually runs out well before you get there. Always match the van to the real, loaded numbers – not the headline towing figure.
One thing worth knowing if your numbers are tight: some popular tow vehicles can be fitted with a certified GVM or GCM upgrade, which legally lifts those limits. It is an engineered modification that has to be approved and certified for your state, not a sticker, so it needs to be done properly through a recognised specialist – but it can be the difference between a setup that works and one that does not. Worth asking about before you rule a van out.
How to check your real weights
Two habits keep you safe. First, load the van the way you actually travel – water, gas, gear, the lot – and take it over a public weighbridge. That gives you your true ATM, GTM and ball weight instead of a plate that assumes empty tanks. Do the same with the car so you can check GVM and GCM.
Second, do the maths before you commit to a van or a trip. That is exactly what our free caravan weight calculator is for. Punch in your van’s tare, ATM and ball weight and your car’s towing capacity, GVM, GCM and kerb weight, and it tells you at a glance whether your setup stays inside every limit. It is the quickest way to turn all this into a simple yes or no.
If the numbers are tight, that is worth knowing before you buy. Our team can walk you through the plates on any van in the yard and help you find one whose payload suits how your family really travels.
The rules behind these definitions come from the national Vehicle Standards Bulletin 1, and towing laws are enforced by your state transport authority – in South Australia that is mySAGOV / the Department for Infrastructure and Transport. It is always worth a quick check before a big trip.
Frequently asked questions
What is the difference between ATM and GTM?
ATM is the total weight of your fully loaded van when it is standing on its own, unhitched. GTM is the weight carried on the van’s wheels once it is hitched to the car, because the tow ball then takes a share of the load. GTM is always a little less than ATM, and the difference is your ball weight.
How do I work out my caravan’s payload?
If the payload is not stated outright on your caravan’s VIN or compliance plate, subtract the tare (empty weight) from the ATM. Both of those figures are on the plate. The result is the total weight you can add in water, gas, food and gear. Remember that water and gas are usually counted as payload, not tare, so they use it up fast.
Can I change my caravan’s tare weight?
No. Tare is set at the factory, where the finished van is weighed before it leaves. It is a fixed fact about that particular van. What you can control is the payload – what you choose to load in, and how you distribute it.
What is a good ball weight for a caravan?
Aim for around 10 per cent of your van’s ATM. Much less than that and the van can start to sway; much more and you risk overloading the rear of your tow vehicle. You can nudge it up or down by where you load heavy items relative to the van’s axles. Check your car as well as your van: your tow vehicle has a maximum tow ball download of its own, often 350 kilograms or 10 per cent of towing capacity, and it is listed on the vehicle’s plate.
Can I tow a caravan up to my car’s full towing capacity?
Usually not once it is loaded. The braked towing capacity is a headline figure. In practice your Gross Combination Mass, GVM and tow ball download limits will cap you well below it once the car and van are packed. Always check the loaded numbers rather than the sticker. If you are close, some vehicles can take a certified GVM or GCM upgrade to lift those limits legally – it has to be an approved, engineered job, so speak to a recognised specialist.
Do I really need to visit a weighbridge?
It is the only way to know your true weights. The compliance plate assumes empty tanks and factory-standard fit-out, so a loaded, accessorised van almost always weighs more. A quick weighbridge visit, fully packed, tells you exactly where you stand.
Ready to check your setup?
Weights should never be the thing that holds up your next trip. Run your numbers through our caravan weight calculator, then come and see the team at Explorer Caravan Sales in Gepps Cross. We will help you match a van to the way your family travels and make sure your rig is safe, legal and ready for the road. Call us on (08) 8278 5771 or drop into the yard – we would love to help you plan your next adventure.