Towing legality and towing stability are two different tests and a vehicle can pass one while failing the other. The legal test is the manufacturer braked towing limit and the gross train weight, both on the vehicle plate. The stability guideline is the ratio of laden trailer weight to towing vehicle kerbweight, where 85 per cent is the widely quoted comfort threshold rather than a law.
The weight that matters for the trailer is its actual laden weight, not its unladen figure. A caravan quoted at 1200 kg empty will often be at or near its maximum technically permissible laden mass once the awning, water, gas and everything else is aboard, which is the number to enter here. Gross train weight then limits the loaded car and loaded trailer added together, and is frequently the binding constraint on a well-loaded car.
A 1,350 kg trailer behind a car with an 1,800 kg limit uses 75.0 per cent of that limit, leaving 450 kg of headroom. Against a 1,650 kg kerbweight the ratio is 81.8 per cent, just inside the 85 per cent guideline. Noseweight is a separate limit again, typically 5 to 7 per cent of the trailer weight and capped by the towbar and the vehicle.
The formula
towing limit- Manufacturer braked towing capacity in kilograms
laden trailer- Actual weight of the loaded trailer or caravan, the MTPLM
kerbweight- Weight of the towing vehicle, used for the 85 per cent guideline
gross train weight- Maximum for loaded car plus loaded trailer combined
How to use this calculator
- Take the braked towing limit and gross train weight from the vehicle plate or handbook.
- Enter the actual laden weight of the trailer, not its unladen figure.
- Enter the kerbweight and choose which stability guideline to test against.
- Enter the noseweight limit and the measured noseweight to check the coupling load.
Worked examples
A 1,350 kg caravan behind a 1,650 kg car
The trailer uses 75.0 per cent of the 1,800 kg braked limit, leaving 450 kg spare. The kerbweight ratio is 81.8 per cent, inside the 85 per cent guideline of 1,402 kg. Combined, the outfit weighs 3,000 kg against a 3,200 kg train weight — 200 kg of headroom.
Where the gross train weight bites first
Load the same car with four passengers and luggage at 320 kg and the combination becomes 3,320 kg, which is 120 kg over the 3,200 kg train weight even though the trailer is still well inside the 1,800 kg towing limit. The plate figure, not the towing limit, is the binding constraint.
How to read your score
Frequently asked questions
Is the 85 per cent rule actually the law?
No. It is a long-standing guideline from towing organisations, not a legal requirement. The legal limits are the manufacturer braked towing capacity, the gross train weight and your licence entitlement. The 85 per cent figure exists because a trailer approaching or exceeding the towing vehicle mass is far more prone to snaking.
Should I use the unladen or the laden trailer weight?
Laden, every time. Use the MTPLM for a caravan or an actual weighbridge ticket for a trailer. Water, gas, awning, bikes and the contents of every locker routinely add 150 to 300 kg over the advertised unladen figure.
How do I measure noseweight?
With a proper noseweight gauge under the hitch, with the trailer level and on level ground. Aim for 5 to 7 per cent of the trailer weight, and never exceed the lower of the towbar rating, the vehicle limit and the trailer coupling rating. Too little noseweight is as destabilising as too much.
Does the gross train weight include passengers?
Yes. It covers the fully loaded vehicle plus the fully loaded trailer, so passengers, luggage, a roof box and a full tank all count. This is why a family car that can legally tow on paper often cannot once it is actually packed for a holiday.
Does my driving licence allow this combination?
Entitlement depends on when you passed your test and the rules of your country, and it changed in Great Britain in December 2021. Check your licence categories against the combined maximum authorised mass rather than assuming, because the penalties for towing outside entitlement are severe.
Towing margin reference
| Laden trailer | Limit used | Spare capacity | Kerbweight ratio | 85% verdict |
|---|---|---|---|---|
| 700 kg | 38.9% | 1,100 kg | 42.4% | Within |
| 900 kg | 50.0% | 900 kg | 54.5% | Within |
| 1,100 kg | 61.1% | 700 kg | 66.7% | Within |
| 1,300 kg | 72.2% | 500 kg | 78.8% | Within |
| 1,400 kg | 77.8% | 400 kg | 84.8% | Within |
| 1,500 kg | 83.3% | 300 kg | 90.9% | Above |
| 1,650 kg | 91.7% | 150 kg | 100.0% | Above |
| 1,800 kg | 100.0% | 0 kg | 109.1% | Above |
| 1,950 kg | 108.3% | -150 kg | 118.2% | Above |
The 85 per cent guideline for this car is 1,402 kg. Substitute your own plate figures in the inputs.