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Ductless Mini Split Sizing Guide: Heat Load & BTU Calculator

How to choose a ductless mini split system - BTU calculator

A ductless mini split system is an ideal way to heat and cool several areas of your home in one convenient and energy-efficient package. However, the system is only as good as the size — too big and you risk wasting power and utility bills; too small and it might not do the job sufficiently. So how do you size a mini split?

Here we’ll help you find the ideal mini split size based on the specific dimensions of the rooms that need cooling.

The Importance of Proper Mini-Split Sizing

When it comes to mini splits, bigger isn’t always better. There are several reasons why the system should be proportionate to the room size:

Comfort – An underpowered unit might not be enough for the space, or may struggle if there are additional occupants in the room

Utility – An oversized unit may not be efficient in terms of energy, initial and operating cost, space occupied, and power consumed from the total system load. Similarly, an undersized unit can also cause bigger bills, since they will need to run for longer periods and on higher settings just to cool the room.

Short cycling – Because they are designed for bigger cooling requirements, oversized units can struggle to maintain temperature in smaller spaces. In such cases, they tend to quickly cycle on and off, resulting in more wear and wasted energy.

What Size Mini Split Do I Need?

Once you know the number of rooms that need cooling, the next question is “What size of mini split do I need?” 

First you need to know how many square feet a ductless air conditioner can cool. This capacity is measured in British thermal units, or BTUs. A BTU is the heat needed to raise the temperature of one pound of water by one degree Fahrenheit. The larger the space, the more BTUs are needed.

For air conditioners, there is a BTU-to-room sizing rule of thumb. This can provide a rough estimate of the mini split BTU per square foot you need to efficiently service the room. One major advantage of mini splits over portable air conditioners with the same BTU rating is that they can cool a larger area.

Space in square ft. BTUs per hour needed
0-150 5,000
150-250 6,000
250-300 7,000
300-350 8,000
350-400 9,000
400-450 10,000
450-500 12,000
550-700 14,000
700-1,000 18,000
1,000-1,200 21,000
1,200-1,400 23,000
1,400-1,500 24,000
1,500-2,000 30,000
2,000-2,500 34,000

Getting the Square Footage

We’ve tackled sizing a mini split. Now it’s time to size up the space. While BTU ratings are straightforward, calculating the room size is more complicated. Since rooms come in all shapes and sizes, here’s how to determine the square footage:

Square and rectangular spaces: 

  • Multiple the length by the width

Triangular spaces: 

  • Multiply the length by the width, then divide by two

Non-standard linear spaces:

  • Divide the room into squares, rectangles or triangles as applicable
  • Determine the size of each subplot, then add everything together

Circular spaces:

If you happen to have an oval office, don’t worry – there’s a formula for that too.

  • Measure the diameter (the largest width of the room)
  • Divide it by two
  • Square it
  • Multiply the result by π (3.14159265)

Other Room Factors to Consider

Note that Square-foot-to-BTU is only one part of the cooling equation. Another factor to consider is the heat load, which is the heat gained by the room from different sources. These include the insulation, windows, floors, ceiling, and even the room’s specific location and direction with respect to the sun.

Ceiling height – if  it’s higher than 8 ft., you will to need multiply by the difference. For example, 10 foot high ceilings are 25% taller so you will need to increase your BTU total by 25%.

Sunlight – if the room gets lots of sun, add 10%. Conversely, reduce the BTU by 10% if the room is heavily shaded.

Room type – If it’s a kitchen, add 4,000 BTUs to compensate for kitchen appliances that emit heat like the fridge, cooktop and oven. If you’re looking for a mini split for your garage, you’ll also have to consider the level of insulation and weather. Add 20% for old or drafty garages, or for hot seasons like summer.

Capacity – If more than two people occupy the room regularly or for long periods (such as a family den), add 600 BTUs for each additional person.

Insulation – If the room is poorly insulated, add 20% to compensate for heat loss.

Location – If you live in the south or in climates that experience seasonal temperatures over 90 degrees Fahrenheit, consider adding 30% BTU.

For more information on heat gain and how to calculate the heat load of a room, refer to our check out our Heat Load BTU Calculator.

