Question 1

What size mini split do I need?

Accepted Answer

As a rule of thumb, a room needs about 20–30 BTU per square foot depending on climate, so a 400 sq ft room lands around 9,000–12,000 BTU and a 600 sq ft room around 15,000–18,000. Sun exposure, ceiling height, a kitchen's appliance heat, and extra occupants all raise it. Enter your room above to get a size rounded to a real mini-split unit, then confirm with a Manual J load calculation.

Question 2

How many BTU per square foot for a mini split?

Accepted Answer

Roughly 20 BTU/ft² in a cold northern climate, about 22–25 in moderate-to-warm areas, and up to 30 in the hot Deep South or Southwest — for cooling. A very sunny room adds about 10%, high ceilings add more air to condition, a kitchen adds ~4,000 BTU, and each occupant beyond two adds ~600 BTU.

Question 3

Is it bad to oversize a mini split?

Accepted Answer

Yes. An oversized unit cools the air quickly but shuts off before it removes much humidity, so the room feels cold and clammy and the system short-cycles — which wastes energy and wears out the compressor. Mini splits modulate, so a correctly sized or only slightly larger unit is far better than a big one. Bigger is not better here.

Question 4

Do I need one mini split per room?

Accepted Answer

Each indoor head conditions the open space it's mounted in, so closed-off rooms generally each need their own head (a multi-zone system) or you keep doors open. Size every zone separately and add the heads up — this calculator sizes one room at a time. For the whole-home picture, compare a ducted system in our ductless vs central guide.

Mini Split Sizing Calculator

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