what size air conditioner unit do I need

What Size Air Conditioner Unit Do I Need?

  • April 29, 2020
  • By MR COOL

With summer quickly approaching, you might be wondering if your air conditioner will get you through the hottest months of the year. You need a cooling system that’s the right size for the space, uses the right amount of energy and keeps your whole house comfortable.

If you’re just replacing an old, the size you used previously is the best place to start. It is also important consider the model and energy-efficiency of any AC unit you might utilize. This guide can help you figure out the proper cooling capacity for your home air conditioner.

How Are Air Conditioner Units Rated?

AC units are rated typically by BTU production and by the SEER rating. While BTUs measure the cooling capacity per hour of an air conditioner, the SEER measures its energy efficiency. Typically, the bigger the cooling system, the higher the BTU and the lower the SEER.

1. What Does BTU Stand For?

A BTU, or British thermal unit, measures the energy that your air conditioner uses to cool the room. When you count calories, you’re measuring how much energy you’re putting into your body and how much you need to burn to maintain your weight. In the same way, when you measure BTUs, you estimate how much energy your appliance can process to regulate the temperature of your home.

To convert BTU to tons, keep in mind that there are 12,000 BTU in a ton. Room air conditioners have a cooling capacity as little as 5,500 BTU per hour. The cooling capabilities of our DIY ductless mini-split systems range from 12,000 to 34,500 BTU per hour. Our Signature Series central air conditioners can go up to 60,000 BTU per hour.

Use these factors to determine if you may need a more significant BTU rated AC unit than typical:

  • Your ceilings are higher than 8 feet.
  • You live in a warmer climate.
  • You’re putting the unit in direct sunlight.
  • You have little insulation.
  • You have large windows in the room.
  • You have a large, open concept house.

This number affects the sizing for your air conditioner because of its energy expenditure in one hour. You would need an AC unit with a smaller BTU for a compact room so you don’t use too much energy to cool your space. A cooling system with a larger BTU for a large room will also adequately cool the area.

2. SEER Ratings

While to does not measure the size of the air conditioner, the seasonal energy efficiency ratio (SEER) number measures how efficient a central AC unit is over the summer. The higher the SEER rating, the more efficient the product.

You might also see information about the energy efficiency ratio (EER), which measures how efficient a cooling system will be at a certain outdoor temperature. The SEER more accurately reflects the overall efficiency of a unit seasonally, but the EER will tell you how the AC works during peak operations.

Energy-efficient appliances have a yellow tag that displays their SEER rating. To have a higher energy-efficiency for your AC unit:

  • Schedule preventative maintenance at least once a year.
  • Change your air filters monthly to protect against debris.
  • Set a programmable thermostat to manage comfort correctly.
  • Seal and insulate windows and ducts, to prevent air leaks.

While appliances with higher SEER ratings have a higher upfront cost, you’ll save money in energy bills each month with an energy-efficient cooling system. Companies also offer rebates on their energy-efficient products as incentives to purchase them.

The minimum SEER rating for most of the country is 13. If you’re in a state that gets warmer summer climates, the score is 14, so that your home could save energy and money.  Some of our air conditioning units have a SEER rating of up to 23.

What Size AC Do I Need?

Figuring out what size air conditioning unit you need is as simple as taking a tape measure and looking at the length, width and height of your living space. You could also call in a professional to help you find the air conditioner you need to cool off the room. Use these calculations to choose the right size for your AC system.

1. Square Footage AC Calculator

To figure out the square feet within your room, measure the length and width of the room. Use these tips to calculate the area of your living space:

  • In rectangular and square areas, multiply the full width by the length.
  • In triangular places, use the same formula but cut it in half.
  • If you have an abnormal room shape, divide the room into rectangles, triangles and squares and use the previous equations. Add the total square footage of all the various dimensions.

2. AC BTU Calculator Chart

How many BTUs do you need? After you’ve measured the dimensions of each room in the home where you want to put an air conditioner, you can figure out what you need based on the BTU conversion chart.

Find your room’s square footage on the left column, and then use the right column to get the proper capacity for the AC unit. The cooling capacity is measured in BTU per square foot. A 3-ton AC unit can cool a small house, while a 5-ton AC unit can cool a bigger home.

3. Make Adjustments as Needed

While this process seems pretty straightforward, you also need to make some adjustments based on these factors:

  • Decrease the cooling capacity of the unit by 10% if the area has shade.
  • Increase the cooling capability by 10% if the area is in direct sunlight.
  • Increase the total by 600 BTUs for every extra person in a room if the room is occupied by more than two people regularly.
  • Increase capacity by 4,000 BTUs if the air conditioner is in the kitchen.

4. Heat Zone HVAC Calculator

Throughout the world, various climate zones indicate the heat that each region experiences. Contractors and builders use climate zones to get the most energy savings in a home. Here are the heat zones in the United States to determine which one you live in and how it affects the size of your AC unit:

  • Hot-humid: You are in a hot-humid climate if you live in the southeastern part of the United States.
  • Mixed-humid: This climate consists of the central-eastern region of the country.
  • Hot-dry or mixed-dry: You may feel hot and dry air if you live in the southwestern part of the United States. The difference between hot-dry and mixed-dry is that mixed-dry sees less warm temperatures throughout the year.
  • Cold or very cold: In the northern part of the country, you get colder days more frequently.
  • Marine: On the west coast, the weather is pretty consistent, but you’ll see rain each month.

