What this calculator does
A tow vehicle has four separate limits and the one that stops you is rarely the one on the advertisement. Take a 2,200 kg vehicle rated to 3,000 kg gross, 5,500 kg combined and 3,000 kg of towing. Put 300 kg of people and gear in it, hitch a 2,500 kg trailer with a ten per cent ball download, and it is legal with 250 kg of payload spare. Add another 300 kg to the trailer and the combination limit is exceeded, even though the towing capacity still says there is room.
The reason is the tow ball download, which is weight the trailer puts on the vehicle rather than on its own axles. At ten per cent of a 2,500 kg trailer that is 250 kg out of an 800 kg payload, before a single passenger gets in. That is why the advertised towing capacity so often cannot be used: reaching it would need a payload the vehicle does not have.
The formula
The ball download is trailer weight × ball percentage, and it counts against the vehicle, not the trailer. The loaded vehicle is kerb weight + people and gear + ball download, which must stay under the gross vehicle mass. The combination is loaded vehicle + trailer weight, which must stay under the gross combination mass. The heaviest trailer allowed solves all three limits at once, allowing for the fact that a heavier trailer also puts more weight on the ball.
| Term | Meaning |
|---|---|
| Kerb weight | The vehicle empty, with fluids and usually a full tank. |
| Gross vehicle mass | The most the vehicle itself may weigh, loaded, with the ball download included. |
| Gross combination mass | The most the vehicle and trailer may weigh together. Usually the binding limit. |
| Tow ball download | Weight the trailer presses onto the tow ball. Comes out of vehicle payload. |
The inputs explained
| Field | What to enter |
|---|---|
| Kerb weight (kg) | Kerb weight from the manual, or better, from a weighbridge with the vehicle as you normally use it. |
| Gross vehicle mass limit (kg) | Gross vehicle mass limit from the compliance plate. |
| Gross combination mass limit (kg) | Gross combination mass limit from the manual. It is usually less than the two limits added together. |
| Maximum towing capacity (kg) | Maximum towing capacity, braked, from the manual. |
| People and gear in the vehicle (kg) | People, luggage, tools, water, accessories and anything bolted on since purchase. |
| Loaded trailer weight (kg) | The trailer loaded as you will tow it, not its empty or nominal weight. |
| Tow ball download (% of trailer) | Ball download as a percentage of trailer weight. Ten per cent is a common target; measure it rather than assume. |
When to use it
Before buying a caravan
Enter the vehicle you own and the loaded weight of the van you are considering. The heaviest trailer allowed is the figure to shop against.
Packing for a trip
Add up what is really going in the vehicle, including water and the people. This is where the payload usually goes.
Checking a borrowed trailer
A plant or machinery trailer can carry far more download than a caravan. Measure the ball weight rather than assuming ten per cent.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How heavy a trailer can this vehicle actually tow?
Only the loaded trailer weight changes.
| Loaded trailer | Payload left in the vehicle | Against the combination limit | Tow ball download |
|---|---|---|---|
| 1,500 kg | 350 kg | within, 1,350 kg spare | 150 kg |
| 2,000 kg | 300 kg | within, 800 kg spare | 200 kg |
| 2,500 kg | 250 kg | within, 250 kg spare | 250 kg |
| 2,800 kg | 220 kg | OVER by 80 kg | 280 kg |
| 3,000 kg | 200 kg | OVER by 300 kg | 300 kg |
What does load in the vehicle do to towing?
The trailer is fixed and only what you put in the vehicle changes.
| People and gear | Payload left in the vehicle | Against the vehicle mass limit | Heaviest trailer these limits allow |
|---|---|---|---|
| 0 kg | 550 kg | within, 550 kg spare | 3,000 kg |
| 150 kg | 400 kg | within, 400 kg spare | 2,864 kg |
| 300 kg | 250 kg | within, 250 kg spare | 2,727 kg |
| 500 kg | 50 kg | within, 50 kg spare | 2,545 kg |
| 800 kg | 250 kg OVER | OVER by 250 kg | 0 kg |
Questions
Why does the ball weight count against the vehicle?
Because that is where it sits. The trailer carries most of its own weight on its own axles, but the download rests on the tow ball, which is part of the vehicle, so it uses vehicle payload exactly as a passenger would.
Why is the combination limit less than the two limits added up?
Because the vehicle has to be able to start, stop and control the whole rig. Manufacturers set a combination figure below the arithmetic sum on purpose, and it is the limit that binds first on most modern tow vehicles.
What ball download percentage should I use?
Caravans are commonly around ten per cent and some trailers run quite differently. It is worth measuring with ball scales, since too little download causes sway and too much eats payload you may not have.
Can a GVM upgrade fix this?
Sometimes, and it is jurisdiction-specific. A GVM upgrade may raise the vehicle limit without raising the combination limit, in which case the ceiling on the trailer does not move at all. Check both plates before paying for one.
Is this the legal answer?
No, it is the arithmetic. Limits, licence classes and enforcement differ by jurisdiction, and the only figures that count are the ones on your compliance plate and in your manual, confirmed on a weighbridge.
For what the trip itself costs, see trip fuel cost and true cost per mile. Power to weight ratio covers how the rig will actually pull.