GVM & GCM Compliance Calculator
Check a car-and-trailer combination against its Gross Combination Mass (GCM) — the legal ceiling for the tow vehicle and loaded trailer together. Optionally check the vehicle's own GVM too. Exceeding either limit is illegal and voids insurance, so it pays to confirm before you hitch up.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the loaded tow-vehicle mass (ideally a weighbridge figure with occupants, fuel and gear on board).
- Enter the trailer's ATM (Aggregate Trailer Mass) and the vehicle's GCM rating from the compliance plate.
- Optionally add the GVM rating to also flag whether the vehicle alone is overloaded, then read the combined mass, margin and verdicts.
How it works
Combined mass = loaded tow-vehicle mass + trailer ATM. The GCM margin = GCM rating − combined mass; a negative margin means the whole rig is over its GCM. If you supply the GVM, the tool separately compares the loaded vehicle mass against it, because GVM (vehicle alone) and GCM (vehicle plus trailer) are independent limits that must both be met.
Worked example
Worked example. A 4WD loaded to 3000 kg tows a van with an ATM of 2500 kg, and the vehicle's GCM is 6000 kg. Combined mass = 3000 + 2500 = 5500 kg, leaving a GCM margin of 500 kg — legal. Load the same rig to 3200 kg towing a 3000 kg ATM van and the combined mass is 6200 kg, which is 200 kg over the 6000 kg GCM.
Common mistakes
- Using the trailer's Tare (empty) mass instead of its ATM — the ATM is the fully loaded figure the GCM check requires.
- Assuming that being under GCM means you are legal, while the vehicle itself is over GVM — both limits apply at once.
- Estimating the tow-vehicle mass from the handbook kerb weight rather than weighing the fully loaded, occupied vehicle.
Frequently asked questions
What is the difference between GVM and GCM?
GVM (Gross Vehicle Mass) is the maximum the tow vehicle alone may weigh, fully loaded. GCM (Gross Combination Mass) is the maximum the tow vehicle and the loaded trailer may weigh together. You can be within GVM but over GCM, or vice versa, so both must be satisfied for the rig to be legal.
Why does towing a heavy van sometimes force me under my own GVM?
Many vehicles have a GCM lower than GVM + max tow rating added together. To stay under GCM while towing at the maximum ATM, you often have to carry less in the vehicle, effectively reducing your usable payload below the GVM figure.
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