These climates got their names based on the amount of precipitation and the frequency of warmer temperatures compared to the rest of the country. You could figure out what type of air conditioning system you need depending on how often you have sunny days in your region. A house in a hot-humid climate would need a more powerful AC unit than one in a cold part of the country.

5. Bring in a Professional

A professional HVAC contractor will know how to size your cooling system for your living space. They can give you an in-house estimate and explain how much it would cost to install an AC unit that would work best for your living conditions.

If you’re checking out our products, you could contact an expert from MRCOOL. We offer free lifetime tech support on all of our merchandise, so if you’re ever stuck, just give us a call. After years of experience, we know where our products fit best, especially our DIY® Multi-Zone, which can cool the whole house.

Why Does the Right Size Air Conditioner Matter?

If you order the wrong size AC unit, you could end up spending more money than necessary to cool your home. Here are some of the reasons why you shouldn’t install a cooling system that’s much too big or far too small.

1. How Do I Know If My AC Unit Is Too Big?

While homeowners tend to believe that a bigger cooling system is better, that doesn’t always apply. A 4-ton AC unit in a tiny space could reduce the air quality of your home without providing additional benefit.

Here are some of the reasons why a large AC unit may be hurting your home:

  • An uncomfortable room: The HVAC system cools the air in your home and the compressor takes out the moisture from the atmosphere. When the AC is too big for the room, it’ll turn off faster than usual, so the compressor won’t have time to remove all the humidity. As a result, you’ll have a sticky, stuffy room instead of a comfortable atmosphere.
  • Dust mites and mold: When the room has too much humidity in it, the result is mold and mildew. You may also see many more dust mites throughout your living space, along with all that moisture in the air.
  • A loud air conditioner: A cooling system has shorter cycles if it’s too big for the room. To find out if it’s too big, time a sequence on a hot day, and if the system only cycles for a few minutes, it’s too big. You can also tell by the sound. If the AC unit sounds like a freight train, it’s too big for the living space. A properly-sized air conditioner makes very little noise, if it makes any noise at all.
  • Higher energy bills: A large air conditioner will cost more money upfront, including the price of installation, because of its size. It can also increase your energy bills. Since an oversized AC unit starts and repeatedly stops to cool down the house, it’ll spend more energy, and your energy bills will increase over time.
  • Signs of wear and tear: An oversized cooling system starts and stops periodically throughout the day, so the compressor will take more of a hit than ones that are the right size. The more wear and tear on your AC unit, the more you’ll have to pay in repair costs.
  • Structural damage: You might see structural damage in the drywall or wood products you may have, especially if your home is in a humid climate. A massive cooling system won’t remove enough moisture from the air, so it can damage these areas of your home that aren’t resistant to moisture.

2. How Do I Know If My AC Unit Is Too Small?

Like having a unit that’s too big, you don’t want one that’s too small, either. Here are some of the reasons why a small room air conditioner may be costing you money:

  • Your AC never stops running: The air conditioner absorbs warm air and moisture inside the home and cools it until the air reaches the number on your thermostat. If the AC is too small for the house, it’ll keep trying to reach the temperature, but it won’t ever have the capacity to cool the whole space. Once the thermostat reaches the proper temperature, the unit should turn off, but since it never reaches the thermostat level, it keeps running.
  • Low airflow: If you keep up with the cleaning of your air filters and there still isn’t a lot of air coming out of the unit, the AC unit might be too small for the house. Since the AC is supposed to reduce the humidity in your home, you may also experience humid air throughout the house if it’s the wrong size.
  • Your home is never cold enough: Your AC unit could fail for several reasons, including dirty air filters and a clogged condenser unit. If your AC is clean, your house should be a comfortable temperature, but if you don’t have adequate size, the air will be warm and stuffy.
  • Your home has inconsistent hot or cold spots: Walk through your house and feel the temperature of each room. Keep in mind that the kitchen, living room or bathroom might be a different temperature and humidity if you’re cooking or someone is taking a shower. Baths and cooking appliances make heat, so they’ll affect the atmosphere of your living space. Otherwise, if one area of the home is much warmer or colder than the rest, then your AC unit isn’t powerful enough to reach the whole house.
  • High energy bills: If the cooling system never shuts off, it’ll use more energy to run each month. Since a small AC unit will never complete a cooling cycle, it will have to crank out air and start all over again each time. You will see a rise in your energy bills as your unit struggles to keep up with the heat in your ample space.

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45 thoughts on “What Size Air Conditioner Unit Do I Need?”

