GarageCalc ATM: Aggregate Trailer Mass
ATM is the trailer's maximum loaded mass, including the mass that will rest on the tow vehicle. The actual whole-trailer mass can be measured unhitched. Do not confuse that measurement with the maximum printed on the plate.
GVM: Gross Vehicle Mass
GVM limits the loaded vehicle. When towing, the vehicle carries the coupling download as well as its own fuel, occupants, accessories and cargo. Check axle loads separately.
A fictional combination
Assume a 3,200 kg vehicle GVM, 3,500 kg braked towing rating, 3,200 kg trailer ATM and a trailer actually loaded to 3,000 kg, including 300 kg of ball load. Assume the vehicle already weighs 2,600 kg with its people, fuel and luggage before hitching.
- Trailer: 3,000 kg is 200 kg below the assumed trailer ATM and 500 kg below the assumed braked rating.
- Vehicle: 2,600 + 300 = 2,900 kg hitched vehicle mass, leaving 300 kg against the assumed GVM.
- Combined: 2,600 + 3,000 = 5,600 kg. Compare that with a separately verified GCM, counting ball load once.
These assumed ratings describe no particular model. The calculation does not check axle, tyre, coupling, brake or other equipment requirements.
Quick reference
| Term | Applies to | Meaning |
|---|---|---|
| ATM | Trailer | Maximum whole loaded trailer mass, including coupling download. |
| GTM | Trailer | Rated maximum mass supported by the trailer axles when coupled. Compare actual axle mass against this separate rating. |
| GVM | Vehicle | Maximum loaded vehicle mass, including coupling download when towing. |
| GCM | Combination | Maximum combined vehicle and trailer mass. |
Actual trailer axle mass equals actual whole-trailer mass minus actual ball load for this simple example. That arithmetic does not establish a missing plated GTM rating.
Definitions: Transport WA GVM and GCM FAQs. Loaded-trailer and equipment checks: NSW Government towing guidance. Checked October 25, 2026.
Frequently asked questions
Is ATM the same as towing capacity?
No. ATM is the trailer manufacturer's maximum loaded trailer mass. The vehicle towing rating limits what the tow vehicle may tow. Compare the actual loaded trailer mass with both limits and with the fitted equipment ratings.
Does the trailer count toward vehicle GVM?
The load transferred through the coupling is carried by the vehicle and counts toward its loaded mass. The trailer mass supported by its own axles is included in the combined-mass check, not added again to the vehicle GVM check.
Where do I find the correct limits?
Use the trailer plate and build documents for trailer ratings, and the vehicle plate, matching owner's manual or manufacturer specification for vehicle ratings. Check approved modification documents where applicable.