Trailer weight numbers tell you two important things: whether your vehicle can tow the trailer and how much cargo the trailer can legally carry. The key terms are tare, payload, Aggregate Trailer Mass, Gross Trailer Mass, and ball load.
Why trailer weights matter
If you do not check the trailer's ratings and expected loaded weight, you could overload the trailer, exceed your vehicle or towbar limits, or create an unsafe ball load. Owners should provide the key weight details in the listing, but hirers should confirm them against the compliance plate / VIN plate before leaving.
Where to find the numbers
Look for the compliance plate / VIN plate, usually fitted to the drawbar, A-frame, or side of the trailer. It should show the trailer's ATM and may also show tare and GTM.
Local Trailer Hire listings should summarise the key weights where the owner has supplied them. Confirm the information against the plate at pickup, particularly if the trailer has a cage, ramps, spare wheel, or other accessories that may have been added later.
Older trailers may not display all of these figures. Ask the owner for clarification rather than estimating them.
Tare, payload, ATM, GTM, ball load
| Term | Meaning |
|---|---|
| Tare | The trailer’s stated unladen mass. Later accessories or modifications may increase its actual empty weight. |
| Payload | The trailer’s carrying capacity. It is generally calculated as ATM minus tare. |
| ATM | Aggregate Trailer Mass: the maximum permitted total mass of the loaded trailer, including the part of its weight supported through the coupling by the tow vehicle. |
| GTM | Gross Trailer Mass: the maximum mass transmitted to the ground through the trailer’s tyres when it is coupled to a tow vehicle. |
| Ball load | The actual downward load the trailer places on the towball. It changes depending on the trailer’s total weight and how the load is positioned. |
In simple terms:
- ATM = weight carried by the trailer wheels + weight carried through the coupling
- Payload = ATM − tare
ATM is not the same as GTM
ATM covers the entire loaded trailer, including the portion supported by the tow vehicle through the coupling. GTM covers only the portion supported by the trailer's tyres while it is hitched.
For example, a loaded trailer weighing 750 kg in total might place 675 kg through its tyres and 75 kg through the towball. Its actual loaded mass is still 750 kg.
At the trailer's rated maximum mass, the numerical difference between its ATM and GTM ratings represents the portion supported through the coupling. That does not tell you the trailer's current ball load, which changes with the actual load and its position.
Maximum ratings are not actual weights
ATM and GTM are maximum manufacturer ratings. They do not tell you what the trailer weighs today.
Tare is also a stated unladen figure and may no longer reflect the trailer's actual empty weight if accessories or modifications have been added. When weight margins are tight, use a weighbridge rather than relying only on the plate.
Worked example
Assume the compliance plate / VIN plate shows:
- Tare: 280 kg
- ATM: 750 kg
Its stated maximum payload is:
750 kg ATM − 280 kg tare = 470 kg payload
This means the trailer, its accessories, and everything loaded into it must not exceed 750 kg in total.
If later accessories have increased its actual empty weight to 310 kg, its remaining real-world carrying capacity would be closer to:
750 kg − 310 kg = 440 kg
If the trailer is unbraked, your vehicle's unbraked towing capacity must also be sufficient. Check the vehicle handbook and towbar rating rather than assuming every car can tow 750 kg.
Practical cargo examples
| Cargo | Weight risk |
|---|---|
| Branches and green waste | Bulky but often relatively light |
| Furniture | Weight varies; secure against movement |
| Soil, sand and gravel | Very dense; trailer payload can be reached quickly |
| Concrete and rubble | Extremely easy to overload |
| Water | Approximately 1 kg per litre, excluding the container |
Common trailer-weight mistakes
- Assuming any trailer that fits the towball is suitable for the vehicle.
- Treating the stated tare as the trailer's guaranteed current empty weight.
- Loading up to the payload figure without allowing for added cages, ramps, or accessories.
- Filling the trailer by volume without estimating the cargo's weight.
- Loading too far rearward and creating insufficient ball load and instability.
- Loading too far forward and exceeding the vehicle or towbar ball-load limit.
- Assuming the difference between rated ATM and GTM is the trailer's current ball load.
- Forgetting that soil, rubble, sand, and water are much heavier than branches or household goods.
Choosing the right body style still matters after weights check out. See box vs cage trailer.
Trailer weight FAQ
Is trailer payload ATM minus tare?
Usually, yes. However, accessories or modifications added after the stated tare was established may reduce the trailer's remaining real-world carrying capacity.
Is ATM the actual weight of the trailer?
No. ATM is the trailer's maximum permitted loaded mass. The trailer may weigh less than its ATM when empty or partially loaded.
Does ball load form part of ATM?
Yes. ATM includes the portion of the trailer's loaded mass supported through the coupling by the tow vehicle.
How do I know the actual loaded trailer weight?
Use a suitable weighbridge or weighing service. Plate figures show ratings and stated weights, not necessarily the trailer's current loaded mass.