Indoor-to-outdoor Unit Ratio

Multi-zone units allow you to connect multiple indoor units provided that their total BTU load falls within the outdoor condenser’s capacity. Depending on the model, it is often acceptable for the total load of the indoor units to exceed the outdoor rating by up to 30%.

For example, an outdoor condenser rated for 36,000 BTUs may be packaged with four 12,000 BTU indoor units, for a combined load of 48,000 BTUs. Each outdoor unit will list its maximum connection capacity. This will vary by brand and model so be sure to check the documentation when planning out your system.

This is because each indoor unit functions independently, so they can adjust according to energy need. Unless all rooms are fully occupied in the middle of a heat wave, it’s rare that all units will operate at full capacity simultaneously. So getting a bigger condenser may just be overkill and wasted money.

What About Mini Split Heat Pumps?

We’ve tackled cooling. What about heating? The good news is that when on heating mode, mini splits are much more energy-efficient than traditional electric heaters. This is because they move heat from the outside condenser instead of generating it. 

In fact, their heating mode is so efficient that mini splits actually consume less power than when on cooling mode. So if you’re looking for a mini split heat pump sizing chart, just use the above sq foot-to-BTU table or mini split calculator, and subtract 1,000 BTUs when on heating mode.

Since mini splits last for a very long time, getting the right BTU capacity for the room size is essential. This will ensure many years of comfort, savings and efficiency. When it comes to mini splits, the right size matters to get the best satisfaction.

Find Your Mini Split – Next Steps

Be sure to check out our Multi-Zone Mini Split Buying Guide for a thorough explanation of how to find the right mini split for your needs. Then, hop on over to to browse our collection of ductless mini splits.

If you have any other questions or concerns regarding BTU ratings and choosing the best ductless mini-split for you, please do not hesitate to contact us and speak with one of our qualified sales representatives.

Kristen Turner

Featured blogger for Total Home Supply.

28 thoughts on “Ductless Mini Split Sizing Guide: Heat Load & BTU Calculator”

  1. SEER/EER Ratings: How Efficient is Your AC? says:

    […] efficiency is determined by the unit’s cooling output (measured in BTUs, a topic we covered previously) divided by how much energy the unit consumes (in watts) during one hour of cooling. The higher the […]

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    […] is determined by BTU rating, which we have written about before in regards to air conditioners, and the same basic rules apply to heating BTUs — the larger […]

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    […] each day on our website, and, on our blog, have previously written about all the benefits of and features to consider when purchasing a mini-split AC of your […]

  5. Billy Neilan says:

    We have a four seasons room facing east which is open to the rest of the house on two sides. The room is 15×19. The rest of the house has a central trane air conditioner. What size mini split should we have installed? Thanks for your help

    1. Mickey Luongo says:

      Since the room is open on two sides, it is hard to give an exact estimate of what is needed. I would estimate that you would need about 12,000 BTUs.

  6. holly smith says:

    i have an old philly home, 1500 sq feet the duct work does not go up into 2nd and 3 floor well, 2nd floor some in bedroom and 3d floor nothing. I was thinking of getting rid of central air system, really old and buying a multi level unit i dont know what BTU or size or whwere to put in home to cool entire house, 3 floors, kitchen and bathroom are of concern for me-

    any ideas?

  7. We have several estamites on mini-splits (heating/cooling) for our great room which is 800 sq ft. We have one outside door and one window facing north and one outside door and one window facing south. We have estamites going from 15000 btu Panasonic high performance unit to a couple of 24 000 btu unit quotes. From Mitabechi and Carrier. We are confused as to what size to go to. Can you help us? We are told that the Panasonic 15000 btu unit surpasses the other 18,000 units.

    1. Mickey Luongo says:

      Based on the size of the space, I would recommend something in the 24,000 BTU range. A BTU is a BTU, so the 15,000 BTU Panasonic would not be equivalent to a larger unit.

