ATM vs GTM: the trailer's two ratings

ATM and GTM are the two mass ratings on every Australian trailer plate. One includes the tow ball mass and one does not. Here is how to tell them apart.

Two ratings, one trailer

Every trailer or caravan built for Australian roads carries a plate listing three figures: tare, ATM and GTM. Tare is what the trailer weighs empty. ATM and GTM are ratings, maximums set by the manufacturer. The Department of Infrastructure's trailer bulletin (VSB1) says it directly: both are manufacturer-specified ratings, so they cannot be measured on a weighbridge. What you weigh is your trailer's actual mass, and you compare that against them.

The difference between the two is the tow ball.

ATM: aggregate trailer mass

ATM is the total mass of the trailer when carrying the maximum load its manufacturer recommends. It includes the part of the trailer's weight that presses down on the tow vehicle through the coupling.

Put another way, ATM is the whole trailer, on its own, unhitched. Transport for NSW describes weighing against ATM with the caravan unhitched and resting on its jockey wheel, so that the whole trailer, jockey wheel included, is on the scale.

GTM: gross trailer mass

GTM is the maximum mass the trailer may put on the ground through its own tyres, fully loaded, while hitched to the tow vehicle. It excludes the tow ball mass, because that part of the load is being carried by the vehicle.

So GTM is always less than ATM for a conventional trailer with a tow ball. Some of the trailer's weight is always on the car.

How they relate

When the trailer is hitched, its total mass splits two ways:

down through the coupling onto the tow vehicle: the tow ball mass (TBM)
down through the trailer's own axles: the load that GTM limits

ATM = GTM + tow ball mass. VSB1 adds a qualifier: this holds for the ratings when the trailer is at maximum load and loaded evenly, or as the manufacturer specifies. Load the same trailer differently and the split changes, even though the total stays the same.

The payload is ATM minus tare

VSB1 puts it simply: ATM minus tare mass is the most the trailer can carry. A caravan with a 2,300 kg tare and a 2,800 kg ATM can carry 500 kg, and that 500 kg covers everything added after it left the factory.

That includes the things people forget:

Water. VSB1 and Transport for NSW both say caravan water and waste tanks are measured empty for tare. A 100 litre tank adds 100 kg of load once full.
Gas bottles, batteries, awnings, annexes and anything fitted after purchase
Food, clothes, bikes, the generator, and whatever you pick up along the way

Two limits, two ways to go over

Because they measure different things, you can exceed one without exceeding the other. Invented figures, for illustration only:

	Rating	Actual
ATM (whole trailer, unhitched)	2,800 kg	2,760 kg
Tow ball mass	—	200 kg
GTM (trailer axles, hitched)	2,500 kg	2,560 kg

This van is under its ATM and over its GTM. The load has been packed too far back, so too little of it is on the tow ball and too much is on the trailer's axles. Moving heavy items forward, toward and over the axles, puts more on the ball and less on the axles. See front and rear axle limits for why position changes the split.

It works the other way too. A trailer packed heavily at the front can sit well within GTM while its tow ball mass exceeds what the vehicle or towbar is rated to carry.

Check all of them

Your trailer's actual mass must be within its ATM and its GTM, and its tow ball mass must be within the vehicle's, the towbar's and the coupling's ratings. Transport for NSW's advice is that the lowest applicable limit is the one that governs. Rules on registration, brakes and inspection by ATM band differ between states, so check with your state road authority.

Model your own rig

LoadSavvy's Load check models how your trailer's mass splits between the tow ball and its axles as you place each load. You can see ATM and GTM move together or apart. It is a planning tool, not a weighbridge reading.

Model your rig in the Load check →