The 21st Century Radiator Company offers a great range of styles including cast iron or designer radiators, towel rails or flat panel radiators. To calculate the BTU of a room, you need to work out the cubic feet or the volume of the room where the radiator is situated by multiplying the length, by the width, by the height. If you have the dimensions in metres, first multiply the figures by 3.281. Once you have this figure, you will need to apply one of the following multipliers, depending on the usage of the room: 3 for halls, stairs, bathrooms & kitchens; 4 for bedrooms; 5 for sitting and dining rooms. Deduct 10% for double glazing; add 15% for north-facing rooms and 20% for French windows. For large rooms, more than one radiator should be considered to distribute heat evenly. It’s a good idea to get the figure verified by an experienced plumber or heating engineer.