What is the difference between ATM and GTM on a trailer?

ATM (Aggregate Trailer Mass) is the maximum the trailer may weigh fully loaded and uncoupled, standing on its own jockey wheel. GTM (Gross Trailer Mass) is the maximum allowed on the trailer's own axles while it is hitched, and is lower than ATM because some weight transfers to the tow ball once coupled.

How do I work out a trailer’s payload?

Payload is ATM minus tare. Tare is what the empty trailer weighs as built; ATM is stamped on the compliance plate. Payload is not printed on the plate, so you have to calculate it yourself.

What is tow ball download?

It is the share of the loaded trailer pressing down on the tow ball. Around ten per cent of the loaded trailer is the usual rule of thumb. Too little and the trailer sways; too much and the back of the tow vehicle squats. The tow bar and the vehicle each have their own maximum ball weight.

The numbers that matter

ATM, GTM, tare and payload explained

Every legal trailer carries a compliance plate stamped with these figures. They are the difference between a trailer that suits the job and one that is overloaded the first time you use it, so they are worth ten minutes of your attention.

Tare: What the empty trailer weighs, as built. A tandem cage trailer weighs considerably more empty than a small single-axle box trailer, and every kilo of tare is a kilo you cannot carry.
ATM — Aggregate Trailer Mass: The most it is allowed to weigh fully loaded, measured uncoupled and standing on its own jockey wheel. This is the headline number, and the one to compare against your vehicle’s towing capacity.
GTM — Gross Trailer Mass: The most that is allowed to sit on the trailer’s own axles while it is hitched up. It is lower than ATM, because once coupled some of the weight transfers onto the tow ball.
Payload: ATM minus tare — what you can actually load. It is the number nobody prints on the plate and the one most people assume is bigger than it is.
Ball weight (tow ball download): The share of the load pressing down on the tow ball. Roughly ten per cent of the loaded trailer is the usual rule of thumb. Too little and the trailer sways; too much and the back of the car squats. Your tow bar and vehicle both have their own maximum ball weight.

A worked example. A trailer plated at 2,000 kg ATM with a tare of 450 kg leaves 1,550 kg of payload. Wet soil runs to roughly 1,600 kg per cubic metre — so a single cubic metre of it will put that trailer at its limit. Volume fills up long after weight does, which is why a half-empty-looking trailer can still be overloaded.

In practice

Where people get caught

Dense loads reach the limit first

Soil, sand, gravel, pavers, firewood and concrete all hit the payload ceiling while the trailer still looks like it has room. If you regularly carry any of them, size on weight, not on volume.

Bulky loads are the opposite problem

Green waste, boxes, furniture and packaging run out of space long before weight. That wants height and sides, not a heavier chassis — see choosing a trailer.

Accessories eat payload

A cage, a canopy, a toolbox, a spare wheel and a jockey wheel are all tare. Fit them and the payload drops by exactly that much, permanently.

The plate is the authority, not the ad

“2-tonne trailer” in a listing might mean ATM, GTM or the axle rating. Read the compliance plate. If a used trailer has no legible plate, treat that as a serious problem — see registration.

Getting these figures right at the design stage costs nothing. A trailer built to order can be specified with the payload you actually need — retro-fixing an under-specified trailer usually cannot be done at all.

More guides

Keep reading

How much can your car actually tow? Braked and unbraked towing capacity, tow bar and ball weight ratings, GCM, and when a trailer legally needs its own brakes. Choosing a trailer size and type How to pick a trailer size you will not regret, when to go tandem instead of single axle, and which of the seventeen trailer types suits your job. Galvanised or painted? Rust, frost and the Canberra climate Why hot-dip galvanising outlasts paint, where trailers actually rust, and what frost, UV and the run to the coast do to a trailer around Canberra. Registering a trailer around Canberra Which side of the border you register with, what a new trailer needs before it can be registered, and what to check when buying a used trailer. Servicing and repairing a trailer What a trailer service covers, how often to do it, the parts that actually wear, and why a trailer specialist is better set up for the work than a general workshop.

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