Caravan & Towing Weight Terms Explained: ATM, GTM, GVM, GCM & More

If you’ve ever stared at your caravan’s compliance plate and thought it looked like alphabet soup, you’re not alone. ATM, GTM, GVM, GCM, Tare, Payload, Tow Ball Mass — the list goes on. These terms aren’t just jargon dreamt up by manufacturers. They’re legal definitions that affect how much you can load, how safe your rig is, and whether your insurance will cover you in an accident.

Many first-time (and even seasoned) travellers get tripped up by the differences. Mix them up, and you could overload your vehicle or caravan without realising. But once you understand what each acronym means — and how they fit together — the whole picture becomes clear. In this blog, we’ll break down each term, explain why it matters, and give you practical tips to keep your Big Lap safe and legal.

Why Weight Terms Matter

Caravan weight terms might feel overwhelming, but here’s why they’re crucial:

• Safety: Exceeding limits makes your rig unstable, harder to brake, and more prone to sway.
• Legality: Overloading can mean fines or being ordered off the road.
• Insurance: Exceed a limit and your insurer may refuse claims after an accident.

Getting these right is the difference between smooth towing and a roadside headache.

The Caravan Weight Terms

• Tare Mass: The caravan’s weight as built in the factory. No personal gear, no water, no gas (unless permanently fitted). Aftermarket accessories often increase the Tare.
• ATM (Aggregate Trailer Mass): The maximum allowed weight of your caravan when fully loaded and unhitched — includes everything inside plus tow ball load.
• GTM (Gross Trailer Mass): The maximum allowed weight on your caravan’s wheels when it’s hitched (ATM minus tow ball load).
• Payload: The carrying capacity of your caravan: Payload = ATM − Tare. Accessories and options eat into this number fast.

The Vehicle Weight Terms

• Kerb Mass: The vehicle’s weight with fuel and standard equipment, but no passengers or extra load. Sometimes called “curb weight.”
• GVM (Gross Vehicle Mass): The maximum the loaded vehicle can weigh — includes passengers, gear, fuel, and the caravan’s tow ball load.
• Axle Loads: Each axle has its own maximum capacity. Overloading one axle (often the rear when towing) can be dangerous and illegal.

The Combination Terms

• Tow Ball Mass (TBM): The downward weight the caravan puts on your tow ball. Ideally 6–10% of the caravan’s loaded weight. Too light = sway, too heavy = overloaded rear axle.
• GCM (Gross Combined Mass): The maximum combined weight of your loaded tow vehicle and loaded caravan together.

Here’s how these terms fit together in practice:

Common Misunderstandings

• ATM vs GTM vs GVM: Many confuse these three. Remember: ATM = caravan max unhitched, GTM = caravan on wheels hitched, GVM = vehicle max loaded.
• Tare vs Payload: Tare is the empty van. Payload is how much you can add.
• “Marketing” towing capacity: Vehicle ads often highlight max towing, but you must also consider GVM, GCM, and tow ball limits.

How to Find & Check Your Numbers

It’s not enough to guess — here’s how to be sure:

1. Check compliance plates: Both caravan and vehicle plates list Tare, ATM, GTM, GVM, and tow limits.
2. Read the vehicle handbook: Includes GVM, GCM, axle loads, and towing specs.
3. Visit a weighbridge: Load your rig as you’d travel, then get real-world numbers. Mobile weighing services can come to you.
4. Keep a log: Write down your figures so you don’t need to recalc each trip.

Handy Glossary (Copy or Print)

• Tare Mass: Empty caravan as built.
• ATM: Max caravan weight loaded and unhitched.
• GTM: Max caravan axle weight when hitched.
• Payload: ATM − Tare (your gear allowance).
• Kerb Mass: Vehicle with fuel, no people/gear.
• GVM: Max loaded vehicle weight.
• Axle Loads: Max per-axle capacity.
• Tow Ball Mass: Caravan downward force on tow ball.
• GCM: Max combined car + van weight.