Free · Equipment sizing
Ductless Mini Split Sizing Calculator
Size every zone in BTUs from room area, sun and use, then see the indoor head for each and the multi-zone outdoor unit that carries all of them. Add up to five zones.
Whole house
Zone 1
Zone 2
Outdoor unit
18,000BTU
Nominal tons
1.5
12,000 BTU per ton
Smallest head
6,000 BTU
Most brands' minimum
Typical installed ranges and what drives them: Mini Split Installation Cost (2026) →
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How mini split sizing works
Every zone is a separate load. The calculator runs a block load for each room using the climate's BTU-per-square-foot rate, then adjusts for insulation, ceiling height, sun and how the room is used. A sunroom carries 35% more load than a bedroom of the same size; a kitchen adds 4,000 BTU/h for the appliances. Each zone is then rounded up to the next standard head: 6, 9, 12, 15, 18, 24, 30 or 36 kBTU.
Rooms run hotter per square foot than a whole house because there is no averaging across shaded and sunny sides, so the calculator never uses less than 20 BTU per square foot for a single room.
Picking the multi-zone condenser
Multi-zone outdoor units are rated in nominal BTU, and manufacturers let you connect heads that add up to more than that, typically 100–130% of nominal, because the zones rarely peak at the same time. The calculator picks the smallest common condenser (18 to 60 kBTU) that keeps the connected ratio under 130%, and warns if the ratio drops under 80%, where some units will not modulate low enough.
Check the brand's connection table before ordering. Some manufacturers cap the ratio at 120%, and every one limits the largest head a given port can serve.
Common sizing mistakes
- Oversizing small bedrooms. A 9k head in a 120 sq ft bedroom will short-cycle and never dehumidify. Use the 6k head when the load allows it.
- Ignoring the garage bonus room. Rooms over or beside unconditioned space pick up heat from three sides. The calculator adds 20%.
- Forgetting heating. In cold climates the heating load usually governs. Check the unit's rated capacity at 5°F, not the nominal number.
For a price, run the same zones through the mini split installation estimator. Sizing a ducted system instead? Use the HVAC load calculator.
Frequently asked questions
What size mini split do I need for a 300 square foot room?
Usually a 9,000 BTU head in a mixed or cool climate and a 12,000 BTU head in a hot climate or a sunny room. A 300 sq ft kitchen or sunroom needs 12,000 BTU almost anywhere.
How many square feet does a 12,000 BTU mini split cover?
About 400–550 square feet in a mixed climate with average insulation, 350–450 in a hot climate, and 550–700 in a cool climate. Sunrooms and kitchens cover less.
Can I connect 36,000 BTU of heads to a 30,000 BTU condenser?
On most brands, yes. That is a 120% connected ratio, inside the usual 100–130% allowance, because the zones do not all run at full load at once. Confirm on the manufacturer's connection chart, and do not expect every head to hit full capacity simultaneously.
Is one big multi-zone system better than several single-zone units?
Single-zone systems are more efficient and keep working when one fails, and are often cheaper for two zones. Multi-zone wins when there is only one good spot for an outdoor unit or the line set runs would be long. Above three zones, price the two approaches against each other.
Does ceiling height change the head size?
Yes. The calculator adds 5% per foot above 8 feet, so a 300 sq ft room with a 12-foot vaulted ceiling carries 20% more load than the same room with a flat ceiling, which is often enough to move it up one head size.