What size mini split do I need?
Punch in your room and I'll give you the size I'd start with on a real job — using the same sizing guide I've used for years. Then I'll show you where to put it, why clearances matter, and how to install it right.
How I size a mini split
A "ton" of cooling is 12,000 BTU. How many square feet one ton covers depends on how well the space holds temperature. Here's what I use in the field to get in the ballpark.
Standard residential
A normal home with normal insulation. This is where most jobs land.
Spray-foam insulated
A tight, foam-insulated building holds temperature — you need less unit.
Sunrooms, retail, commercial
Lots of glass, doors opening, people and lights — more load per square foot.
Data & server rooms
The equipment is the load. These run hard all year.
Square feet times 25 gives you a rough BTU number with a little cushion built in. Good for a sanity check.
I won't put more than a 12,000 BTU unit in a room under 250 square feet. It'll short-cycle and never dehumidify.
Rules of thumb get you close. A Manual J load calc gets you right. I use LaserCalc Mobile and Wrightsoft.
Mini split sizes: 9,000 to 36,000 BTU
Single-zone mini splits start at 9,000 BTU and top out around 36,000 BTU. Square footage below uses my sizing guide for a standard home and a spray-foam home.
| Size | Tons | Standard home | Spray foam | Sunroom / commercial | Voltage | Typical line set |
|---|
Square footage is a starting point, not a guarantee — windows, sun, climate, ceiling height and how many people use the room all change the load. Line set sizes vary by manufacturer; always use the install manual. Some brands make 6K–7K units, but 9K is the smallest common single-zone size.
What size mini split for my square footage?
The most-asked sizing question I get. Here's where I'd start for one open room or zone, using my sizing guide.
| Room size | Standard home | Spray-foam home | Sunroom / commercial |
|---|
Estimates for one zone at 8 ft ceilings. Lots of windows, west sun, or a kitchen push you up a size; a shaded, tight room can come down. Confirm with a load calc.
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Where to put a mini split — and why clearances matter
Right size in the wrong spot still won't work. The indoor unit pulls air in the top and throws it out the bottom, so where it hangs and how much room it has decides how well it heats and cools.
Indoor unit — do this
- Mount it high on an interior or exterior wall, level, into studs.
- Leave clear space above it — the return air comes in the top.
- Aim it to blow across the longest part of the room.
- Keep the drain running downhill the whole way out.
- Don't hang it over a bed or right above where someone sits.
- Don't tuck it in a corner, behind a beam, or in an alcove.
- Don't put it above a heat source (stove, TV, lamp) — it reads the wrong temp.
- Don't try to cool three rooms from one hallway head.
Outdoor unit — do this
- Give it breathing room behind it and keep the fan discharge clear.
- Set it level on a pad, stand or wall bracket — up off the ground and snow line.
- Leave a service loop in the line set and keep the copper off the house.
- Use rubber isolation pads so vibration doesn't carry into the wall.
- Don't box it in with a fence or shrubs.
- Don't put it under a roof drip line or where dryer vents blow on it.
- Don't aim the discharge at a wall, a window, or a neighbor's patio.
💨 Airflow is capacity
A unit choked by a wall or a ceiling can't move the air it was rated for. Less air means less heating and cooling — you paid for BTUs you'll never get.
🔁 Short-cycling & humidity
When supply air bounces right back into the return, the unit thinks the room is satisfied and shuts off early. The room stays clammy.
🔧 Service & warranty
Manufacturers spell out clearances in the install manual. Ignore them and you'll fight high-pressure trips, ice, callbacks — and maybe your warranty.
Installing it yourself? Start here → Learn installation step by step
115V vs 230V, cold climate & multi-zone
⚡ 115V vs 230V
Only 9K and 12K units come in 115V (they're also made in 230V). 18K and up are 230V only and need a dedicated circuit and disconnect. 115V doesn't save energy — it just pulls more amps.
❄️ Cold-climate units
A standard unit may only heat down to about +5°F and loses capacity as it gets colder. In cold country, buy a cold-climate / low-ambient unit and check the rated heating capacity at low temps — not just the 47°F number.
🏠 Bigger than 36K?
Past 36,000 BTU — or several separate rooms — you're looking at multiple single-zones, a multi-zone system, or ducted equipment. One head can't cool a whole house of closed-off rooms.
My mini split sizing videos
How I size real jobs — houses, shops, garages, spray-foam builds — plus voltage and heat load calcs.
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Hey, I'm Taddy.
I'm James T. Fuller — most folks know me as Taddy from Taddy Digest. I'm a licensed Tennessee mechanical contractor, I own Fuller Heating and Cooling, and I've spent 20 years in the field installing and fixing mini splits. Everything on this site is what I actually do on real jobs.
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Mini split sizing questions
What size mini split do I need?
In a standard insulated home, plan on about 500–600 sq ft per ton (12,000 BTU). Spray-foam homes stretch to about 800–900 sq ft per ton; sunrooms, retail and commercial drop to about 400–450. Use that to get close, then confirm with a Manual J load calc.
What size mini split for 600 square feet?
In a standard home, usually a 12,000 BTU (1 ton). In a tight spray-foam home a 9,000 BTU may be enough; a sunroom or a lot of glass may need 18,000 BTU.
Is it better to oversize or undersize a mini split?
Oversizing is the worse mistake. An oversized unit short-cycles, wastes energy, wears out faster and never pulls the humidity out — the room ends up cold and clammy.
What's the smallest and largest single-zone mini split?
Common single-zone sizes are 9K, 12K, 18K, 24K, 30K and 36K BTU. Past 36K you're into multi-zone, multiple systems, or ducted equipment.
Can a mini split run on 115 volts?
Only 9K and 12K units are offered in 115V. 18K and up are 230V only with a dedicated circuit and disconnect. 115V doesn't save energy — it pulls more amps.
How high should a wall mount mini split be installed?
High on the wall — typically around 6–7 ft off the floor — with at least about 4–6 inches of clear space above for return air. Follow your unit's install manual for exact clearances.