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How To Correctly Size Your Ductless Mini-Split System

The most important step in a mini-split ductless system installation is choosing the correctly sized unit for your home. Inverter technology makes mini-splits one of the most efficient HVAC options on the market. A ductless mini-split produces only the required amount of heating or cooling to maintain your desired temperature, making them extremely efficient and effective. Below you will find a mini-split sizing guide to help you configure the best match-up to fit your needs.

Mini Split Installer Measuring and Sizing

Why is Properly Sizing Your Ductless Mini-Split So Important?

It is crucial that a mini-split heat pump is sized properly to fit your space. The system will be able to heat or cool your home without issue if sized correctly. To do so, you must choose a system with a BTU rating that is appropriate for your the size of your space and your heating/cooling needs.

What is a BTU?

BTU stands for British Thermal Unit. A BTU is unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.

As a general rule of thumb, the BTU rating of your unit will be higher if the room is larger. A mini-split's rating can range from 6,000 to 56,000 BTUs. Note that a mini-split should have a higher heating capacity than their cooling capacity.

You should also be aware that the system you choose should not be too large or too small for the space you are trying to heat/cool. You could experience unwelcome consequences that can affect your daily comfort, heating, cooling, and overall function of your mini-split.

What happens when a mini-split is too big or too small for your space?

Importance of sizing your ductless mini split correctly

A ductless mini-split system that is too small for your environment can lead to inefficiency and discomfort in the space you are looking to condition. Being too small for a space will cause the system to run continuously without effectively conditioning the space it's in, which will end up wasting money and energy.

If the system you choose is too big for your space, it will likely also end up wasting money and energy. This would happen due to short cycling, which causes a system to turn off and on constantly. It may also lead to quicker wear and tear and shorten the life cycle of the system.

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How To Size Your Ductless Mini-Split System

Sizing Your Ductless Mini Split System Correctly

Mini-split systems can be installed in either a single zone or multi-zone situation. A single zone system will have a compressor and air handler that have the same BTU output. A multi-zone ductless mini split will have a condenser (outdoor section) that is a different, but complementary size for the associated indoor air handlers i.e. a 36k BTU outdoor unit is compatible with four 9k BTU air handlers.

Note that since each room in your home may not be the same size you will likely have different sized air handlers. The BTU rating for each indoor air handler will most likely vary.

Although we highly recommend getting professional assistance when determining the right mini-split size, an expert technician might use the following steps to determine the BTUs needed.

  1. First, you will start by measuring the square footage of each room. Multiply the room's length and width by their measurement. If the room measures 30 feet by 40 feet in length, it will be 30 x 40 x 1,200 square feet.
  2. A technician will multiply the square footage by 25 to get an approximate BTU rating. If you have 1,200 square feet, multiply it by 25 to get 30,000 BTUs.

These calculations work for rooms with a rectangular shape. Triangular and circular room shapes are also common.

No matter what shape your room is, a professional can help. It is a good idea for you to call your trusted technician to determine the right ductless mini-split size for your home, office, warehouse, industrial space, etc.

Below is an estimate BTU size chart for mini-split system:

Ductless BTU Sizing Chart

Area Size BTU
up to 375 sq. ft. 9,000
375 to 550 sq. ft.  12,000
550 to 800 sq. ft. 18,000
800 to 1,050 sq. ft. 24,000
1,050 to 1,550 sq. ft. 36,000
1,550 to 2,650 sq. ft. 48,000


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Considerations that Affect Sizing a Mini-Split

Considerations when sizing your mini split

Remember that room size is only one factor in determining the BTU power you require. You will also need to consider the environment and other factors when determining the size of your mini-split.


Temperature indoors can be affected by the weather outside. Your mini-split may be different sized depending on where you live. If you live in an area with temperatures exceeding 90 degrees Fahrenheit frequently, the BTU rating for the system that you purchase should be increased by at least 30%.

Insulation and Age of Building

Due to modern building codes, newer buildings tend to be more insulation-efficient than older ones. Poor insulation in older buildings means that it takes more energy to heat or cool. To be comfortable, the BTU rating must be increased by at minimum 30%. If you have a new house with excellent insulation, it will require fewer BTUs.

Ceiling Height

Ceiling height increases the size of the room. Eight feet is the standard ceiling height in most homes. If your ceilings are taller than this, it is recommended to increase the BTUs of the mini-split that you purchase by at minimum 20%.

Other Components

The following factors can affect the BTU load calculation for your home:
Windows and Doors: Windows and doors do not have insulation. They can potentially have cracks, or are not sealed well and can certainly affect the BTU load.

Direct Sunlight: It can heat up a room and in turn impact the size you choose.

Kitchen: A room with appliances tends to be warmer due to refrigerators, stoves, and other heat sources. You may need to increase BTU load in your estimate and size up in order to adequately cool it in the hot months.

Room Occupancy: The number of people in the room can have an impact on the size of your mini-split. You should therefore take into account the number of people who live in the space you're putting the equipment.

Lightbulbs: Lightbulbs can also produce heat. Some types of bulbs generate more heat than others. If a large number of lightbulbs are in a space, it will affect the size of the unit.

Floors: Carpeted floors are more insulated than hardwood floors. This could affect the space's required size.

Browse MRCOOL DIY Multi-Zone Ductless Split Systems


Load calculation is used to determine the right size mini-split for your living space. This calculates how much warm or cool air is needed to maintain a comfortable temperature in the space.

You will find load calculators available online, but they can result in miscalculations. A professional HVAC technician can help you determine the right size for your space. A qualified local ductless technician will give you an accurate and professional sizing of your space to ensure you get exactly what you need.

Have Questions?

Leave them in the comments below or reach out to us directly.

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If a builder installs an undersized mini split on a new build is he responsible to replace with a larger btu unit?

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