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Mitsubishi MSZ-hj25va mini split indoor unit with its filter cover open

What Size Mini Split Do I Need for My Home?


Choosing the right size unit is critical when installing a new air conditioning system in a home. Ensuring your new mini split system is the correct size for your living space is essential if you want maximum comfort, energy efficiency, and reliability.

However, finding the right mini split system size can be challenging if you don’t know where to start. Learn what factors to consider when choosing a mini split for your space, including BTUs, room measurements, and zone cooling capabilities. 

a mitsubishi electric minisplit condenser

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AC Sizing, BTUs, and Tons

When shopping for a new high-quality air conditioning system such as a mini split, you will see two units of measurement on virtually all products: BTUs and tons. Both these units refer to the system’s capacity or size. 

A British Thermal Unit (BTU) is an imperial unit of heat. The HVAC industry has moved to BTU measurements as mini split systems have become more popular. One BTU equals the energy required to heat one lb. of water by one degree Fahrenheit.

In air conditioning, a ton refers to a ton of refrigeration and is equivalent to the cooling power of a short ton (2,000 lbs.) of pure ice melting over 24 hours. One ton is equal to 12,000 BTU. For example, if you’re looking at a Mitsubishi 9,000 BTU mini split system, its capacity in tons is 0.75.

a close up of a mitsubishi electric mr. slim mini split logo


How Many BTUs Do I Need?

The BTU capacity or tonnage of your mini split must be proportional to the amount of space you want to cool or heat. However, unlike traditional central air conditioning systems, a mini split is zoned, meaning it will only provide climate control in rooms with indoor units.

For this reason, you cannot use your house’s overall square footage to calculate the ideal size for your unit. Instead, you must determine the number of zones you’ll use, measure each zone’s square footage, and calculate your BTU requirements based on those measurements. 

Step 1: Determine the number of zones.

Mini splits are available in single-zone and multi-zone units. For example, the Mr. Slim family of Mitsubishi mini splits is available with up to five zones.

Single-zone units provide climate control in a single room, whereas multi-zone models can typically control between two and five different areas. Assess your space and determine the zones for your mini split. For example, if you decide on three zones, you’ll need three units for the multi-zone split setup.

Step 2: Measure the square footage of each zone.

To get a unit with the correct BTU capabilities, you need to measure the square footage of each zone. Measure the length and width of the foot in feet. Then multiply the room’s width by length to get the total square footage. If your zone encompasses more than one room, add the length of each room, then the width of each room, and multiply the totals to get the total square footage. 

Step 3: Calculate your initial BTU requirements.

According to the Department of Energy, each square foot of living space requires 20 BTU. Although the guidelines indicate this method only considers room size, the estimate offers a valuable starting point to determine your space’s ideal mini split sizing.

For instance, if your living room is 360 ft2, multiply 360 by 20 to obtain the ideal BTU capacity for this room: 7,200 BTU.

You can also use estimates in the other direction to estimate the ideal living space size of a mini split system with a known capacity. For example, if you’re looking at a single-zone Mitsubishi 6,000 BTU mini split, divide 6,000 by 20. This gives you a relatively accurate assurance that this system is ideal for a 300 ft2 area.

Other Factors That May Affect Your Mini Split Size

The square footage of each room is only a starting point. You’ll need to check for alternate elements like your room’s internal climate and insulation to get a more accurate estimate of your ideal mini split sizing needs. Some of the most critical factors include:

  • Ceiling height: The 20 BTU per square foot rule assumes a standard ceiling height of eight feet. Every additional foot over eight increases the amount of living space by 12.5%; consequently, multiply the corresponding room’s BTU needs by 12.5%.
  • Climate: If you live in a colder region, reduce your overall BTU needs by 10 to 30%. If your region is warmer with hot and humid summers, increase them by 10-30%.
  • Sunlight: The more sunlight the room receives, the warmer it becomes, increasing that room’s BTU requirements by 10-20%. Heavily-shaded rooms are cooler and may require 10-20% fewer BTUs.
  • Insulation: You can install a mini split indoor air handler in less well-insulated rooms and zones, such as an outdoor shed, workshop, or garage. If you do, increase that zone’s BTU needs by 20%.


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