Reading a weighbridge ticket
Three passes, and every plated limit comes from them.
Weigh the whole rig, then the car with the van still hitched, then the van alone.
Those three readings check GCM (Gross Combination Mass), GVM (Gross Vehicle Mass) and ATM (Aggregate Trailer Mass).
Updated . 8 min read • Technical Guide
Weigh the rig in three passes, and each figure checks a different plated limit: GCM, GVM and ATM.
GTM (Gross Trailer Mass) and your tow ball mass come from subtracting one pass from another.
- The three passes - whole rig, car alone still hitched, van alone (in that order, in one visit)
- The van's GTM - whole rig minus the car (subtracted, not read off a plate)
- Your tow ball mass - the van's ATM minus that GTM (subtracted, not read off a plate)
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Three passes, and every number falls out of them
Three passes over the same plate, in this order, give you every figure your compliance plates ask about. They take a few minutes longer than a single weigh.
Come loaded exactly the way you travel, with the tanks at your normal touring level and the people who normally ride in the car. Do the arithmetic in the car park afterwards rather than on the plate, so the queue keeps moving.
- Pass 1
The whole rig
- How to position it
- Car and van coupled, everything and everyone aboard, all wheels on the plate.
- What the reading is
- The combination mass: the total weight of the thing you drive down the highway.
- Which limit it checks
- The car's GCM, the most the car and van are rated to weigh together.
- Pass 2
The car, van still hitched
- How to position it
- Roll forward until only the car sits on the plate. The van stays coupled with its own wheels off the plate.
- What the reading is
- The loaded car with the tow ball load included, which is how the car actually travels.
- Which limit it checks
- The car's GVM.
- Pass 3
The van on its own
- How to position it
- Uncouple, wind down the jockey wheel, and put the van alone on the plate.
- What the reading is
- The van's real all-up weight, loaded the way you tour.
- Which limit it checks
- The van's ATM.
The two numbers that come from subtraction
Pass 1 minus pass 2 is the load sitting on the van's own axles, which is its GTM.
Pass 3 minus that GTM figure is your tow ball mass, measured rather than assumed.
That second line is worth the extra pass on its own. Ball mass is the figure most often estimated at a flat percentage, and it is the one that quietly spends the tow car's payload.
There is a print-and-keep version of this sequence in our printable checklists, sized for the glovebox.
Reading the ticket
Weighbridge tickets are written for freight, not for caravanners, so the wording can look stranger than the job you came to do. A docket usually carries a gross figure, and often a tare and a net alongside it.
Gross is simply what was standing on the plate at that moment, so on a caravan weigh the gross figure is your figure, every time. Tare and net belong to the freight job: an operator weighs a loaded truck, tips the load, weighs the empty truck, and the difference is the freight. Ignore both unless you paid for a second weigh yourself.
Caravan tare is a different thing again, and the word doing double duty here catches plenty of people out. On your compliance plate, tare means the van as the manufacturer weighed it before you owned it. Our caravan weights guide sets out each term, and the compliance plate guide shows where to find the plated figures you are comparing against.
Write the date and the time on the docket, note the tank levels, and keep it. A ticket you cannot place in time is a number without a rig attached to it, and the comparison a year later is the useful one.
Which number checks which limit
Your figures have to sit under every one of these at the same time. Being comfortably inside one limit does not buy any room in another.
GCM
The most the car and van may weigh together.
- The limit is printed on
- The car's placard or owner handbook.
- Your figure comes from
- Pass 1, straight off the ticket.
GVM
The most the loaded car may weigh, ball load included.
- The limit is printed on
- The car's compliance placard.
- Your figure comes from
- Pass 2, straight off the ticket.
ATM
The most the van may weigh, uncoupled and standing on its own jockey wheel.
- The limit is printed on
- The van's compliance plate.
- Your figure comes from
- Pass 3, straight off the ticket.
GTM
The share of the van's weight carried by its own axles when it is hitched up.
- The limit is printed on
- The van's compliance plate.
- Your figure comes from
- Pass 1 minus pass 2.