  8. Art Bonnell says:

    I have a 12 x 14 Sun Room one side open to main house other three sides have 6 foot high Thermopane double glass patio doors and slider windows facing S/W Could you please tell me the BTU I would need in Split Unit for Heat & AC Thanks

  9. Types of Mini Splits: A Guide says:

    […] types of mini splits available, and picking the right one can be a confusing process. Visit our mini split sizing calculator to determine the best size mini split for your needs, then check out our Multi Zone Mini Split […]

  10. How Much Do Mini Splits Cost? A Guide to Mini Split Prices says:

    […] it comes to choosing a mini split heat pump system, size is important. Keep in mind your BTUs per square foot, room size, and general expectations for your unit when purchasing so that you can get the most […]

  11. Hi my living room and my kitchen combine is 14 ft X 23 ft how much btu would I need to cool and heat these two room , thanks

  12. We are having a Mitsubishi 12000 BTU ductless A/C without heat pump installed next month (October). The company that’s doing it is family run and I believe trusted. I’m concerned because the company felt this would cool the whole house (1012 sq ft). The box will go in our bedroom. The Air will have to go around to the living room, to the kitchen to cool all. The house is in sun all day (no trees). We live in NH. Everything I’m reading states we should be using at minimum 15000 BTUs. When I spoke to the company, they said they are conservative but feel this will do the job. What are your thoughts?

    1. Mickey Luongo says:

      Mini splits are designed to cool the room they are located in. Regardless of BTUs, the bedroom that the unit is installed in will be signaiclty cooler than the rest of the house even if the door is left open all the time. If the door is closed, the rest of the house will be unconditioned. What you really need is a multi-zone unit where you would have a single outdoor unit and at least two indoor units to help distribute the air.

      If you are concerned about the sizing, which you should be, ask them to perform a Manual J heat loss calculation to determine what you really need.

  13. What are EER & SEER Ratings? An Energy Efficient AC Guide says:

    […] efficiency is determined by the unit’s cooling output (measured in BTUs, a topic we covered previously) divided by how much energy the unit consumes (in watts) during one hour of cooling. The higher the […]

  14. Barry Hawkins says:

    We have a Daikin 18000 BTU unit and are looking at enclosing the outside compressor unit with a heat pump shelter as we live in a cold weather climate and are subject to chilly north winds. Any thoughts on heat pump shelters in colder climates? Any suggestions on what we should be aware of in installing a shelter?

    Thank you

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  16. My beach house is open concept, 700 sq ft open area, and 160 sq foot bedroom.

    I’ve only seen 9000 BTU units for smaller rooms. Isn’t 9000 BTU too much for 160 ft bedroom? Why don’t they make smaller air handlers?


  17. Cheryl Duguay says:

    We have vaulted ceilings in 450 sq ft…..what size mini split should we install. We’ve been told 12,000 and 15,000 BTU’S by two different companies

  18. I live in South Carolina where the humidity is very high at times. According to my gauge it is currently 70% inside the house now. We added on to the house a few months ago and now our central a/c is to small. Our house is 1400 sf. Would a duck less unit help dehumidify the house and would completely replacing to central with a duck less unit be advisable?

    1. Mickey Luongo says:

      You have a few options. A ductless system would be a big upgrade, but would definitely help with the problem. You can also consider adding a whole house dehumidifier to your existing central system to assist with removing excess humidity.

  19. I have a 220 sq ft ground floor tiny house, with high (gable) ceiling in living area. Living area opens to a loft of approx 120 sq ft.
    There are many windows (about 15, many are small) in the house, nearly all are single pane. The house gets lots of central Texas sun.
    I currently have a 9,000 btu mini split that runs continuously. During the day when it’s 95 outside, my mini split can’t get it below around 85 in the house.
    I will be doing a little more insulating around windows etc.
    Walls and ceiling are already insulated with wool.

    I need to replace my month old 9,000 btu as it is clearly too small. Do you think a 12,000 BTU is enough?

  20. Ductless Mini Split AC Line Sets: Which One Do I Need? says:

    […] various air conditioning systems available for purchase at Total Home Supply, including a guide to understanding BTU ratings, understanding energy efficiency and SEER/EER ratings, how to select the right multi-zone mini […]

  21. Deborah Slusher says:

    Hi, I have a 500 sq foot shotgun house that’s in need of mini split heat/air unit. can you help with what BTU do i need and how many zones? it is a one bdrm, kitchen, livingrm, and bath. You can walk in front door and go through to the back door

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