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The 85% rule is the industry safety guideline for caravan stability, not law: the caravan's loaded weight (MTPLM) should ideally not exceed 85% of the car's kerbweight, and never the car's legal towing limit. Noseweight, driving licence rules and outfit length have their own limits.
What The Caravan Towing Weight Calculator Works Out
- 85% guideline: kerbweight multiplied by 0.85 gives the recommended maximum loaded caravan weight (MTPLM). A 1,500kg car matches caravans up to 1,275kg. The guideline comes from the caravan clubs and industry rather than law, because snaking (trailer oscillation) worsens directly as the trailer outweighs the tug.
- 100% of kerbweight: the upper figure considered acceptable for experienced towers; anything heavier than the car is inadvisable regardless of experience. Kerbweight definitions vary annoyingly between makers on fuel and driver, so use the handbook or V5C figure.
- Legal ceiling: the car's type-approved maximum towing limit from the V5C and VIN plate may never be exceeded. It occasionally comes in under the 85% figure, in which case it wins; some cars are not rated to tow at all.
- Matching guideline (advanced): change the 85% to the 100% experienced-tower figure, or to a stricter 75% that some clubs suggest for novices, and the recommended weight recalculates.
- Caravan MTPLM and match ratio (advanced): enter the caravan's maximum weight from its plate and the card shows it as a percentage of kerbweight with a plain verdict, flagging anything over the legal towing limit as illegal.
- Caravan payload (advanced): MTPLM minus the MiRO (mass in running order, the unladen weight). This is what awnings, water, gas, bedding and everything in the lockers consume, typically 100kg to 250kg and used up faster than owners expect. A loaded weighbridge visit gives the honest answer.
- Recommended noseweight (advanced): the download on the towball belongs at 5% to 7% of the caravan's actual weight, within the car's towball limit (often 75kg to 100kg). Measure it with a cheap gauge rather than guessing; too little is as unstable as too much.
- Train weight allows a trailer of (advanced): the car's gross train weight minus its gross vehicle weight (MAM) is the heaviest trailer it can legally pull with the car fully loaded. If that is smaller than the MTPLM, you must leave weight out of the car or the caravan.
- Other limits: since the 2021 rule change a standard car licence covers trailers up to 3,500kg MAM; the towing speed limit is 60mph on motorways and dual carriageways and 50mph elsewhere; mirrors must be extended when the caravan is wider than the car; and heavy items belong low and over the axle for stability.
Further reading: GOV.UK: towing with a car.