Tow ball mass
The downward load the van puts on the ball.
- The limit is printed on
- The towbar and the tongue, and often the car handbook as well.
- Your figure comes from
- Pass 3 minus your GTM figure.
Two more limits the ticket will not show you
A towbar carries its own rated capacity and its own maximum ball load, and those ratings can sit below what the car itself is rated to tow. Tyres carry a load rating too, and it applies per tyre rather than per axle. Both are worth reading off the hardware while the plates are fresh in your mind.
What to do if the rig is over
Plenty of rigs come off the plate over something the first time, and finding it in a weighbridge car park is a much better place to find it than the side of a highway. Work through these in order.
1. Check the ticket describes the rig you actually tour
A weigh taken with empty tanks, without the kids, or with the boat still at home is not the number to act on. If the load was not typical, the fix is another weigh, not a re-pack.
2. Name the limit that is over, because each one has a different fix
Over ATM and over GVM are different problems with different answers, and it is common to be inside one while sitting over another. Take them one at a time.
3. Over on the van (ATM or GTM): take weight out of the van
Water is usually the largest single lever, at one kilogram per litre, so a part-full tank changes the figure immediately. After that it is the gear that lives in the van year-round and never gets used.
4. Over on the car (GVM): move load, or carry less of it
Gear in the boot, a loaded roof rack, a bullbar, a second battery and the people in the seats all spend the car's payload, and so does the tow ball load. Shifting boot gear into the van helps the car and costs the van, so check both figures afterwards.
5. Over on the combination (GCM): weight has to leave the rig
GCM counts the car and the van together, so moving gear between them changes nothing. Something comes out, or the pairing itself is the problem.
6. Ball mass wrong: rearrange inside the van before you change anything else
Heavy items belong low and close to the axles. Load carried well behind the axles reduces ball mass and makes a van less settled at speed; load stacked over the drawbar sends the ball figure the other way.
7. Weigh it again once you have changed something
A re-pack you have not measured is a guess with more effort in it. The second visit is usually the short one.
Why it is worth acting on
Loading a trailer past its ATM is an offence, and an overweight rig can put an insurance claim in question after a crash. Those are facts about the rules rather than a verdict on your van, and the ticket is the only thing that settles which side of them you are on.
If the numbers say the pairing itself is the problem rather than the packing, the tow calculator shows which vans sit inside your car's published limits, and the payload calculator estimates where a load will land before you pack it.
Weighbridge ticket questions, answered
How do I work out my tow ball mass at a weighbridge?
Subtract the van's axle load from the van's all-up weight. Weigh the whole rig coupled, then the car alone with the van still hitched: the difference is the load on the van's axles, its GTM. Weigh the van again uncoupled for its ATM, and ATM minus GTM is the tow ball mass. No separate ball scale needed.
My van is over its ATM. What happens now?
Loading a trailer past its ATM is an offence, and an overweight rig can put an insurance claim in question after a crash. The practical fix is to take weight out of the van and weigh it again. Water is the biggest single lever at one kilogram per litre, and the rest is usually gear that lives in the van and never gets used.
Can I get the plated ATM raised instead of unloading?
Plated figures are set by the manufacturer, and changing one is a road authority process rather than a paperwork exercise. The rules and the engineering evidence required differ by state, so ask your state road authority what applies to your van before you count on it.
Do I need all three passes?
One pass with the whole rig on the plate tells you whether you are inside GCM, and nothing else. The car alone gives GVM and the van alone gives ATM, and those two are the limits most rigs run closest to. Three passes is a few extra minutes for the complete picture.
The ticket says gross, tare and net. Which one is my weight?
Gross is what was standing on the plate at that moment, so on a caravan weigh that is your figure. Tare and net exist for freight, where an operator weighs a truck full, tips the load and weighs it empty. If a docket shows only a gross figure, that is the number you compare against the plate.
Haven't weighed it yet?
Our weighbridge finder lists every licensed public weighbridge in Australia, straight from the national register. Filter it by state, suburb or postcode, then come back here with the ticket.