Cassettefrom $1,312
What size MRCOOL mini split do I need for my room?
Short answer: For MRCOOL mini split sizing, start at 20 to 25 BTU per square foot for a room with 8 ft ceilings, then adjust for ceiling height, sun, a kitchen, insulation and your climate. In round numbers, a 9K handles up to about 350 sq ft, a 12K up to 550, an 18K up to 800, a 24K up to 1,050 and a 36K up to 1,550. When your result lands between two sizes, the room details below decide which way to go.
Sizing is the question we get asked most on the phone. Usually someone has measured one room and wants to know if it takes a 12K or an 18K.
This guide walks you through the same math we use on those calls, then works out six real rooms so you can see each adjustment in action.
How many BTU do I need per square foot?
Multiply the floor area by 20 to 25 BTU. Measure length and width in feet and multiply them. For an L-shaped room, split it into two rectangles and add the areas together.
Use the 20 end for a shaded, well insulated room in a mild climate. Use 25 for an average or sunny room, an older house, or a place with hot summers or hard winters.
| Room area | At 20 BTU/sq ft | At 25 BTU/sq ft | Size to start with |
|---|---|---|---|
| 150 sq ft (10 x 15) | 3,000 | 3,750 | 9K |
| 250 sq ft | 5,000 | 6,250 | 9K |
| 350 sq ft | 7,000 | 8,750 | 9K |
| 450 sq ft | 9,000 | 11,250 | 12K |
| 550 sq ft | 11,000 | 13,750 | 12K, or 18K if sunny or tall |
| 700 sq ft | 14,000 | 17,500 | 18K |
| 800 sq ft | 16,000 | 20,000 | 18K, or 24K if it runs hot |
| 1,000 sq ft | 20,000 | 25,000 | 24K |
| 1,300 sq ft | 26,000 | 32,500 | 36K |
| 1,550 sq ft | 31,000 | 38,750 | 36K |
These are starting points. A load calculation done room by room is more precise, and we cover when that's worth paying for further down.
What do the max square feet on your product pages mean?
Our 5th Gen listings name a max room size for each unit: 350 sq ft for the 9K, 550 for the 12K, 800 for the 18K, 1,050 for the 24K and 1,550 for the 36K. Those work out to roughly 22 to 26 BTU per square foot, so they assume an average room with 8 ft ceilings.
Treat them as limits, not targets. A 540 sq ft room with a 12 ft ceiling is a very different load from a 540 sq ft bedroom.
| Size | Listed max room | Power | Single zone lines | Typical rooms |
|---|---|---|---|---|
| 9K | 350 sq ft | 115V | DIY Select, DIY EasyPro, DIY 5th Gen | Bedroom, office, nursery |
| 12K | 550 sq ft | 115V | All three | Primary suite, small living room |
| 18K | 800 sq ft | 230V | All three | Living room, rec room, garage |
| 24K | 1,050 sq ft | 230V | DIY Select, DIY EasyPro, DIY 5th Gen | Open kitchen and living area |
| 36K | 1,550 sq ft | 230V | DIY 5th Gen only | Great room, large shop |
Next: What does BTU mean, and why is a bigger unit not better?
Why does getting the size right matter so much?
An undersized unit runs flat out on the hottest afternoon and the room still drifts up to 78°F. In winter it's worse, because a heat pump puts out less heat as the outdoor temperature drops, right when the house needs more.
An oversized unit causes a different kind of trouble. The inverter compressor slows down to match the room, but every unit has a minimum speed. When the room needs less than that minimum, the system starts and stops, the coil never stays cold long enough to pull moisture out, and 72°F feels clammy.
You also pay for capacity you won't use. Going from 18K to 24K adds about $200 to $420 depending on the line (prices as of November 2026), and moving from 12K to 18K means a 230V circuit instead of 115V.
How do I adjust for high ceilings?
The 20 to 25 BTU rule assumes 8 ft ceilings. For anything taller, multiply your result by the ceiling height divided by 8.
- 9 ft ceiling: multiply by 1.13
- 10 ft ceiling: multiply by 1.25
- 12 ft ceiling: multiply by 1.5
For a vaulted or cathedral ceiling, use the average height. Walls at 8 ft rising to a 14 ft peak average out to 11 ft, so multiply by about 1.38.
Here's how much it moves the result. A 400 sq ft room at 22 BTU needs 8,800 BTU, which fits a 9K. Give that same room a 10 ft ceiling and it needs 11,000, which is 12K territory.
How do sun, kitchens, insulation and climate change the size?
Apply these after the ceiling adjustment, in this order:
- Very sunny room with large south or west windows: add about 10%.
- Kitchen in the space: add about 4,000 BTU for the heat from cooking.
- Poor insulation, single pane glass, or a room tucked under the roof: use the 25 BTU end, and go one size up if you're close to the line.
- Very hot summers or very cold winters where you live: lean to the high end the same way.
- Shaded room, good insulation, newer windows: use the 20 BTU end and size down a little.
Don't stack every adder by reflex. If you've applied all of them and you're 1,000 BTU over a size, take a second look at the room before you jump up.
Do I size for heating or for cooling?
In the South, cooling sets the size. In the North, heating usually does, if the mini split is going to be your main heat for that room.
All three of our lines heat down to −13°F. "Heats down to" means the unit keeps running and making heat at that temperature. It does not mean you get the full rated output, which drops as it gets colder. The single zone specifications page has the numbers for each size.
If your winter nights regularly hit single digits and this is the main heat source, size toward the high end and keep a backup heat source for the coldest week of the year. Below −13°F, call us for cold-climate sizing.
