What Size Mini Split Do I Need? BTU by Room Size
Start with the room, not the unit
A mini split that is too small runs all day and never catches up. One that is too big cools the air fast, shuts off before it pulls the humidity out, and leaves the room cold and damp. The right size is the one that matches the room's load, and for a single room you can get close with a tape measure and the table below.
Measure length times width in feet. If the room opens into another space with no door, add that space; the unit will be conditioning it whether you planned to or not.
BTU by square footage
| Capacity | Rated range | Room size (starting point) |
|---|---|---|
| 9k | 9,000–9,100 BTU | 350–450 sq ft |
| 12k | 11,500–12,000 BTU | 450–750 sq ft |
| 18k | 17,000–18,000 BTU | 750–1,100 sq ft |
| 24k | 22,500–24,000 BTU | 1,100–1,500 sq ft |
These figures assume 8 ft ceilings, average insulation, and a room that is not fighting a west-facing wall of glass. They are the same numbers we use on our Sizing Guide page, and in the middle of each range they work out to roughly 20 BTU per square foot. That rule of thumb is fine for a shortlist. It is not a load calculation.
Adjust for the room's loads
Each of these adds heat the unit has to remove, or lets conditioned air escape:
- Ceiling height. The table assumes 8 ft. A 10 ft ceiling adds a quarter more air to condition; a vaulted room adds more.
- Sun. A room facing west or south with large windows takes a heavy afternoon load. Blinds help; they do not remove it.
- Insulation. A room over a garage, under a roof with no attic, or in an older house with thin walls loses what you pay to condition.
- Kitchens. Ovens, cooktops, and refrigerators all dump heat into the room. Size a kitchen up.
- Florida and Gulf Coast humidity. Moisture is a load. A correctly sized unit runs long enough to wring it out; an oversized one short-cycles and leaves the room clammy.
- People and equipment. A home office with two monitors and a printer, or a family room full of people on a weekend, is warmer than an empty bedroom.
If one of these applies, aim for the top of the range. If 2 or more apply, go up a size. Do not go up two sizes on a hunch; oversizing has its own cost, described above.
When a Manual J is worth it
A Manual J is the load calculation HVAC contractors use. It accounts for orientation, window area and glazing, insulation values, air leakage, and local climate, and it produces a number in BTU per hour rather than a range. It costs money and takes a site visit or a detailed set of plans.
For one bedroom or office with standard construction, the table above is enough. A Manual J earns its cost when:
- You want one system to condition a whole floor or several rooms through open doorways.
- The space is a converted garage, sunroom, or addition where nobody knows what is in the walls.
- Heating matters as much as cooling. Heat pump output drops as the outdoor temperature drops, and the rated BTU on the box is measured at standard test conditions, not at 5°F.
- You are choosing between two sizes and the difference is a 208-230V circuit you do not have yet.
Pick voltage before capacity
The size you land on decides the electrical work. 9k and 12k units come in 115V and 208-230V; 18k and 24k units are 208-230V only, which means a dedicated two-pole breaker and a disconnect at the condenser. If you have not settled the circuit, read 115V vs 208-230V: which circuit do you have? before you order.
Where to shop by size
- 350–450 sq ft: 9,000 BTU mini splits
- 450–750 sq ft: 12,000 BTU mini splits
- 750–1,100 sq ft: 18,000 BTU mini splits
- 1,100–1,500 sq ft: 24,000 BTU mini splits
Every unit we sell is a single-zone system: one indoor head, one outdoor condenser. If your numbers say two rooms need two heads, that is two systems, and a tech should confirm the plan. We sell the equipment and recommend installation by an EPA Section 608 certified technician; our Installation page lists what to ask for.