Window Air Conditioning Chart (BTUs for Room Size)

Selecting the proper size window air conditioner for your room is crucial to ensure efficient and effective cooling. The general rule of thumb is that for every square foot of living space, you need 20 BTUs (British Thermal Units) of cooling power. However, many other factors come into play when determining the ideal window AC unit size. This comprehensive guide will provide a detailed window air conditioning BTU chart tailored to room dimensions and other considerations.

How Many BTUs Per Square Foot Should an Air Conditioner Have?

As a baseline, you should have 20 BTUs per square foot of room space. However, this is just a general guideline, not an exact science. The optimal BTU rating depends on several other aspects as well:

  • Room size – Measure length x width to get total square footage.
  • Room height – Ceilings over 8 feet need more BTUs.
  • Window size – Larger windows mean more heat/light enters.
  • Window orientation – South- and west-facing rooms get more sun exposure.
  • Shading – Rooms with shaded windows need fewer BTUs.
  • Insulation – Well-insulated spaces stay cooler.
  • Occupants – More people means more body heat.
  • Electronics – Computers, TVs emit heat when operating.
  • Geography – Warmer climates need higher BTU power.

With all those factors at play, the 20 BTU per square foot rule serves only as a baseline. The optimal BTU rating could be higher or lower depending on your room conditions.

Window Air Conditioner BTU Chart by Room Size

Refer to this window AC BTU chart for guidelines tailored to room sizes:

100 to 150 Square Feet (Small Room)

  • Minimum BTUs: 5,000
  • Ideal BTUs: 6,000 to 8,000

Typical small rooms include home offices, small bedrooms, kitchens, and bathrooms. A 5,000 BTU window air conditioner can sufficiently cool 100 to 150 square feet. For maximum comfort, look for a unit with 6,000 to 8,000 BTUs.

150 to 250 Square Feet (Medium Room)

  • Minimum BTUs: 6,000
  • Ideal BTUs: 8,000 to 10,000

In a medium-sized room like a dining room, living room, or main bedroom, an 8,000 to 10,000 BTU AC unit will efficiently cool 150 to 250 square feet. You can get by with 6,000 BTUs in a pinch.

250 to 400 Square Feet (Large Room)

  • Minimum BTUs: 8,000
  • Ideal BTUs: 10,000 to 12,000

For larger spaces between 250 and 400 square feet, such as master bedrooms, studio apartments or home gyms, look for a window air conditioner with a 10,000 to 12,000 BTU rating. Make sure the unit has sufficient power for peak cooling days.

400 to 700 Square Feet (Extra Large Room)

  • Minimum BTUs: 10,000
  • Ideal BTUs: 12,000 to 18,000+

Extra large rooms over 400 square feet, like open concept living/dining combos, require an equally powerful AC unit, typically with 12,000 to 18,000+ BTUs. Dual hose models or multiple window units may be needed to effectively cool these big spaces.

Adjust BTU Recommendations Based on Other Factors

Use the window AC size chart above as a starting point. Then fine-tune your BTU requirements based on these additional considerations:

1. Room Height

For rooms with 8-foot ceilings or lower, the chart above will suffice. If you have cathedral ceilings or two-story great room spaces, increase BTUs by 8% for every additional foot above 8 feet.

2. Window Size

The more window space you have, the more heat and light can enter. Add 600 BTUs for every square foot of window space. For example, a large 5’ x 3’ window adds 15 sq ft of window space, meaning an extra 9,000 BTUs needed.

3. Window Orientation

South- and west-facing windows get much more direct sun exposure from overhead and the setting sun. Add 10% more BTUs if your room’s window faces these directions. North- and east-facing windows get less sun.

4. Window Shading

Shading from awnings, overhangs, trees or other buildings reduces solar heat gain significantly. You can decrease the BTU rating by 10% for rooms with well-shaded windows.

5. Insulation

Properly insulated spaces stay cooler and require less AC power. For well-insulated and weatherstripped rooms, reduce BTUs by 8-10%. Poor insulation requires higher BTU units.

6. Occupants

The more people in the room, the more body heat. Add 600 BTUs for every extra person over two occupants.

