For a standard 400-square-foot room, you generally need between 9,000 and 12,000 BTUs.

However, 400 square feet is a “borderline” size in the HVAC world. It sits right on the edge of what a smaller unit can handle and what a larger unit provides. Choosing the wrong one is easy to do:

If you choose 9,000 BTUs

It might struggle to keep up on extremely hot days if the insulation is poor.

If you choose 12,000 BTUs

It might be too powerful for a well-insulated room, causing it to shut off too quickly (short-cycle) before removing humidity.

To decide between the two, you need to look at how the room is built.

9,000 vs 12,000 BTU: Making the Choice

Since you are right on the line, you need to determine if your room has a “Low Load” or a “High Load.” Here is how a technician decides:

When to choose a 9,000 BTU Unit

A 9k unit is usually the perfect choice for a Master Bedroom or a standard Living Room.

If your home has average-to-good insulation, standard 8-foot ceilings, and you aren’t in an extreme climate, stick with the 9,000 BTU unit.

The Technician’s Logic: In a bedroom, comfort is key. You want the unit to run at a low, steady speed to pull moisture out of the air. If you oversize a bedroom unit to 12,000 BTUs, the room will cool down in 5 minutes, the unit will shut off, and you will be left with cold, clammy air.

When to choose a 12,000 BTU Unit

You should bump up to the 12k unit if the room has “heat gain” factors that standard math ignores:

High Ceilings: If ceilings are 10+ feet, you have more air volume.
Kitchens: Open-concept kitchens add load from ovens/fridges.
Poor Insulation: Single-pane windows or drafty walls.

In terms of 12,000 BTU mini split coverage, these units are workhorses, but they need enough heat load to run efficiently.

The “Garage Factor”: 400 Sq Ft Garage Sizing

This is the most common question I get. If you are converting a 2-car garage into a gym or workshop, throw the standard rules out the window.

The Floor: Concrete slabs radiate cold/heat.
The Doors: Garage doors are much leakier than walls.
No Insulation: Most garage walls are just studs.

The Verdict:

For a 400 sq ft garage, start with a 12,000 BTU unit. If you live in an extreme climate (like Phoenix or Northern winters) and have no insulation, you may need 18,000 BTUs. A 9k unit will likely never reach your target temp.

400 Sq Ft Mini Split Sizing Scenarios

If you are still on the fence, find the row below that matches your room type best.

Room Type	Insulation	Ceiling Height	Recommended Size
Master Bedroom	Good / Average	8 ft (Standard)	9,000 BTU
Living Room	Average	8 ft (Standard)	9,000 BTU
Living Room	Average	10-12 ft (Vaulted)	12,000 BTU
Kitchen / Dining	Average	8 ft	12,000 BTU
Garage / Workshop	Poor / None	8-10 ft	12k – 18k BTU
Sunroom	Glass Walls	8 ft	12k – 18k BTU

Still Not Sure? Don’t Guess.

Because 400 square feet is such a tricky “borderline” size, guessing creates a high risk of being uncomfortable. Don’t flip a coin between 9k and 12k.

Use the MiniSplitSizer Tool

Want to understand the math? Read our comprehensive Ultimate Sizing Guide for a deep dive.