Caravan & Towing Weights Explained: GVM, ATM, GTM, TBM & GCM

This page is the reference for the whole vocabulary — caravan weights explained once, in one place. Every term gets a plain-English definition, the formula behind it where one exists, and a link to a deeper guide where the detail lives. One worked rig runs through every number on the page, so the arithmetic always reconciles. Start here, then follow the links down.

Every one of these is a ceiling set by a manufacturer, not a target to load up to. The sections below take them side by side: vehicle, caravan, the ball between them, and the combination.

The worked rig used on this page

Every figure below comes from one illustrative rig, so the numbers always add up — the same dual-cab ute as our Gross Vehicle Mass guide, now with a van behind it. The figures are illustrative; your compliance plates and owner's manual are the real authority.

For the trip that runs through this page, the ute carries 520 kg of people, accessories and gear, and the van is packed to its plate: 3,000 kg all-up with 300 kg on the ball.

Which weights belong to the tow vehicle?

Three numbers cover the vehicle's side: its empty weight (kerb or tare), its loaded ceiling (GVM), and the payload between them. Kerb weight is the empty vehicle with all fluids and a full tank; tare is the leaner factory figure with about 10 L of fuel aboard. Know which one a spec sheet is quoting before you do any arithmetic — the difference is mostly fuel, and diesel runs about 0.85 kg per litre, so a full 80 L tank against tare's 10 L is roughly 60 kg of phantom payload if you mix them up.

GVM is the ceiling for everything the vehicle carries: occupants, accessories, cargo — and, when towing, the ball mass pressing down on the back. The deep dive on what counts towards it, where to find it and when a GVM upgrade makes sense is in the Gross Vehicle Mass guide.

One usage note that clears up half the confusion online: GVM belongs to the vehicle. Forum shorthand for a van's "GVM" is really its ATM.

What is the payload formula?

Payload = GVM − kerb (or tare) weight — it's the number you actually manage, because the GVM itself is fixed by the manufacturer. On the worked ute: 3,050 − 2,130 = 920 kg for people, accessories, gear and ball mass combined.

Towing reshapes it. Hitch a van that puts 300 kg on the ball and the working version becomes:

Payload once hitched = GVM − kerb − tow ball mass 3,050 − 2,130 − 300 = 620 kg for everything and everyone else.

The van has its own version: van payload = ATM − tare. This van: 3,000 − 2,400 = 600 kg — and it goes faster than it sounds, because water is payload at 1 kg per litre, and gas, bedding, the awning, tools and the second battery all queue ahead of your luggage.

Which weights belong to the caravan?

Three numbers again: tare (empty), ATM (the loaded ceiling) and GTM (the share on its wheels) — joined by the tow ball mass. ATM, the Aggregate Trailer Mass, is the most the van may weigh fully loaded and unhitched — everything aboard, including the weight that will rest on the ball. It's stamped on the van's compliance (VIN) plate, usually on the drawbar. GTM, the Gross Trailer Mass, is the most that may sit on the van's wheels once hitched, after the ball mass has moved across to the vehicle.

One formula joins them: ATM = GTM + TBM.

Two details worth keeping: axle, suspension and tyre ratings on a van are typically specified against GTM, not ATM; and GTM moves as you load — shift weight forward of the axles and the ball takes more while the wheels take less, with the ATM unchanged. For ATM upgrades and the rest of the trailer-side story, see the Aggregate Trailer Mass guide.

Where does tow ball mass fit?

Tow ball mass (TBM) is the share of the van's weight pressing down on the vehicle's tow ball — and it counts on both sides of the rig. Unhitched, it's part of the van's ATM. Hitched, it transfers to the vehicle and counts towards the GVM. That double life is why it's the most common blind spot in the whole vocabulary.

Two rules govern it. First, the guidance band: 8–12% of the van's loaded weight, with about 10% a common target — enough on the nose to keep the van stable, not so much that it crushes the vehicle's rear. Second, the vehicle's maximum ball loading, listed in the owner's manual: on the worked ute it's 300 kg, and this rig's 300 kg ball sits exactly on it.

The ball also works a lever. The hitch sits behind the rear axle — the rear overhang plus roughly 150 mm of hitch — so ball weight multiplies through it like the long end of a seesaw. On the worked ute (1,250 mm overhang, 3,000 mm wheelbase) the factor is 1 + (1,250 + 150) ÷ 3,000 ≈ 1.47: the 300 kg ball puts about 440 kg on the rear axle and lifts roughly 140 kg off the front. The axle-by-axle detail is in the GVM guide linked above.

How does GCM cap the whole combination?

Gross Combination Mass (GCM) is the most your tow vehicle and van may weigh together — set by the vehicle manufacturer, and usually less than GVM and ATM added together. On the worked rig: GVM 3,050 + ATM 3,000 = 6,050 kg, but the GCM is 5,950 kg. Max both plates at once and you're over, with neither individual limit broken.

Adding up the combination has one trap: the ball. On the road, the combination's weight is the vehicle on its axles plus the van on its axles. Count it either way — unhitched vehicle plus full van (2,650 + 3,000), or hitched vehicle plus GTM (2,950 + 2,700) — and this rig comes to 5,650 kg both times. Add the full ATM to the hitched vehicle's weight instead and you've counted the 300 kg ball twice, inventing an overweight problem you don't have.

