British thermal units (BTUs) are used to measure the capacity of heating and cooling equipment. Scientifically, a BTU is defined as the amount of energy needed to raise the temperature of 1 pound of water by 1 degree Fahrenheit. That's probably the last thing on your mind when it's 37.8 degrees C outside and your AC unit just bit the dust. But getting the right measurements for your BTU requirements will keep you at just the right temperature and, more importantly, keep the heat off your wallet.
Measure the length and width of the room and multiply the figures to determine the square feet.
Multiply the result in step one by 25.
For each occupant in the room, add 400. For example, in a couple's bedroom, the total would be 800. Add this to your total from above.
Add 1,000 for each window in the room.
The total should give you a rough estimate of the amount of BTUs needed to cool the room and keep it at a comfortable temperature.
If you want to get really serious about this, you could forgo the pencil, paper and calculator and plug everything into a spreadsheet. There are a number of online calculators that also factor in window glazing, shades/drapes and insulation into their figures.
Don't get more BTUs than you need. For example, if you got an air conditioning unit that was too powerful, the unit would overly humidify the room, resulting in an damp and unpleasant environment, not to mention the higher cost of utility bills.