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Plenty of websites have calculators to help you figure out your heat need. Search for a radiator sizing calculator. Most of them can do the math in both feet and meters. You'll need to know your room dimensions as well as its features, such as the type of windows and insulation. These calculators use the same math you can do by hand. They're good for saving time or checking your calculations. Get a tape measure if you don't already know the dimensions of the room. Measure the length, width, and height of the room. Write down the measurements so you don't forget them. Multiply together the length, width, and height of the room. This gives you the cubic volume of the room. For example, a room 13 feet (4m) wide, 12 feet (3.7m) long, and nine feet high has a cubic volume of 1,404 cubic feet. You can measure your room in either feet or meters. Multiplying by a room factor is the easiest way to estimate the heat needed for a type of room. For lounges, living rooms, and dining rooms, multiply the cubic volume by five. For bedrooms, multiply by four. For common areas and kitchens, multiply by three. This gives you the needed radiator output in BTUs. When measuring in meters, you can multiply the cubic volume by 153. This gives you a general idea of the heat you need for any room. For example, a 1,404 cubic foot living room requires about 7020 BTUs. Some rooms hold more heat than others. For rooms facing north, add 10% to the BTU score. For rooms with French windows, add 20%. Subtract 20% for foam-filled cavity walls and 5% for double glazed windows. Continue making adjustments until you have an estimate for the number of BTUs your room needs. There are plenty of other factors that determine heat loss and retention. For example, outside walls raise the BTUs needed by 15-40%. Having a bedroom above the room lowers the score by 15%. To figure out how big a boiler you need, you have to calculate the BTU requirement of every room. Follow the same steps you used for the first room. Total all the BTU measurements together. This number is the amount of BTUs the boiler needs to put out to heat your home. Add 20% to the BTU total to account for a hot water circulating tank. Add on 10% to account for heat loss. Divide your final BTU measurement by 3.41. The heating power of radiators is often calculated in watts. Keep both numbers on hand in case you need them while picking out a radiator. When in doubt, it's always best to get the opinion of a heating expert. They'll be able to figure out your heating needs for you. In addition, they can help you change your heating system. Since water is used in radiators, some plumbers may also be able to do this.

Summary:
Use an online calculator. Measure out the room. Multiply the room measurements together. Multiply the cubic volume to account for room size. Adjust the heat requirement for room features. Add all room heat requirements to come up with a boiler size. Convert BTUs to watts. Consult a heating contractor.