What Size Mini-Split Do I Need? How to Calculate

outdoor ac unit
Photo: Bartek/ Adobe Stock
outdoor ac unit
Photo: Bartek/ Adobe Stock
Melissa Graham
Written by Melissa Graham
Contributing Writer
Updated May 30, 2025
Highlights

  • Mini-split sizes are determined by the number of British Thermal Units (BTUs) required. 

  • You’ll need to measure your room’s square footage first. 

  • To calculate the number of BTUs you need, multiply the square footage by 25. 

  • Your local HVAC professional can help you install a mini-split.

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Picking the right mini-split size is essential for efficiency, comfort, and cost savings. A unit that’s too small won’t cool effectively, while an oversized unit can waste energy and cycle on and off too frequently. To get that perfect level of comfort, you’ll need to do a little math—but don’t worry, no geometry degree is necessary for this project. It all starts with your room's square footage and ends with finding the perfect number of British Thermal Units (BTUs) to keep things cool. Let’s take a deeper dive. 

Mini-Split Sizes Based on Square Footage

When it comes to cooling a room, size matters—in this case, it’s all about the square footage. Pick a mini-split unit with too few BTUs, and your system will be working overtime just to keep up. Go too big, and you'll waste energy (and money) faster than you can say “climate control.” The chart below shows the sweet spot for your space—so that comfort isn’t a guessing game.

Square FootageRecommended BTUs
100-1505,000-6,000
150-2506,000-9,000
250-3509,000-12,000
350-45012,000-15,000
450-60015,000-18,000
Read more

Calculating Mini-Split Sizes

Using your square footage measurements, apply the following formula to determine how many BTUs your mini-split needs:

BTUs = Square Footage x 25

How to Measure Square Footage

Before selecting the perfect mini-split, you’ll need to determine your space’s square footage. Luckily, it’s easier than it sounds: just grab a tape measure, measure the length and width of the room, multiply the two, and there you have it—your square footage. 

Length × Width = Square Footage

For oddly shaped rooms, break the space into squares or rectangles, calculate the square footage of each individual shape, and then add them all up. 

Who to Hire to Install a Mini-Split

tech performing maintenance on mini split ac
Photo: Tomasz Zajda/ Adobe Stock

Once you do the heavy lifting of determining what size mini-split you need, stop there—now’s the time to hire your local HVAC professional to install the unit for you. Although experienced DIYers can likely handle smaller installations, it’s best to leave this job to an expert who can have your new mini-split up and running in no time. This way, you won’t break a sweat and can instead enjoy the cool reward once the unit’s installed. 

Frequently Asked Questions

A mini-split unit works by providing room-by-room temperature control, so you know exactly how cool your bedroom will be on a sweltering summer day. It consists of an indoor and outdoor unit connected by cables and refrigerant lines. In cooling mode, it pulls warm air from the room and sends it outside, returning cool air into your space.

The cost to install a ductless mini-split system ranges from $2,000 to $14,000, depending on the type of unit and the complexity of the installation process. The more zones you want your mini-split to cover, the higher the cost you can expect to pay. If you don’t already have an AC system in place, you can expect to add the cost of permits, an electrician, and a concrete pad. 

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Learn more about our contributor
Melissa Graham
Written by Melissa Graham
Contributing Writer
Melissa has spent over ten years writing about home design, style, and DIYs for regional and national publications.

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