  1. We have a building that is 12’x8′ with 10′ ceilings. We will be insulating the whole building. It gets afternoon sun at the hottest part of the day. We live in the Southeastern United states and have highs near 100°F or more during the summer and lows in the teens during the winter. Do you all have a unit that would keep it cool in the summer and warm during the winter. What size unit would you all recommend? We have been researching for months.

    1. A 12×8 is certainly a more compact space than we design our mini-splits to handle. Your best bet might be a traditional window unit.

  2. I have a detached insulated work shed that is 12×16 (192 sq ft) with a cathedral ceiling. Would the 9000btu mini split be too big?

  3. What is the mrcool 3rd Gen 36k btu mini split capacity range in cooling btu’s. Knowing this will make my sizing decision much easier. Thanks.

  4. Hello, Mr. Cool!
    I am trying to size a 3rd Gen DIY unit for a 200 sq foot room with a vaulted ceiling, which goes from 8’ to 17’. This is a top floor space with The ceiling located right underneath the roof with no vented attic. Th roof is exposed to direct sun all afternoon and the room can get as hot as 90F in the summer. There is a single zone central air system but the room temp difference between the ground floor (where the thermostat is) and the top floor is too great for any efficient cooling of the top floor. The indoor unit will hang at the top of the tallest wall (17’) blowing cool air down.
    Thank you very much for your help.

  5. Help! We live in SW Michigan. Appx 2800 sf 2-story plus unfinished (hope to finish) basement. 4br+dining room, family room. Our current heating system is very inefficient and AC unit does not work. We got an estimate from pro for replace/ install of new furnace/AC for $15k+! It’s it feasible to DIY a whole house solution with only MRCOOL products? Can a multi- zone system completely replace the traditional furnace/AC we have or can it at least be primary and the traditional stuff be back- up? We were freezing in the blizzard-y winter and wasn’t spring. Now we’re burning up here and need to get these sorry window units out of here! I tried to find a way to order the DIY multi- zone unit, but don’t see that option. Hellllp!!

    1. You could definitely use a DIY Multi-Zone for all your cooling needs. However, if you’re in Michigan, we would highly recommend a good furnace for heating. Any heat pump, ours or anybody’s, suffer in very low temperature conditions. Your best bet would be to replace your current heating system with a new, high efficiency furnace, and use a Multi-Zone for cooling and supplemental heat.

  6. Hi,

    I live in Charlotte NC. I have a two car garage, 3 walls exposed to sun at various parts thought the day (E, W, N), the other wall is attached to the house, and there is a finished room above the garage as well as knee walls on the edges with supper attic type insulation. The garage is ~25x26ft with 9.5 ft ceilings; dry walled with 19880’s 1/2 inch wall board behind it serving as very, light insulation; new insulated garage door; one east facing window (double pane and low e) ; and one half light door facing with normal glass facing SW/W. How many BTUs would you recommend in the Gen 3? I am not sure if 12k is enough, afraid 18k is too much, and think anything over 20k is for sure too much. What is your recommendation?

  7. I have a 30ft. by 40ft. pole building that is my garage and woodshop in south Texas. It is very well insulated with foam on all walls and roof and the two 10 x 10 garage doors have Styrofoam insulation. In the 100 degree heat of summer, it will get up to about 88 degrees in the garage. Heating in the winter is a non issue as the coldest day we have seen was only 26 degrees and I kept the garage at nearly 60 degrees with 2 halogen flood lights. If I closed off the open area above the 11 ft. high rafters, do you think I could get by with a 24,000 btu diy mini split to cool the rest of the area to below 75 degrees in the heat of our summers?

    1. You might be able to get by with a 24k. It’s hard to say without doing a Manual J. What do you have in place that is maintaining 88 degrees in summer?

      1. Just a heavy layer of blown in expanded foam insulation, insulated steel doors and garage doors, and no windows.

  8. Hi, I have a game rm attached to the main house that is 550sq ft. The rm is next to the kitchen with just a counter separating the rooms. The game rm is typically over 90f in this South Florida heat. The main part of the house has central ac but does not cool the game rm even when set to 70f. The ceiling in the game rm is 8ft with no insulation, I want to keep the rm at 78f. What size should I buy 18k or 24k.

  9. I am looking to size a Advantage 3rd Gen mini split unit for an attic turned living space. 750 sq ft with sloped ceilings and open stairway to 1st floor. Walls and ceiling have some insulation, but not great. 1st floor AC doesn’t cool entire 1st floor and thinking about going with 24K unit to help counter heat coming up from the 1st floor and cooling dropping from 2nd floor.

    Does this seem like a good option? Will eventually add another minisplit in kitchen to help 1st floor after remodel.

    1. With poor insulation, a 24k could be the right choice for that space. Since you plan to install the Advantage, you should consult with the installer before you settle on a particular size.

  10. If an 18,000 BTU condenser feeding one air handler is needed to cool a 900 sq. ft. space , would a single 18,000 BTU condenser feeding two air handlers be the correct size to cool two 450 sq. ft. spaces? Or, would a larger condenser be needed since there are two air handlers?

    Thank you.

    1. The overall cooling capacity of an 18k condenser will be the same regardless of whether you have one air handler or two.

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