What size mini-split for a 400 sq ft room?
At 25 BTU/sq ft with average insulation, 400 sq ft needs about 10,000 BTU — round up to a 12,000 BTU (1 ton) head. Sunny or poorly insulated rooms may need 18,000 BTU.
Mini-Split Size Calculator
Ductless mini-splits are sized in BTU per indoor head. This calculator totals your space load and recommends the next standard single-zone size — 9,000, 12,000, 18,000, or 24,000 BTU.
Space dimensions
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Sizing settings
Why mini-split sizing is different from central AC
Central systems split capacity across ducts and multiple rooms. A single-zone mini-split must handle the entire connected space alone — open floor plans work; closed floor plans often need one head per primary room or a multi-zone system.
Compare options in our single-zone vs multi-zone guide before you buy one large head for a whole house.
Inverter mini-splits and oversizing
Modern inverter units modulate down on mild days. Slightly oversizing a bedroom is common because retail minimums are 9,000 BTU. Severely oversizing a small insulated room causes short cycling and poor dehumidification — size to calculated load, not “biggest available.”
See common mini-split sizing mistakes before you commit to a unit.
Heat pump heating BTU
Most mini-splits sold today heat and cool. Our heating estimate is for planning only — cold-climate performance depends on HSPF rating and install quality. Northern cabins may need supplemental heat below design temperature.
Frequently asked questions
Can one mini-split cool multiple rooms?
Open floor plans sometimes work with one head. Closed floor plans usually need multi-zone systems or one head per primary space. This calculator totals area for a single zone.
Do mini-splits provide heat?
Most modern mini-splits are heat pumps that heat and cool. Our heating BTU estimate is a rough planning number — cold-climate performance depends on model HSPF and install quality.
Related calculators
HVAC Calculators · https://hvaccalculators.net/mini-split-calculator/