What size mini-split do I need for 400 square feet?

About 12,000 BTU. In a mixed climate like Atlanta the estimate is 12,000 BTU (cooling load 10,100 BTU/h, heating load 7,600 BTU/h). Hot climates need more cooling capacity and cold climates more heating capacity, as the table shows.

How many BTU do I need for 400 square feet?

For cooling, roughly 11,000 BTU/h in a hot, dry climate and 8,700 BTU/h in a cold one. For heating, from 6,000 to 12,000 BTU/h depending on how cold winters get. Older, drafty homes need up to 30% more.

Is it better to oversize or undersize a heat pump?

Neither, but slight undersizing is usually less harmful for cooling. An oversized unit short-cycles and leaves the air clammy. In cold climates, size for the heating load and check the unit’s output at 5°F, or plan for backup heat on the coldest nights.

What size mini-split for 400 sq ft?

Short answer: 12,000 BTU, depending on your climate and insulation. In a mixed climate like Atlanta: 12,000 BTU.

By climate

Average insulation, 8 ft ceilings, three occupants.

Best 12,000 BTU mini-splits

Most efficient

Fujitsu AOUH12KTAP1

12,000 BTU mini-split · AIRSTAGE H-Series · cold-climate certified

SEER2: 31.5
HSPF2: 13.7
Heat at 5°F: 16,200

Top-rated certified model in the size most 400 sq ft rooms need.

Full specs and running cost

Best for cold winters

Fujitsu AOUH12KTAP1

12,000 BTU mini-split · AIRSTAGE H-Series · cold-climate certified

SEER2: 31.5
HSPF2: 13.7
COP at 5°F: 2.62

Cold-climate certified, best efficiency at 5°F in this size.

Full specs and running cost

Compare all 12,000 BTU mini-splits

Work it out for your room

Pre-filled with 400 sq ft. Pick your city and adjust insulation, sun and ceiling height.

Your space

Area to heat and cool sq ftCeiling heightInsulationSun exposurePeople who usually use itIncludes the kitchen?

Your climate

StateCity

Mini-split size: 12,000 BTU
Central / ducted size: 1.5 ton
Cooling load: 11,000 BTU/h
Heating load: 6,000 BTU/h

See 12,000 BTU mini-splits See 1.5-ton central heat pumps

Rule-of-thumb estimate: cooling follows the ENERGY STAR room sizing table, heating uses typical BTU per square foot for the climate (NOAA 1991-2020 heating degree days). For a whole-house system, ask your installer for a Manual J load calculation.

More detail

Cooling and heating loads by climateBTU/h

Hot and dry (Phoenix)cooling 11,000 · heating 6,000
Hot and humid (Houston)cooling 11,000 · heating 6,000
Mixed (Atlanta)cooling 10,100 · heating 7,600
Cold (Chicago)cooling 9,600 · heating 10,800
Very cold (Minneapolis)cooling 8,700 · heating 12,000

Cooling follows the ENERGY STAR sizing table; heating uses BTU per sq ft for the climate (NOAA 1991-2020 normals).

How insulation changes the sizein Atlanta

Poor12,000 BTU · heating 9,900 BTU/h
Average12,000 BTU · heating 7,600 BTU/h
Good12,000 BTU · heating 6,100 BTU/h
Very good9,000 BTU · heating 4,900 BTU/h

Top 5 12,000 BTU mini-splitsranked by SEER2

#	Model	SEER2	EER2	HSPF2	COP at 5°F
1	Fujitsu AOUH12KTAP1	31.5	16.7	13.7	2.62
2	Fujitsu AOUH12KZAH1	30.5	16.0	12.7	2.16
3	Hisense AO-12U3T25Palso sold as 1 other model number	30.0	15.4	11.8	2.25
4	Hisense AS-12UW3TBW00	30.0	15.4	11.8	2.25
5	Mitsubishi Electric MUZ-FX12NLHZalso sold as 1 other model number	29.9	15.4	12.4	2.20

Questions3

What size mini-split do I need for 400 square feet?: About 12,000 BTU. In a mixed climate like Atlanta the estimate is 12,000 BTU (cooling load 10,100 BTU/h, heating load 7,600 BTU/h). Hot climates need more cooling capacity and cold climates more heating capacity, as the table shows.
How many BTU do I need for 400 square feet?: For cooling, roughly 11,000 BTU/h in a hot, dry climate and 8,700 BTU/h in a cold one. For heating, from 6,000 to 12,000 BTU/h depending on how cold winters get. Older, drafty homes need up to 30% more.
Is it better to oversize or undersize a heat pump?: Neither, but slight undersizing is usually less harmful for cooling. An oversized unit short-cycles and leaves the air clammy. In cold climates, size for the heating load and check the unit’s output at 5°F, or plan for backup heat on the coldest nights.

Other sizes

All sizing guides