What size do I need? Six rooms worked out
1. Guest bedroom, 12 x 14 ft, shaded
168 sq ft times 25 is 4,200 BTU. The smallest single zone unit is a 9K, so that's the pick. It will spend most of its time at low speed, which is fine for a bedroom.
2. Living room, 16 x 22 ft, 9 ft ceiling, west windows
352 sq ft times 25 gives 8,800. The ceiling factor of 1.13 brings it to about 9,940, and 10% for afternoon sun makes it about 10,940. On floor area alone this looked like a 9K. With the ceiling and sun, it's a 12K.
3. Open kitchen and family room, 24 x 30 ft, 8 ft ceiling
720 sq ft lands between 14,400 and 18,000 BTU, and the kitchen adds 4,000, so you're at 18,400 to 22,000. We'd go 24K. An 18K could keep up in a shaded house in a mild climate, but on a July Sunday with the oven on it will fall behind.
4. Detached garage shop, 24 x 24 ft, 10 ft ceiling, thin insulation
576 sq ft times 25 is 14,400, and the ceiling factor of 1.25 makes it 18,000. That sits right on the 18K line with weak insulation, so the safe pick is a 24K.
The other route is to insulate the garage door and ceiling first, then buy the 18K. In a cold climate, an 18K in an insulated shop is a better setup than a 24K in a drafty one.
5. Great room, 30 x 40 ft, 10 ft ceiling
1,200 sq ft at 22 BTU is 26,400, and the ceiling brings it to 33,000. That's a 36K, which only the DIY 5th Gen line offers in single zone. See the 36K single zone systems.
6. Finished basement, 28 x 32 ft, 7.5 ft ceiling
896 sq ft at 20 BTU is about 17,900, and a ceiling under 8 ft trims it slightly, to about 16,800. Below grade rooms stay cooler in summer, so the 20 end is right here. An 18K fits. If it's a walkout with a wall of glass, lean toward 24K.
| Room | Area | Adjusted BTU | Our pick |
|---|---|---|---|
| Guest bedroom | 168 sq ft | 4,200 | 9K |
| Living room, 9 ft, sunny | 352 sq ft | about 10,900 | 12K |
| Kitchen and family room | 720 sq ft | 18,400 to 22,000 | 24K |
| Garage shop, 10 ft | 576 sq ft | 18,000 | 24K, or 18K after insulating |
| Great room, 10 ft | 1,200 sq ft | 33,000 | 36K |
| Basement | 896 sq ft | about 16,800 | 18K |
Next: How do I size a multi-zone system for several rooms?
What if my room is long, L-shaped or open to other rooms?
One indoor unit pushes air from one wall. It does a good job in a rectangle and a weaker job around corners.
In a room 40 ft long, mount the head on a long wall near the middle and let it blow across, not down the length from one end. For an L-shaped space, add both legs to your area and place the unit where it can see into both. Expect the far end of the short leg to run a degree or two off.
Open stairs change things too. Warm air rises up a stairwell in winter, and cool air pours down it in summer, so a living room open to a second floor needs more capacity than its floor area suggests.
Bedrooms off a hallway are the most common sizing mistake we see. A big unit in the living room will not cool or heat a bedroom with the door closed. If those rooms matter, they need their own indoor units, which is what a multi-zone system is for.
What if I'm between two sizes?
Lean up when the room has strong sun, a kitchen, high ceilings or poor insulation, or when the unit will be the main heat in a cold climate.
Lean down for a shaded room, a basement, a well insulated newer home, or a bedroom where humidity control and low fan noise matter more than fast cool-down.
One practical point between 12K and 18K: the 12K runs on 115V and the 18K needs a 230V circuit. Both need their own dedicated circuit, and the electrical work for each is different. Breaker and wire sizes are in the installation manual for your size.
What if I want to heat and cool the whole house?
A single zone system covers one space. In a 1,400 sq ft ranch with three bedrooms, even a 36K in the living room won't reach the back bedrooms.
For that, look at a DIY 5th Gen multi-zone: one outdoor unit serving 2 to 6 rooms. You size each room with the same method above, then pick an outdoor unit big enough for the rooms that run at the same time on the hottest or coldest day.
For a whole-house plan or a new addition, a Manual J load calculation from an HVAC contractor or energy auditor is the formal way to size. It looks at every window and wall, and it's worth the fee when you're spending several thousand dollars.
Which line should I buy once I know the size?
The size comes first. The line decides efficiency, cold weather features and warranty.
- DIY Select, from $1,249: the lowest price, 9K to 24K, an 11.5 ft line set in the box, 1 year parts and 2 year compressor warranty. Good for a mild climate or a room you use part of the year.
- DIY EasyPro, from $1,487: ENERGY STAR certified at 12K, 18K and 24K, leak detection, Follow Me, and a 3 year parts and 5 year compressor warranty.
- DIY 5th Gen, from $2,329: the widest SEER2 range (18.0 to 23.6), a 25 ft line set, the only 36K option, and a 5 year parts and 7 year compressor warranty.
The full line comparison chart puts every spec side by side, and our article on Select vs EasyPro vs 5th Gen explains who each line suits.
Ready to pick your size?
Measure each room: length, width, ceiling height, which way the big windows face, and whether a kitchen is part of it. Run it through the steps above and write down your number.
Then shop by size in our single zone systems. If you're stuck between two sizes, call us at +1 (844) 999-9725, Monday to Friday, 9 AM to 5 PM EST, or text your measurements to 302-385-6142, and we'll tell you which one we'd install in that room.
