Trailer GVM, ATM & Towing Capacity Explained

Tare and payload: what the trailer weighs empty and what you can add

Tare is the mass of the trailer as it left the factory with nothing loaded on it, sitting on its own wheels. A box trailer, a plant trailer and a tandem car carrier all have very different tare weights, so it is the first number to check on the compliance plate rather than a figure you can guess.

Payload is simply the load you are allowed to carry: the difference between the trailer's maximum rated mass and its tare. If a trailer is rated to carry more when loaded than it weighs empty, that gap is your usable capacity for tools, timber, a mower, or a small machine. Every litre of fuel, every strap and every accessory bolted on after purchase eats into that payload, which is why people are often surprised how quickly a lightly loaded trailer fills its rating.

ATM: the trailer's total loaded limit

ATM stands for Aggregate Trailer Mass. It is the total weight of the fully loaded trailer when it is not coupled to the tow vehicle, standing on its own wheels and jockey wheel. In other words, ATM covers the tare plus the full payload, and it is the single most important number for deciding what the trailer can legally weigh when loaded.

ATM is set by the manufacturer and shown on the trailer's plate. You must never load a trailer past its ATM, even if the tow vehicle could physically drag more. Exceeding ATM overloads the axles, brakes and chassis the trailer was engineered around, and it puts you outside the terms of your registration and insurance.

GTM and towball download: how the weight is shared

GTM stands for Gross Trailer Mass. It is the weight carried by the trailer's own axles when it is loaded and coupled to the vehicle. GTM is always a bit less than ATM, because once the trailer is hitched, part of its weight transfers onto the tow vehicle through the coupling.

That transferred portion is the towball download, sometimes called ball weight. Roughly speaking, ATM equals GTM plus towball download. As a rule of thumb many trailers sit best with around ten percent of the loaded weight on the ball, and how you position the load changes that figure. Too little ball weight lets a trailer sway; too much can overload the rear of the vehicle. Both the towbar and the vehicle have a maximum towball rating you must stay within.

GVM: the vehicle side of the equation

GVM stands for Gross Vehicle Mass, and it belongs to the tow vehicle, not the trailer. It is the maximum the loaded vehicle itself may weigh, including passengers, fuel, cargo and the towball download pressing down on the rear. Hitching a heavy trailer adds ball weight to the vehicle, so a fully loaded ute can reach its GVM before the tray is anywhere near full.

There is a second figure to respect as well: GCM, or Gross Combination Mass, the most the vehicle and loaded trailer may weigh together. It is common to find that you cannot run the vehicle at full GVM and tow the maximum trailer at the same time, because the combination would exceed GCM. Reading the vehicle's own plate and handbook is the only reliable way to know these limits.

Braked vs unbraked towing capacity

Every vehicle has a maximum towing capacity, and it is usually quoted as two numbers: unbraked and braked. Unbraked capacity applies to trailers that rely only on the vehicle's brakes and is much lower. Braked capacity is higher and applies once the trailer has its own braking system, which Australian Design Rules require above a set trailer mass.

As trailer mass rises, the required braking gets more capable, moving from override brakes on lighter trailers to electric or other systems on heavier ones, with breakaway protection on the heaviest. Your legal towing limit is always the lowest of the relevant numbers: the vehicle towing capacity, the towbar rating, the towball rating and the trailer ATM. Match the trailer to the weakest link, not the strongest.

Working out what you can tow, step by step

Start with the trailer plate and note the ATM and towball download. Then check the vehicle plate and handbook for GVM, GCM, braked towing capacity and maximum towball load, and check the towbar for its own rating. Compare them: the trailer's ATM must not exceed the vehicle's braked towing capacity, the ball weight must sit within the towball limit, and the loaded vehicle must stay under GVM while the pair stays under GCM.

Because ratings are set by the trailer and vehicle makers and by ADR for road trailers, they are not figures to invent or round up. Trailer electricals such as lights and connectors also fall under Australian requirements, marked under the RCM framework, and trailers are registered through your state or territory authority. Swinton Equipment Depot supplies trailers and equipment Australia-wide from Port Botany in Sydney with flat-rate delivery to mainland destinations, so if you are unsure which rating suits your vehicle and load, check the plates and matching numbers before you buy.

Frequently asked questions