How to Measure BTUs for Air Conditioning

Air conditioner power is measured in BTUs.

BTUs, or British Thermal Units, is a unit of energy measurement, used to rate the power of air conditioners and heaters. One BTU denotes the amount of heat it takes to raise the temperature of 1 pound of water 1 degree Fahrenheit at an atmospheric pressure level of 1. A BTU is also equivalent to 251.997 calories of energy. To determine the proper BTU level of an air conditioner -- how powerful a unit you need -- measure your room and take into consideration existing windows and the number of people usually in that room.

Things You'll Need

• Measuring tape
• Calculator

Instructions

1. Find the square footage of the room. Measure the length and width of the room with the measuring tape and multiply the two numbers together to get square footage. For example, a room measuring 10 feet by 10 feet is 100 square feet.

2. Multiply the square footage by 25.

   Example: 25 x 100 = 2,500.

3. Add 400 for every occupant in the room. For example, a master bedroom that usually houses two people would necessitate adding 800 to the total.

   Example: 2,500 + 800 = 3,300.

4. Add 1,000 per window.

   Example: 2 windows x 1,000 = 2,000. 3,300 + 2000 = 5,500.

5. Use the number you calculated in step 4 as the least amount of BTUs that your air conditioner should provide. In this example, 5,500 BTUs is the least amount.

6. Add room totals together for a whole house BTU rating. For whole house cooling, calculate the size of a central air conditioning unit that will cool the entire house. Central air units are rated by the ton, which each ton being equivalent to a 12,000 BTU rating. A 2-ton unit puts out 24,000 BTUs and a 3-ton unit puts out 36,000 BTUs. By adding all of your rooms BTU totals together, you will determine the correct central air unit for your home.

Tips & Warnings

• Other factors such as ceiling height and type of flooring can come into play when measuring BTUs. There are many online calculators and charts that can help you correctly estimate your air conditioner size.

References

• Inspectapedia: How to Choose an Air Conditioner - BTU Chart

Resources

• Consumer Reports: How powerful an air conditioner do you need?