What size mini split do I need?
The honest version: it's mostly square footage, and then four things that move the number. Here's how to get it right the first time.
The short answer: plan on roughly 20 BTU per square foot. A 500 sq ft room lands on a 12,000 BTU unit. A 900 sq ft open living area lands on 18,000.
Then adjust up for tall ceilings, poor insulation, heavy sun or a garage — and adjust down for a well-sealed, shaded room.
Start with the square footage
Measure length times width of the space the unit has to condition. If it opens into another room without a door, count that space too — air doesn't respect doorways.
| Room size | Unit size | Typical use |
|---|---|---|
| 300–450 sq ft | 9,000 BTU | Bedroom, office, small shop |
| 450–650 sq ft | 12,000 BTU | Large bedroom, garage, bonus room |
| 700–1,000 sq ft | 18,000 BTU | Open living area |
| 1,000–1,300 sq ft | 24,000 BTU | Small home, large shop |
| 1,300–1,600 sq ft | 30,000 BTU | Whole floor |
Size it for your room
Square footage in, unit out — adjusted for the things that actually matter.
Four things that change the number
1. Ceiling height
The rule of thumb assumes 8 ft. At 9–10 ft you're conditioning 15–25% more air, and a vaulted ceiling can add a third. Size up.
2. Insulation
An uninsulated garage or a room over a crawlspace loses heat far faster than a sealed bedroom. This is the single most common reason a correctly-sized-on-paper unit disappoints.
3. Sun
A room with west-facing glass takes a real afternoon heat load. Heavy shade goes the other way.
4. What you're asking it to do
Sizing charts are written around cooling. If this unit is your heat through a Kentucky winter, that's the load that should decide it — see below.
Bigger is not safer. An oversized unit cools the air fast, hits the thermostat, and shuts off before it has run long enough to pull humidity out. You get a room that's cold and clammy, plus more wear from constant stopping and starting. Correct beats generous, every time.
Sizing for heat, not just cooling
Most people shop these as air conditioners and treat heating as a bonus. If you're in western Kentucky and this unit is carrying a garage or a bonus room through January, size it on the heating load instead — and look at the Hyper Heat line, which keeps producing heat down to −22°F rather than fading out when it gets truly cold.
It matters on the bill, too. Electric resistance heat — a plug-in heater or baseboard — runs about 75¢ an hour at Kentucky power rates. A heat pump moving the same heat runs closer to 28¢, because it moves heat rather than making it. (MRCOOL DIY 4th Gen 12K vs 1500W resistance, 15¢/kWh.)
Still not sure?
Tell us the room — size, ceiling, insulation, what it's for — and we'll size it for you. If the answer is a smaller unit than you were about to buy, we'll say so.
Common questions
Can I really install one myself?
The DIY lines ship pre-charged with quick-connect line sets, which removes the step that normally requires HVAC tools. Most people finish in a day.
What if my room is between two sizes?
Go with the smaller one if the room is well insulated and shaded; go bigger if it's a garage, has vaulted ceilings, or takes hard afternoon sun.
Do mini splits actually heat in cold weather?
Standard models lose capacity as it gets colder. The Hyper Heat line is built for it and keeps heating down to −22°F.
One unit for the whole house?
A single wall unit heats and cools the space it's in. For several rooms you want a multi-zone system or a central system — give us a call and we'll map it out.