Towing safely and legally comes down to a handful of figures. This page explains what they mean and where to find them. Our towing calculators page lets you work them out for your own vehicle and trailer.
D value
The D value measures the coupling strength needed between the towing vehicle and the trailer. It is worked out as D = 9.81 x T x C divided by (T + C), then divided by 1000, given in kN, where T is the towing vehicle's maximum authorised mass in kg and C is the trailer's maximum mass in kg. The towbar's D value, shown on its approval plate, must equal or exceed the D value your combination needs.
Noseweight
Noseweight is the downward force the trailer's coupling places on the towball. Use the lowest of three limits: the car's noseweight limit, the towbar's noseweight limit and the caravan or trailer's noseweight limit. Within that lowest figure, aim for around 7 per cent of the trailer's actual laden weight. Check the actual figure with a noseweight gauge rather than estimating it.
Gross train weight
Gross train weight is the maximum combined weight of the towing vehicle and trailer together, fully loaded. The actual weight of your car plus the actual weight of your loaded trailer must not exceed the car's gross train weight, which is shown on the vehicle's VIN plate.
The 85 per cent guideline
Caravan clubs recommend that a trailer's actual laden weight should not exceed 85 per cent of the towing vehicle's kerb weight, as guidance for less experienced towers. This is a guideline, not a legal limit.
Licence entitlement
Drivers who passed their car test from 1 January 1997 can tow trailers up to 3500kg maximum authorised mass on a standard category B licence, following the rule change in 2021, with no separate B+E test needed.
What the law requires
- Extension towing mirrors when the trailer is wider than the car's own rear view.
- A breakaway cable or secondary coupling fitted correctly.
- A number plate on the trailer matching the towing vehicle.
- Working trailer lights, including indicators, brake lights and rear fog light where fitted.
- A speed limit of 50mph on single carriageways and 60mph on dual carriageways and motorways.
- No towing in the outside lane of a motorway with three or more lanes.
Use our towing calculators to work out your D value, noseweight and gross train weight for your own vehicle and trailer.
Tow Tide is a trading name of AT AUTO & SPARES LTD, a company registered in England and Wales, company number 17176482. Registered and trading address: Unit 1 Main Road, Tydd Gote, Wisbech, PE13 5QD, United Kingdom. Email info@atautoparts.co.uk, telephone +44 7429 827113.