ATM, GTM, Tare & Payload Explained
If you've looked at a trailer's compliance plate or a spec sheet, you've seen a wall of acronyms — ATM, GTM, Tare, Payload, TBM. They decide how much you can legally carry, whether your car can tow it, and whether your trailer needs brakes. Here's what each one means in plain English, how they fit together, and how to check your own setup.
This is a general, independent explainer based on the Vehicle Standards Bulletin 1 (VSB 1) and Queensland Department of Transport and Main Roads (TMR) guidance. It is not legal or engineering advice — always confirm the figures on your trailer's compliance plate and, if in doubt, with TMR or a certified RPEQ engineer.
The five terms
Tare Mass (empty weight)
Tare is the weight of the trailer empty — no load, but with its standard equipment (and often the jockey wheel and spare). It's the starting point for everything else. A typical 7×5 ft single-axle box trailer might have a Tare of around 300 kg.
ATM — Aggregate Trailer Mass
ATM is the maximum total weight of the fully loaded trailer when it is not coupled to your vehicle — i.e. standing on its own jockey wheel and axle(s). It's set by the manufacturer and stamped on the compliance plate. You must never exceed it.
GTM — Gross Trailer Mass
GTM is the maximum weight carried on the trailer's own axle(s) while it is coupled to the tow vehicle. When you hitch up, some of the weight transfers onto the tow ball, so the GTM is always less than the ATM.
Tow Ball Mass (TBM / tow ball download)
Tow Ball Mass is the downward weight the coupling pushes onto your vehicle's tow ball. As a rule of thumb it sits around 10% of the loaded trailer weight, and it must stay within your vehicle's maximum tow ball rating (check the vehicle placard/handbook).
Payload — what you can actually carry
Payload is the bit you care about at the tip or the hardware store: the maximum load you can put in the trailer.
How they fit together
Two simple relationships tie it all up:
- ATM = GTM + Tow Ball Mass
- Payload = ATM − Tare
So the tow ball takes some of the load off the axles (GTM), but it all still counts toward the ATM.
A worked example
Say a trailer's compliance plate reads:
- ATM: 750 kg
- Tare: 300 kg
Then:
- Payload = 750 − 300 = 450 kg you can legally load.
- If your loaded tow ball mass is ~75 kg (10%), the GTM on the axles is 750 − 75 = 675 kg.
- Your tow vehicle must be rated to tow at least 750 kg and accept at least 75 kg on the ball.
Overload any one of these — total (ATM), axle (GTM), or ball (TBM) — and you're non‑compliant, even if the others are fine.
Why it matters
- Legality & insurance: exceeding the ATM or GTM is an offence and can void your insurance after an incident.
- Braking: brake requirements step up with the trailer's loaded mass. A trailer over 750 kg must have brakes on at least one axle, and a trailer over 2,000 kg must have brakes on all wheels plus a breakaway system that automatically applies the trailer's brakes if it disconnects from the tow vehicle. (The Compliance Auditor works out which applies to your setup.)
- Matching your car: the Tow Ball Mass and ATM must fit your tow vehicle's rated towing capacity and maximum tow ball download — not just the trailer's own limits.
- Safety: too little tow ball weight causes dangerous trailer sway; too much overloads the rear of the car.
Quick FAQ
Is ATM or GTM the bigger number? ATM. GTM is ATM minus the tow ball mass, so it's always smaller.
Which number do I compare to my car's towing capacity? Your vehicle's braked/unbraked towing capacity is compared to the ATM, and its max tow ball download to the Tow Ball Mass.
How do I find my Tare and ATM? They're stamped on the trailer's compliance plate (usually on the drawbar or front). Payload is ATM − Tare.
What's a safe tow ball weight? Aim for roughly 10% of the loaded trailer weight, kept within your vehicle's ball rating.
Check your own trailer against these limits. Enter your load, tow vehicle capacity and ball rating into the free Compliance Auditor to see your required ATM, braking and axle requirements instantly — no sign‑up.
Independent reference only. Figures are based on VSB 1 and QLD TMR guidelines and your trailer's compliance plate; verify with TMR or a certified RPEQ engineer before purchasing, building or registering a trailer.