AC Btu Calculator – What Size Air Conditioner Do I Need?

This is an important question to get right – How many BTU air conditioner do I need? We answer that question for three types of air conditioners: Window ACs, portable ACs and single-zone mini-split air conditioners.

This information supports our PickHVAC AC BTU calculator that gives you guidance on how many BTUs your air conditioner should be. There is a room size chart to assist in choosing the right size air conditioner.

  • Window air conditioners range from 5,000 to 24,000 BTUs in most brands (a few are a little larger)
  • Portable air conditioners are available from 8,000 to 14,000 BTUs – but there is a huge difference between the BTU ratings of window ACs vs portable ACs
  • Mini-split ductless units are built from 9,000 to about 42,000 BTUs – though these units require installation, so cost quite a lot more

The point is that your options might be limited in larger spaces. There’s a special issue concerning portable air conditioners and their BTU rating. Please read that section below if you are considering as your options window air conditioners vs portable air conditioners.

What is BTU & What are BTU Ratings

What do BTU ratings mean? Here is the basic explanation.

BTUs are British thermal units. A BTU is defined as “the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.”

OK, but an AC doesn’t raise the temperature, it lowers it, of course.

How to Calculate BTU

BTU ratings specify how many BTUs the air conditioner moves. The rating is actually BTU/hour, or the number of BTUs the air conditioner removes from the space in one hour. A 12,000 BTU AC captures 12,000 BTUs of heat per hour and moves it outside, making the room more comfortable.

BTU Calculator

Everyone quotes the US DOE when it states, “Based on size alone, an air conditioner generally needs 20 Btu for each square foot of living space.” But there is much more to the equation, and we have built those factors into our AC BTU Calculator for improved accuracy in sizing your next air conditioner.

Window & Portable AC BTU Calculator

Window AC BTU Calculator

Our BTU calculator considers three factors. Let’s discuss them, and then we’ll make a few recommendations for “perfecting” the sizing process.

Room Size: How large is the space you’re trying to cool? If you don’t know, measure the length of the room times the width. For example, a room that is 15 feet x 12 feet is 180 square feet.

Insulation Condition: How much insulation your home has and how well sealed it is are important factors in answering how many BTU air conditioner do I need?

  1. Was your home built after 2000? Do you know that your home has house wrap/Tyvek/vapor barrier? If these apply, then choose Good.
  2. Have you replaced the windows and doors in an older home? If you stand by windows, do you feel drafts coming through around the frames? If not, then choose Average.
  3. If you don’t know anything about how well your home is insulated and sealed – or if you definitely feel air coming in around doors and windows, then choose Poor.

Your answer about insulation makes a significant difference. For example, here are the BTUs needed for 500 square feet in each insulation tier:

Good: 10,800 / Average: 12,000 / Poor: 13,200

Sun Exposure: This factor makes an equally large difference. What direction does the room face? If it is west and/or south, choose Very Sunny unless there are huge trees shading the house. In that case, choose Average.

If it is on the north or east, choose Average if there is little shade and Very Shady if that side of the house has tall trees and shrubs around it to keep it cool.  

Pro Insights on Sizing a Room Air Conditioner: Keeping these few issues in mind will help you choose an AC that best suits your needs.

  1. Bigger is better – or at least OK. It doesn’t hurt to have a room AC that is a little too large for your space. You wouldn’t want this with central air conditioning, but it is OK or even recommended with window and portable air conditioners.
  2. How high are the ceilings? If you have ceilings 10 feet high or higher, you’ll need an AC 2,000 to 4,000 BTU larger.
  3. What room are you cooling? Is it the most popular room in your home? Is it the kitchen where hot appliances are often in use? Once again, go with a larger air conditioner because people, pets and appliances create heat!

Portable vs Window Air Conditioner (Must Read!)

This is a very important topic!

The old BTU ratings for portable air conditioners are wrong – but they’re still published as accurate on most product pages.

There’s a complete explanation of this issue in the PickHVAC Portable AC Review and Guide.

