What size mini split do I need?
Size a ductless mini split head by head. Add a room for each head, and the tool sizes each one and totals them for the outdoor condenser. Garages and sunrooms get the extra heat-gain they need. These are heat pumps, so the same head that cools also heats.
Mini split size calculator
per head + condenser totalAdd each room you want to cool to size the heads and the outdoor unit.
On the garage and sunroom numbers: those rooms carry a rough heuristic bump (about 20 percent) for their extra heat gain. That figure is an industry rule of thumb, not a manufacturer or DOE number; the makers point to a Manual J for those spaces. Nothing you enter leaves your browser.
Per head, then the condenser total
A mini split has an outdoor condenser and one or more indoor heads. Each head cools one room, so you size each room on its own, using the same 20 to 25 BTU per square foot cooling load as central air, adjusted for climate and sun. Then you add the room loads together, because the outdoor unit has to carry all of them at once.
Garages and sunrooms are the special case. They run hotter than a bedroom of the same size, uninsulated and sun-baked, so we add a rough bump for them. That bump is an industry convention, not a manufacturer or DOE figure, and the makers themselves point to a full load calculation for those rooms.
Do not oversize the heads
The temptation with multi-zone is to round every head up. Do not. An oversized head cools its room in a burst and shuts off before it pulls humidity out, the same short-cycling problem oversized central AC has. Match each head to its room, and match the condenser to the total.
Most residential multi-zone condensers support four or five heads. If your layout needs more, that is usually two systems, and it is the point where a licensed installer's load calculation earns its keep.
Common questions
What size mini split do I need for a 2-car garage?
A typical 2-car garage runs 400 to 600 square feet, and garages carry extra heat because they are usually uninsulated and sun-exposed. That lands around 12,000 to 18,000 BTU, roughly a 1 to 1.5 ton head. Add the garage as a room in the calculator and it applies the extra heat-gain bump. The bump is an industry rule of thumb, not a manufacturer figure.
What size mini split for a 500 sq ft room?
At the common 20 to 25 BTU per square foot, 500 square feet is around 10,000 to 12,000 BTU, a 9,000 or 12,000 BTU head depending on climate, sun, and ceilings. A sunny room or a hot climate pushes toward the top; a shaded northern room sits lower. Enter the room and your climate zone for a specific number.
How do I size a multi-zone mini split?
Size each room on its own, then add the room loads together to size the outdoor condenser. The calculator does this: add a room for each head, and it shows the per-head BTU and the condenser total. Match the condenser to the total, keep each head close to its room's real load, and remember most condensers support four or five heads.
Can one mini split cool a whole house?
A single head cools one open area well, but it will not evenly cool a house chopped into rooms; doors and walls block the air. For a whole house you either use a multi-zone system with a head per area or stay with ducted central air. The multi-zone builder here helps you see the head count and total a whole-house layout would need.