BTU Calculator – What Size Air Conditioner Do I Need?
Find the right cooling capacity for any room in seconds. Enter the room size and conditions to get a recommended BTU rating for a window AC, portable AC or mini-split.
Results
This is an estimate. For homes with poor insulation or large windows, have an HVAC professional do a Manual J load calculation.
How to Use This BTU Calculator
- Enter the room length and width in feet.
- Adjust the ceiling height if it is not 8 feet.
- Pick the sun exposure and room type — both change the recommendation.
The calculator starts with the standard rule of 20 BTU per square foot, scales it for ceiling height, adds 10% for very sunny rooms (or subtracts 10% for shaded ones), and adds 4,000 BTU for kitchens.
Why Sizing Matters
Buying the biggest unit is a mistake. An oversized AC:
- cools the air so fast that it shuts off before removing humidity, leaving the room cold and clammy;
- cycles on and off constantly, which wears out the compressor;
- uses more electricity despite running less, because startup draws are heavy.
A slightly undersized unit runs longer, removes humidity properly, and feels more comfortable — as long as it can keep up on the hottest day.
Common Window AC Sizes
| Room size (up to) | Recommended capacity |
|---|---|
| 150 sq ft | 5,000 BTU |
| 250 sq ft | 6,000 BTU |
| 350 sq ft | 8,000 BTU |
| 500 sq ft | 10,000 BTU |
| 650 sq ft | 12,000 BTU |
| 800 sq ft | 14,000 BTU |
Frequently Asked Questions
How many BTU do I need for a 12x12 room?
A 12×12 room is 144 square feet. At the standard 20 BTU per square foot, you need about 2,880 BTU, so the smallest 5,000 BTU window unit is the right size.
What size air conditioner do I need for 500 square feet?
A 500 square foot area needs about 10,000 BTU (500 × 20), plus adjustments for ceiling height, sun exposure and kitchen use.
What happens if my air conditioner is too big?
An oversized AC cools the air quickly but shuts off before removing humidity, leaving the room cold, clammy and uncomfortable. Slightly undersized units run longer but dehumidify much better.
Why does a kitchen need more BTU?
Cooking appliances add significant heat. Industry guidance is to add about 4,000 BTU for kitchens, on top of the room-area calculation.