HVAC Calculators · Calculator
Mini-Split Sizing Calculator: BTU per Zone and Indoor Head Size
Size a mini-split one zone at a time. Start near 20 BTU/h per square foot, adjust for ceiling height, sun, climate and people, then choose the smallest head (6,000, 9,000, 12,000, 18,000 or 24,000 BTU/h) that covers the load. A 350 sq ft bedroom typically needs about 7,000 BTU/h, which is a 9k head.
Mini-split zone calculator
How zone sizing works
A ductless system delivers its heating and cooling through one indoor head per zone, so each head must match the room it sits in. The load estimate is the same one used for room air conditioners, applied separately to each zone.
Zone load = Area × 20 × (Ceiling ÷ 8) × Climate × Sun + (People − 2) × 600 + Kitchen
Head = smallest of 6k, 9k, 12k, 18k, 24k BTU/h ≥ Zone load
The last output shows how much bigger the head is than the load. Under about 130% is a good match. Well over 150% means the head will spend much of the season cycling at its minimum speed, or on and off, which hurts dehumidification and efficiency.
Worked example
A 350 sq ft primary bedroom, 8-foot ceilings, mixed climate, average sun and two occupants:
- Load = 350 × 20 × 1 × 1.0 × 1.0 = 7,000 BTU/h
- No extra occupants or kitchen: 7,000 BTU/h
- Smallest head ≥ 7,000 = 9,000 BTU/h (0.75 tons), 129% of the load
A 6,000 BTU/h head would be 86% of the load, and in a mild climate with a shaded room that can be the better choice. Most inverter heads also have a short-term boost range above their nominal rating.
Standard head sizes and typical rooms
| Head size | Tons | Typical zone (average conditions) |
|---|---|---|
| 6,000 BTU/h | 0.5 | Small bedroom, office, up to about 300 sq ft |
| 9,000 BTU/h | 0.75 | Bedroom or den, about 300–450 sq ft |
| 12,000 BTU/h | 1.0 | Large bedroom, small living room, about 450–600 sq ft |
| 18,000 BTU/h | 1.5 | Living area or open kitchen-family room, about 600–900 sq ft |
| 24,000 BTU/h | 2.0 | Large open plan or garage workshop, about 900–1,200 sq ft |
Some brands also sell 15,000 and 30,000+ BTU/h heads, and ducted "slim" air handlers that can serve two or three small rooms from one indoor unit.
Single-zone vs multi-zone outdoor units
A single-zone system pairs one head with its own outdoor unit, so each can modulate down to its own minimum. A multi-zone outdoor unit serves two to five heads. Its rated capacity can be lower than the sum of the heads because not every room peaks at the same time, but manufacturers limit how far the connected heads can exceed the outdoor unit (often to about 130%).
- Multi-zone systems are cheaper to install and use one outdoor footprint.
- They are less efficient at low load, because one big compressor has to throttle down for a single small bedroom.
- If one zone is much larger than the rest, a single-zone unit for that room plus a multi-zone for the bedrooms is a common split.
Heating with a mini-split
In cold climates a ductless heat pump is often the main heat source, and the heating load can exceed the cooling load. Check each zone's heat loss (the heating BTU calculator works per room if you enter the room's area) and confirm that the head's output at your design temperature covers it. Cold-climate models typically keep most of their rated capacity at 5 °F. The heat pump sizing calculator walks through the balance-point check, and heat pump efficiency vs temperature explains how output falls in the cold.
Heat rises and a high-wall head blows across the ceiling. Bedrooms with the door closed and rooms far from the head may need their own small zone rather than relying on one big head in the hallway.
Zone-by-zone worked plan
A two-story, 1,600 sq ft house in a mixed climate might be split like this, using average sun and two occupants per zone:
| Zone | Area | Load | Head |
|---|---|---|---|
| Open living, dining and kitchen | 700 sq ft | 14,000 + 4,000 kitchen = 18,000 BTU/h | 18,000 |
| Primary bedroom | 320 sq ft | 6,400 BTU/h | 9,000 |
| Bedroom 2 | 180 sq ft | 3,600 BTU/h | 6,000 |
| Bedroom 3 / office | 160 sq ft | 3,200 BTU/h | 6,000 |
The heads add up to 39,000 BTU/h. A multi-zone outdoor unit of about 30,000–36,000 BTU/h could serve them, since the bedrooms and living area rarely peak together, provided the manufacturer's connection ratio allows it. Small bedrooms are where oversizing is hardest to avoid, because 6,000 BTU/h is the smallest common head; a ducted slim unit serving two small bedrooms is one way around it.
Installation and refrigerant
Connecting line sets, pulling a vacuum and opening the service valves involves refrigerant and requires EPA Section 608 certification. Pre-charged DIY kits avoid some of that, but line length limits, flare torque and electrical connections still matter. Line-voltage wiring should follow the NEC and local code.
Comparing quotes? Look at SEER2 and HSPF2 ratings for the exact head-and-outdoor combination, not just the outdoor unit. For whole-room sizing tables that apply to window and portable units too, see how many BTU AC you need.
Frequently asked questions
What size mini-split do I need for 500 square feet?
About 10,000 BTU/h in average conditions, so a 12,000 BTU/h (1-ton) head. In a shaded, well-insulated room a 9,000 BTU/h head can be enough; in a sunny room in a hot climate, consider 12,000 with a strong boost range.
Is it bad to oversize a mini-split?
Less bad than oversizing a single-speed AC, because inverter heads slow down. But every head has a minimum speed, and if the room's load sits below it the unit cycles on and off and removes less humidity.
Can one mini-split head cool a whole house?
Only in a small open-plan home. Walls and doors stop air from a high-wall head, so closed bedrooms usually stay warmer. Plan a head for each area that is closed off or has its own load.
Last reviewed 2026-10-28