BTU Calculator

Instructions: Easily measure your BTUs by following the steps below.

Choose one of the following:


This calculator should only be used to estimate your building’s BTU needs. You should get a professional BTU estimate done by your local HVAC company to get a better picture of what size unit you need.

What Size Air Conditioner Do I Need?

Using an air conditioner on a hot day can make the temperatures more bearable, but only if the air conditioner is big enough to cool the area you want it to. On the other hand, purchasing an air conditioner that is too large for the space may mean you’re spending more money, both on the unit and on the energy it requires to run. Get the right-size air conditioner for your needs and make sure you’re comfortable – both physically and financially.

Determining Your BTUs

Air conditioners are sized by the number of BTUs they put out. BTUs stand for British Thermal Units and refer to the amount of energy your air conditioner is using per hour.

To figure out what size air conditioner you require, you first need to determine the amount of BTUs you require. This is done by calculating your square footage.

Break up each room into as many squares and rectangles as you need to be able to measure the entire space accurately. This means separating out closets, bump outs, and hallways each as their own separate area.

Measure the length and width of each separate area in inches. Multiply the two numbers together to get the number of square inches in each area. Divide this number by 144 to get the square feet. Round up to the nearest whole number, then add the various areas together to the get the total amount of square feet your air conditioner needs to cool.

Selecting Your Unit

Air conditioners are clearly labeled with the amount of BTUs they need. Consult the following chart to see where your square footage lands in relation to the amount of BTUs. Purchase the unit that best corresponds to the amount of square footage you have.

Square Footage

BTUs Required

100 – 1505,000
150 – 2506,000
250 – 3007,000
300 – 3508,000
350 – 4009,000
400 – 45010,000
450 – 50012,000
500 – 70014,000
700 – 1,00018,000
1,000 – 1,20021,000
1,200 – 1,40023,000
1,400 – 1,50024,000
1,500 – 2,00030,000
2,000 – 2,50034,000

If you are between unit sizes, go up to the next nearest size to ensure the air conditioner is able to adequately cool your space.

Variations and Variables

While this chart is used as a general rule of thumb for selecting air conditioner size, variations can occur depending on several variables you need to consider.

Shade and Sun

If the room or house you are cooling is heavily shaded, you may need 10 percent fewer BTUs to cool it. If the house or room is very sunny, however, add 10 percent more BTUs.

Room Occupancy

The more people that use the area you are cooling, the higher the BTUs need to be to compete with body heat. As a general rule of thumb, for every person over an initial 2 people, add 600 BTUs.

Hot Appliances

If the room you are planning on installing an air conditioner in is the kitchen, you need to factor in additional BTUs for the stove, oven, and microwave. Add about 4,000 BTUs to the unit size to cover this.

Get a Professional Opinion

If you are unsure about the size of unit you need, or you have trouble calculating the size of area you plan to cool, get a professional opinion. Getting the right-sized air conditioning unit is too important to leave to chance.