Here’s the short version.

Portable air conditioners pull warm from outside into your home as they vent their exhaust. Plus, their electronics create a bit of heat. So, for roughly every two BTUs removed, one BTU is produced or pulled inside through a drafty door or window.

This makes a portable AC about half as efficient as its old ASHRAE rating would have you believe.

As a result, they must now also use the new US DOE Seasonally Adjusted Cooling Capacity (SACC) ratings. For example, see the two ratings listed on this Danby model – the old rating of 10,000 BTUs plus the SACC rating of just 5,300 BTUs. It says:

  • 10,000 BTU (5,300 BTU, SACC*) Portable Air Conditioner cools spaces up to 450 sq. ft.

There is no way a 5,300 BTU air conditioner will serve 450 square feet. The point is this – even though Danby does the right thing by listing the lower SACC rating too, it bases the room size on the old ASHRAE rating that is inaccurate for portable ACs.

BTU Chart for Sizing an Air Conditioner

Feel free to use the BTU Calculator for the most accurate answer to our central question: “How many BTU air conditioner do I need?” At least that is the way the question is often searched online.

However, for a simple and quick view, we’ve completed this BTU table or chart.

80-100 sq ft8x10, 8x12, 10x10, 14x7 rooms5,000 BTU
100-200 sq ft10x12-10x20, 12x15, 14x14 rooms6,000 BTU
200-300 sq ft10x20-15x20 rooms, efficiency apartment8,000 BTU
300-400 sq ft15x20-20x20 rooms, studio apartment10,000 BTU
400-500 sq ft20x20-20x25 rooms, 1-bedroom apartment12,000 BTU
500-600 sq ft20x25-20x30 rooms, ave. 2-bedroom apartment14,000 BTU
600-700 sq ft25x25 room, apartment w/ open floor plan15,000 BTU
800-900 sq ft20x40-30x30 spaces, home w/ open floor plan18,000 BTU
900-1,000 sq ft30x30-25x40 spaces, small home, 3-bdrm apartment20,000 BTU
1,000-1,200 sq ft20x50-30x40 zones, small home,
3-4 bdrm apartment
24,000 BTU
1,200-1,400 sq ft30x40-35x40 zones, medium home,
4-bdrm apartment
25,000 BTU

Note on Apartments and Multi-room Zones

In our opinion, several air conditioners placed in various rooms is a better way to cool and dehumidify the space than choosing one very large room AC. You’ll get better temperature balance, rather than having one area really chilly and the furthest areas a little too warm.

The only time it might be OK to use one very large AC is if the floor plan is quite open. Our chart reflects that.

The Best Air Conditioners for the Room or Zone Size

The question we’ve answered is how many BTUs should your AC be?

There is a second question – which air conditioner types will deliver those BTUs of cooling?

If you’re still wondering what type AC to use, this list might help.

  • Window Air Conditioners / 5,000 to 24,000 BTUs: Best suited for up to 1,000 square feet, but if the floor plan is open, they can be used in spaces up to 1,400 square feet. The chart above is very accurate if you’re considering a window AC.
  • Portable Air Conditioners / 8,000 to 14,000 BTUs: Best suited for up to 400 square feet in most areas. Even if the information you’re reading on a box or product page says, “up to 600 square feet,” don’t believe it. See our section above Portable vs Window Air Conditioner for the reason for making this case. Frankly, you can’t trust the room size recommendations of portable air conditioners even if the SACC BTU rating is given.
  • Single-zone Ductless Air Conditioners / 9,000 to 42,000 BTUs: Up to 2,000 square feet.

For Further Reading and Information

Our Window AC Review and Buying Guide is loaded with information on sizing a window air conditioner, pros and cons vs other AC types, efficiency ratings and what they mean, top brands and the best window ACs currently available.

Our Portable AC Review and Ultimate Guide has similar information on portable air conditioners.

Another option is to hover over the tabs at the top of the page to see guides to specific types of both – Quietest, best at various sizes, brand reviews and more.

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