GCM is also where the brochure tow rating quietly shrinks. At full GVM (3,050 kg), the ute can put at most 5,950 − 3,050 = 2,900 kg on the van's axles — with a 300 kg ball, that's a van of about 3,200 kg all-up, well short of the 3,500 kg headline. You usually can't max the GVM and the tow rating on the same trip.

Is braked towing capacity the number that matters most?

No — it's the biggest number on the brochure, and on this rig it's the limit with the most spare room. Braked towing capacity is the heaviest trailer the vehicle is rated to tow with trailer brakes fitted, measured against the van's actual loaded weight. The worked ute is rated to 3,500 kg and tows a 3,000 kg van — 500 kg of headroom — while the ball and the van's own ATM sit at zero margin.

One buying note from the compliance side: if a van's plated ATM exceeds the vehicle's braked rating, you can still tow it legally while its actual weight stays under the rating — but after an incident, proving that falls on you. Matching the plated ATM to the rating is the cleaner buy. The full story, including what you should tow rather than what you can, is in the braked towing capacity guide, and the model-by-model numbers — braked rating, GVM, GCM and payload at full tow — live in towing capacity by vehicle.

Does your rig clear every limit at once?

A legal tow is five checks cleared at the same time, not one number obeyed. Here's the worked rig against all five:

Two margins on this rig are zero. That's legal — but it means one extra jerry can on the drawbar, or a water tank topped up at the last servo, tips a limit. Axle ratings are a sixth check on top of these, and the lever effect above is why the rear one usually matters most. The difference between a rig that's legal at the weighbridge and one that's legal for the whole trip is margin.

What happens if you tow over a limit?

An over-limit combination is unroadworthy, and the consequences scale with the outcome. At the roadside, weights for light combinations are enforced by state and territory road authorities and police: fines vary by jurisdiction, and a defect notice can park the van until it's fixed. After an incident, your insurer can examine whether the rig was legal — overloading gives a Product Disclosure Statement plenty to work with.

When it goes properly wrong, it becomes criminal. In January 2019, a driver who'd left Tamworth was towing a caravan later found to be more than 800 kg over its maximum weight. On a descent on the Oxley Highway near Walcha, the van began to sway, jack-knifed, and the tow vehicle hit a tree; two passengers died. The driver was convicted of dangerous driving occasioning death, and in 2022 an appeal court increased the sentence, finding the original "patently insufficient" (per court reporting in The Northern Daily Leader). Overweight is not a technicality on a long descent — brakes fade and sway builds exactly when you have the least room to correct either.

Where do you find your rig's real numbers?

The limits live on the plates; the actuals only come off a scale. The tow vehicle's GVM is stamped on its compliance plate (driver's door jamb or engine bay), with GCM, braked capacity and the ball maximum in the owner's manual. The van's ATM and GTM are on its compliance (VIN) plate. If a rating has been changed — a GVM or ATM upgrade — the new figure sits on a modification plate.

Actual weights are a different exercise. A public weighbridge gives you honest totals; a mobile weigh service breaks the rig down properly. A professional weigh report also introduces two terms worth recognising: GVM (your vehicle's actual weight, unhitched) and GVM in combination (its actual weight hitched, ball mass aboard). The in-combination figure is the one that's usually over, because the ball arrives after everything else is packed.

Loading does the rest: heavy items low and over the axles, ball mass held in the 8–12% band, and remember that water and gas are payload too — a full 180 L tank is 180 kg, and two of them spend 360 kg of the worked van's 600 kg allowance (our caravan water tanks guide runs those numbers). The loading habits that quietly wreck good rigs are collected in our common caravanning mistakes guide.

See all five limits in one place

Five ceilings, one rig: that's the shape of the problem this page leaves you with. None of the limits above can be checked on its own — the same 300 kg of ball mass sits inside three of the five sums at the same time, so a change on one side of the coupling moves the arithmetic on the other.

That whole-rig view is what the loadmate app is built around. The free Can I Tow It? check answers the pairing question in one pass: it weighs any vehicle-and-van combination against the braked rating, ball limit, GVM and GCM at once, then returns one of three verdicts — pass (well matched), caution (careful) or fail (no) — with the reasons behind each. It's free and needs no account. From there, loadmate Pro can hold your own rig: every load you add or move re-runs the same five numbers, so the picture on this page stays current as you pack.

Related guides

• Gross Vehicle Mass (GVM) Explained — the vehicle-side deep dive: what counts, the rear-axle lever effect, and when an upgrade makes sense
• Aggregate Trailer Mass (ATM) Explained — the van-side deep dive, including ATM upgrades
• Braked Towing Capacity Explained: A Legal Ceiling, Not a Target — why the brochure number isn't the plan
• Caravan water tanks — how much water to carry, and what it does to your payload
• Common caravanning mistakes — the loading and weight habits that catch people out
• Towing capacity by vehicle — your model's real numbers: braked rating, GVM, GCM and payload at full tow