GVM & GCM Calculator — Check Your Towing Weights Before You Hit the Road

Most Australian caravanners have heard the terms GVM, GCM and ATM. Far fewer actually know their numbers — and fewer still have checked whether their loaded setup is legal before they leave the driveway.

We learned the hard way that towing capacity is not a single number. It’s four different limits that all have to be met simultaneously — and exceeding any one of them means you’re illegal on the road, regardless of how the others look. After our own professional weigh, we built this calculator so you can check your setup before it becomes a problem on the road.

Enter your vehicle and caravan details below to get your safety analysis instantly.

Understanding Your Towing Numbers

GVM — Gross Vehicle Mass This is the maximum legal weight of your tow vehicle when fully loaded. It includes the vehicle itself, every passenger, all your gear, fuel — and critically, the tow ball weight from your caravan. The tow ball weight is often the figure people miss. It counts as part of your vehicle’s weight and eats directly into your GVM payload. You’ll find your GVM on the compliance plate inside the driver’s door jamb.

GCM — Gross Combination Mass This is the maximum combined weight of your fully loaded vehicle and fully loaded caravan together. It’s the limit most people overlook — and it’s often the one that catches you out. You can be under your GVM and under your ATM individually, and still be over your GCM. All four limits have to be met at the same time. Your GCM is in your vehicle’s owner’s manual or on the compliance plate.

ATM — Aggregate Trailer Mass This is the maximum legal weight of your caravan when fully loaded — everything inside including water, gas, food, gear and accessories. It’s set by the caravan manufacturer and stamped on the compliance plate on the A-frame or drawbar. Your loaded caravan must never exceed this figure, full stop.

Tare Weight This is the empty weight of your caravan as it left the manufacturer. The difference between tare and ATM is your payload — the total weight of everything you can add to the van. For full-time travellers this number is often tighter than expected. Our Sunny has a tare of around 2,200kg and an ATM of 3,000kg, which sounds like 800kg of payload — but by the time you add water, food, clothes, tools, solar equipment and everything else you live with, that gap closes faster than you’d think.

Why Ball Weight Matters Your tow ball weight — the downward pressure the caravan applies to your vehicle’s tow ball — should sit between 8 and 12 percent of your loaded caravan weight. Too light and the caravan becomes unstable and prone to sway at highway speeds. Too heavy and you’re overloading your vehicle’s rear axle and tow bar. It’s one of the most important numbers in your setup and one of the easiest to get wrong without realising.

The Number One Rule Never guess. A calculator gives you an estimate based on the numbers you enter — but those numbers are only as accurate as what you put in. The only way to know your actual loaded weights is to use a qualified vehicle and caravan weighing specialist. We do ours at least once a year and after any significant gear changes. It’s the best hundred dollars you’ll spend on your setup.

Keep Your Setup Safe — More From Retired Rascals

If the numbers from the calculator raised any questions about your setup, these guides are the right next step:

• Caravan Tyre Pressure Guide — Safe PSI for bitumen, gravel, corrugations and sand
• Tyre Pressure Monitoring System Review — Why a TPMS is the one piece of safety gear we’d never travel without
• Why Every Caravanner Needs a Torque Wrench — Loose wheel nuts are one of the most preventable causes of wheel failures
• Caravan Pre-Trip Safety Checklist — The exact checks we do before every departure