7. Electronics

Electronics like TVs, computers and other devices emit heat when operating. Add 600 BTUs for every 100 watts of electronic equipment running simultaneously.

8. Local Climate

In extremely hot southern climates, you may want a unit with 10-20% higher BTUs than the chart shows. In cooler northern areas, you may be able to get by with slightly fewer BTUs.

Real World Examples and Recommendations

Here are some real room examples with tailored AC size recommendations:

Case 1: Master bedroom

  • 12 x 15 foot room = 180 sq ft
  • 8 foot standard ceiling
  • North facing window
  • Well insulated
  • Shaded by large tree

Per the chart: 8,000 – 10,000 BTU unit
Reduced for shading: -10% = 7,200 – 9,000 BTU range

Recommended unit size: 8,000 BTU

Case 2: Open living/dining room

  • 20 x 22 = 440 sq ft total
  • Cathedral ceiling up to 12 feet
  • Large south facing bay window
  • Poor insulation
  • 4 occupants

Per chart: 12,000 – 18,000+ BTU unit
Increased for ceiling height: + 8% = 13,000 – 19,000 BTUs
Increased for poor insulation: +10% = 14,000 – 21,000 BTUs
Increased for occupants: + 2,400 BTUs = 16,400 – 23,400 BTU range

Recommended unit size: 18,000 – 24,000 BTU dual-hose or multi-AC setup

Choosing the Right AC Unit Size

When in doubt about AC sizing, always err on the side of more power. An oversized unit simply runs in short cycles and still adequately dehumidifies. But an undersized AC unit will not sufficiently cool and dehumidify the air, leaving your room hot, sticky and uncomfortable.

Refer to this comprehensive window air conditioner BTU chart when shopping for the ideal unit tailored to your room size, window specifics, climate and other cooling needs. Properly sizing your AC unit is crucial to beat the heat this summer!

Frequently Asked Questions about Window AC Unit Sizing

Still have questions about selecting the right window air conditioner BTU size? Here are answers to some commonly asked questions:

What size window AC unit do I need for a 12×12 bedroom?

For a 144 square foot bedroom with a standard 8-foot ceiling, you would generally need between 6,000 to 8,000 BTUs for adequate cooling power. A 5,000 BTU unit may not be sufficient for hotter climates or rooms with inefficient insulation.

What size window AC unit is needed for a 300 sq ft room?

A room that’s 300 square feet falls into the large room category, which requires a minimum of 8,000 BTUs. For ideal comfort in a 300 square foot space, look for a unit with cooling power between 10,000 to 12,000 BTUs.

Can a window AC unit be too big for a room?

Technically, yes an oversized AC unit will cool your room just fine, if not better. However, it may feel uncomfortably cold as it cycles on and off frequently. Oversized units also cost more upfront. Just don’t buy a unit drastically too big for your space.

How do I calculate the correct BTU for window AC?

First calculate room square footage then multiply by 20 BTUs per square foot. Then adjust up or down based on other factors like ceiling height, window size, shading, insulation, occupants and electronics.

Is it OK to buy a used window AC unit?

Buying a used window air conditioner from an individual can be risky since you don’t know its maintenance history. However, retailers often sell discounted refurbished units that have been professionally inspected, tested and repaired as needed. These can be a great option if on a tight budget.

Should I get dual hose or single hose AC for a large room?

For spaces over 400 square feet, a dual hose AC is often recommended since it’s more energy efficient and effective for larger cooling needs. Single hose units work better for smaller rooms under 400 square feet.

How long should a window AC unit last?

On average, a new window air conditioner will last between 8 and 12 years. Making sure your AC is properly sized and installed, cleaning the filter often, and performing annual maintenance will help maximize its useful lifespan.


Determining the right window AC unit size for your room is a crucial first step. While the standard guideline is 20 BTUs per square foot, tailor this further based on room layout, window specifics, insulation, occupants and climate. When in doubt, it’s better to get a unit slightly larger rather than too small. With a properly sized window air conditioner based on this BTU chart, you’ll stay cool and comfortable all summer long.