Caravan weights explained: Tare, ATM, GTM and ball weight in plain English

Caravan weights are the most-googled, worst-explained topic in Australian RV ownership. The terms aren’t intuitive, the consequences of getting them wrong are serious — insurance voids, NHVR fines, brake failure — and the manufacturers don’t always make it easy to find the numbers you actually need.

This guide breaks down the four numbers that matter, how they relate to your tow vehicle, and what to weigh before you sign on a van.

The four numbers you need to know

Tare mass

Tare is the empty weight of the caravan as it left the factory. It includes standard inclusions — the awning, the air conditioner, the fridge — but excludes anything you add yourself: water in the tanks, gas in the bottles, food, clothes, bedding, the kayak on the roof.

The catch: every manufacturer defines tare slightly differently. Some include full LPG bottles, some don’t. Some include a half-full hot water tank, some don’t. Always check the compliance plate on the actual van you’re buying — not the brochure — because the same model can vary by 50 kg or more depending on options fitted.

ATM — Aggregate Trailer Mass

ATM is the maximum legal total weight of the caravan, fully loaded, when it’s hitched up and ready to travel. It’s the hard upper limit. Go over ATM by a single kilogram and you’re driving an unregistered vehicle as far as the law is concerned — your insurance can be voided, you’re personally liable in any accident, and you’ll cop NHVR fines if you’re pulled over.

ATM includes everything inside the van and the ball weight transferred to the tow vehicle. It’s the number that matters for legal compliance.

GTM — Gross Trailer Mass

GTM is the maximum weight permitted on the caravan’s own axles when it’s hitched up. It’s always lower than ATM, because it excludes the ball weight (which is supported by the tow vehicle, not the caravan’s wheels).

A typical relationship: ATM 3,000 kg, ball weight 250 kg, GTM 2,750 kg.

GTM matters because the caravan’s tyres, axles and brakes are rated for the GTM, not the ATM. Overloading the GTM is what causes blown tyres on Stuart Highway shoulders.

Ball weight (towball download)

Ball weight is the downward force the caravan’s coupling exerts on your tow vehicle’s tow ball. It’s typically 8–12 per cent of the loaded ATM on conventional caravans, though some brands engineer for higher (15 per cent+) for towing stability.

Ball weight is critical because your tow vehicle has its own ball weight rating, separate from its towing capacity. A Toyota Prado rated to tow 3,000 kg may have a maximum ball weight of only 300 kg. Pull a van with 350 kg of ball weight and you’re overloaded on the rear axle — even though you’re within the towing capacity.

Payload: the number the brochure doesn’t show you

Payload is the gap between Tare and ATM — everything you can legally add to the van.

Payload = ATM − Tare

Worked example: a Jayco Silverline Outback 21.65-3 has a tare of around 2,704 kg and an ATM of 3,293 kg. Payload is therefore 589 kg.

That 589 kg has to cover water (160 kg if you fill both 95-litre tanks), gas (16 kg for two 9 kg bottles), food and drinks (~30 kg for a week), clothes and bedding (~40 kg), kitchen and bathroom gear (~25 kg), tools and recovery equipment (~30 kg), camp chairs and outdoor mat (~15 kg), and any aftermarket accessories like a Starlink dish, bike rack or generator.

It adds up fast. Many couples find their realistic loaded payload sits at 400–500 kg, leaving 90–190 kg of headroom. Families with kids’ gear regularly exceed factory payload — which is why upgrading the ATM rating (a chassis engineering exercise, not just a sticker change) is a common aftermarket modification.

How your tow vehicle’s numbers interact

Your tow vehicle has its own weight ratings that all have to be respected simultaneously:

GVM (Gross Vehicle Mass) — max legal weight of the tow vehicle itself, fully loaded, including ball weight from the van
GCM (Gross Combination Mass) — max legal weight of the tow vehicle plus the caravan, combined
Towing capacity — max ATM of any trailer the vehicle can tow (often varies braked vs unbraked)
Tow ball rating — max downward force on the tow ball

The trap most new caravanners hit is GCM. A Toyota LandCruiser 300 has a GVM of 3,280 kg and a towing capacity of 3,500 kg. Naive maths suggests it can tow a 3,500 kg van. But GCM is only 6,750 kg. Subtract the 3,280 kg GVM and you have only 3,470 kg left for the caravan — and that’s before you load the tow vehicle with passengers, fuel, water, recovery gear and an awning bag.

Real-world GCM compliance often means the heaviest van a 200/300-Series can legally tow is closer to 3,000–3,200 kg ATM, not the headline 3,500 kg towing capacity.

What to do before you sign

Get the actual compliance plate weights on the specific van you’re buying — not the brochure tare. Options like a slide-out, an extra battery or a washing machine add 30–80 kg each.
Calculate your realistic loaded weight using the payload formula above. Add 20 per cent for overestimate buffer.
Check your tow vehicle’s GVM, GCM, towing capacity and ball weight rating in the owner’s manual — not what the salesperson says.
Run the numbers through GCM math. If you’re tight, you’ll need either a heavier tow vehicle or a lighter van.
Use a public weighbridge after your first big shop-up. CMCA and Big Rigs operate certified weighbridges across Australia; many are free or under $20. This is the single most useful thing you can do in your first month of ownership.

NHVR enforcement and insurance

The National Heavy Vehicle Regulator’s Chain of Responsibility legislation makes weight compliance a shared legal duty — owners, drivers and (in commercial settings) employers can all be prosecuted for overweight breaches. Roadside weighing operations regularly run on major tourist corridors in winter; expect to be checked on the Nullarbor, the Stuart, and the Bruce Highway in peak season.

Insurance is the bigger practical issue. Most caravan policies include a clause that voids cover for any incident involving an overweight van. A rolled van that was 100 kg over ATM is your problem, not the insurer’s.