What is BTU?
BTU, which stands for British Thermal Unit, is a measure of heat energy. One BTU corresponds to the amount of thermal (heat) energy required to heat one pound of water one degree Fahrenheit.
For technical purposes, a BTU also equals:
- 252 heat calories (not the same kilo-calories measured in food)
- one third of a watt-hour
BTUs work in reverse, too. That is, a BTU measures the amount of energy required to lower the temperature of one pound of water one degree Fahrenheit.
BTU in Practical Use
HVAC professionals use BTUs to measure the cooling capacity of an air conditioning unit. The higher the BTU measurement, the more powerful the air conditioner, and, usually, the higher the price, as well. Air conditioners typically start at approximately 1,000 BTU.
When used to describe a unit of power, as is the case with air conditioning systems, BTU is written as BTU per hour, or BTU/h. For instance, a 10,000 BTU/h air conditioner provides 10,000 BTUs per hour to cool a home.
How many BTUs are enough for your home cooling system?
To determine how many BTUs you need for your new air conditioning system, first calculate the size of the space you need to cool. For room air conditioner units – either portable, window, or wall units – multiply the width of the room by the length to figure out the total square footage. To determine your whole house square footage, measure along two outside walls of your home.
For an L-shaped ranch or other irregularly-shaped house where rooms protrude, take one measurement for the basic square or rectangle of the home, and a separate measurement for any rooms not included in that calculation and add the two together.
With your square footage measurement in hand, multiply that number by 35 to get an estimate of the BTUs necessary to cool your home or room.
A few other guidelines will help you purchase the right size unit:
- If the room is shaded, reduce the BTUs needed by 10 percent
- In a sunny room, see if you can shade the area or add 10 percent more BTUs
- Add 4,000 BTUs for an AC in the kitchen since kitchen appliances generate a lot of heat
- If more than two people spend a lot of time in a room (for instance, a family room or dining room), add 600 BTUs per person to cool that room
| To cool this area | Look for this size air conditioner |
| 100 to 150 square feet | 5,000 BTUs |
| 150 to 250 square feet | 6,000 BTUs |
| 250 to 300 square feet | 7,000 BTUs |
| 300 to 350 square feet | 8,000 BTUs |
| 350 to 400 square feet | 9,000 BTUs |
| 400 to 450 square feet | 10,000 BTUs |
| 450 to 550 square feet | 12,000 BTUs |
| 550 to 700 square feet | 14,000 BTUs |
| 700 to 1,000 square feet | 18,000 BTUs |
| 1,000 to 1 400 square feet | 24,000 BTUs |