In 2010 and then again in 2011 there were changes in the way that a caravan's Mass of in Running Order (MRO) is calculated.

For many years the MIRO quoted by most UK manufacturers was little more that the weight of the van as it left the works. The payload was (and still is) calculated by subtracting the MIRO from the Maximum Technical Permissible Laden Mass (MTPLM). Before the changes came in the payload was effectively the maximum weight of items that could be added to the van after it left the factory.

From the 2010 model year the definition of MIRO was changed to line up with the European Standard for Payloads BSEN 1645 part 2. This meant adding in an allowance for items such as gas and water.

The situation has changed again for 2011 with a new definition of MIRO to line up with the European Directive for Masses and Dimensions for vehicles. This change is all part of a new European directive that will oblige manufacturers to start to work towards EC Whole Vehicle Type approval, a requirement that will be introduced for all new build caravans from 2014.

The new definition specifies more thoroughly how much weight should be added for items like water. So, if a caravan or motorhome has an on board fresh water tank as standard, it must be assumed that it will be 90% full. In the case of a 30 litre on board water tank the manufacturer must add 27kg to the MIRO. However, by travelling with the tank empty, this weight can be allocated to other items.

Water heaters are assumed to be full of water (typically 10 litres = 10kg) and loo flush tanks too contain a couple of litres. This is the maximum Thetford recommends when travelling. Again, if you travel with these empty then the weight saved can be assigned to other items.

The allowance made for gas includes 2 x 5kg BP Light gas bottles (16.5kg). These weigh about the same as one standard steel 6kg propane cylinder when full.

Surprisingly the new UK definition of MIRO for caravans does not include a battery, which is classed as a personal effect.  Apparently this is to help maintain the user payload levels that we are used to. An 85 amp hour battery will weigh around 19kg, a 100 amp hour one around 25kg.

Fortunately none of this affects towcar matching as the key factor here is the MTPLM of the caravan and the definition of this has not changed. So, if you find the new definition of MIRO a bit confusing, there is an easy and cast iron way of making sure your van is legal on the road. Simply take it to a public weighbridge and get it weighed. It must never exceed the MTPLM figure given on the plate mounted near the bottom of the entrance doorway. If it does you must remove items to bring the weight down to the MTPLM or less. One point worth noting is that you can legally take a slightly overweight trailer on the road as long as you are going directly to a public weighbridge (best to have an appointment as proof) or directly home again. If grossly overweight you could be considered to be a danger and prohibited from proceeding.

The new regulations are designed to ensure that the MIRO’s quoted by different manufacturers can be compared on a like for like basis. Hitherto each manufacturer has interpreted the regulations differently. 

From 2011 the MIRO of motorhomes should be calculated as the mass of the unladen vehicle plus a 75kg allowance for the driver, plus an allowance for engine coolants, a full fuel tank and water and gas tanks at 90% capacity.

A number of caravans can have their weight plates upgraded to a higher MTPLM. This is because MTPLM’s are sometimes set lower than they need to be in order to extend the range of tow cars that can safely be used and therefore make the caravan appeal to a wider audience.

If your towcar is capable of handling a heavier caravan and you want to carry more weight, ask the supplying dealer if an upgrade is possible. If it is, he will arrange it for you, and supply a new plate and the necessary paperwork to prove that the upgrade is legal. Most manufacturers levy an admin charge for this.

Motorhome weight plates cannot be upgraded as the gross vehicle weight is set by the base vehicle manufacturer. The base vehicle will have been tested and homologated to the declared weight.

Further Reading
Those wishing to find out more about the changes can read the National Caravan Council’s guidance on payload and MIRO allowance in their tourer information website


MTPLM - Maximum Technical Permissible Laden Mass is the maximum weight at which the caravan or motorhome can legally be taken on the road.

MRO is the Mass in Running Order and now has to include an allowance for gas and water but not a battery

EHEP - Essential Habitation Equipment Payload. This is the mass of the items required for the safe and proper functioning of the equipment for habitation as defined by the manufacturer.

OEP - Optional equipment payload. This is an amount of weight provided by the manufacturer for factory fitted options (e.g. air conditioning). It does not include any items that might be fitted by the retailer such as a mover. These items will reduce the overall payload available to the user.

PEP - Personal Effects Payload is the mass specified for the items which a user can choose to carry in a caravan and which are not included as essential habitation equipment or optional equipment. In addition to a battery this payload will include items such as an Aquaroll, Wastemaster, crockery, cutlery, bedding, clothing, TV, toilet fluids and so on.