AC BTU Calculator

Estimate BTU needed for air conditioning.

Result:

Chill Out: How to Right-Size Your Air Conditioner

Buying an air conditioner is a big investment, but surprisingly, bigger isn't always better. The most important specification on any AC unit is its BTU rating. Our AC BTU Calculator helps you find the "Goldilocks" zoneâ€”not too weak, not too strong, but just right for your specific room.

What is a BTU?

BTU stands for British Thermal Unit.
One BTU is defined as the amount of energy required to heat (or cool, by removing heat) one pound of water by one degree Fahrenheit.
In plain English: It's a measure of cooling power. A higher BTU rating means the machine can remove heat from the air faster.

The Goldilocks Problem

Why do calculations matter? Why not just buy the biggest unit you can afford?

1. Too Small (Undersized):
The unit will run 100% of the time, driving up your electric bill, but the room will never reach the set temperature. The machine will eventually burn out from stress.

2. Too Big (Oversized):
This is a common mistake. An AC unit does two things: it cools air, and it dehumidifies air. If the unit is too powerful, it will cool the room in 5 minutes and shut off ("short cycling"). However, 5 minutes isn't enough time to remove moisture. You end up with a room that is cold but clammy and damp, which can lead to mold growth and musty smells.

3. Just Right:
The correct size unit runs long enough to pull the humidity out of the air while maintaining a comfortable temperature.

Factors We Calculate

While the standard rule of thumb is 20 BTU per square foot, real life is more complex. Our calculator adjusts for:

Square Footage: The baseline cooling load.
Occupancy: Humans are heat radiators. Each person adds about 400-600 BTU of heat to a room. If you are cooling a crowded living room, you need more power than an empty bedroom.
Sun Exposure: A south-facing room with big windows is basically a greenhouse. We add 10% capacity for sunny rooms and subtract 10% for shady, north-facing rooms.
Kitchens: (Note: If the unit is in a kitchen, standard advice is to add 4,000 BTU to account for the oven/stove).

Common Sizes

Small Room (100-250 sq ft): 5,000 - 6,000 BTU
Medium Room (250-400 sq ft): 7,000 - 9,000 BTU
Large Room (400-550 sq ft): 10,000 - 12,000 BTU
Open Concept / Master Suite (550+ sq ft): 14,000+ BTU or Central Air

Use our tool to get your baseline number, then check the specific features (like trusted brand warranties and energy saver modes) when you shop.