Mini Split Sizing Calculator (BTU per Zone)
Describe the house, then list the zones you want to serve. The calculator estimates the whole-house load from your ZIP code and insulation, splits it across the zones by floor area and exposure, and suggests an indoor head size for each. It is an estimate only; a real design uses a room-by-room Manual J.
Mini split sizing calculator
Northeast US (default)Zone 5A · 5°F heating / 88°F cooling design
Estimate only. Not a substitute for a room-by-room Manual J.
Heating load (zoned area)
36,700 BTU/h
at 5°F outdoor
Cooling load (zoned area)
24,500 BTU/h
at 88°F outdoor
| Zone | Heating | Cooling | Suggested head |
|---|---|---|---|
| Living / kitchen | 18,600 | 12,400 | 24k (24,000 BTU/h) |
| Bedrooms | 9,900 | 6,600 | 12k (12,000 BTU/h) |
| Primary suite | 8,200 | 5,500 | 12k (12,000 BTU/h) |
Loads in BTU/h. Heads are nominal sizes; total connected 48,000 BTU/h.
Large zone: one wall head of 24k or more is a lot of air in one spot. Consider splitting the zone or using a ducted air handler for it.
Head size is picked to cover the larger of the zone's cooling load and the rated heating capacity it needs at 5°F. Small zones often land on the smallest head; check the minimum turndown so it does not short cycle. The outdoor unit is selected separately from the connected heads and the manufacturer's data.
Assumptions behind this estimate
- Climate
- Northeast US (default) (zone 5A)
- Design temperatures
- 5°F heating / 88°F cooling (approx. 99% / 1%)
- Whole-house load
- 36,700 BTU/h heating / 24,500 BTU/h cooling
- Zone allocation
- By floor area, weighted 0.8 interior / 1.0 average / 1.25 exposed
- Insulation level
- average
- Head sizes considered
- 6k, 9k, 12k, 15k, 18k, 24k, 36k
- Heat pump type
- Cold-climate inverter
Do this room by room in Tradewright
Real equipment matches, a 15-year cost comparison and a signed proposal, all from one visit.
Why per-room BTU charts get mini splits wrong
Charts that say a 12 by 12 bedroom needs a 9,000 BTU head ignore the climate, the walls and the glass. In a tight, well-insulated house that bedroom may need a fraction of the smallest head on the market. In a drafty corner room over a garage it may need more.
Oversized heads are the most common mini split complaint. A head far larger than its room runs at minimum speed and still cycles off, so the room swings in temperature and dehumidifies poorly. Total head capacity can also add up to far more than the outdoor unit or the house needs.
The fix is to start from the load. Work out what each zone actually loses and gains at design conditions, then pick the head whose capacity range covers it, checking the minimum output as well as the maximum. Selection limits for heat pumps are set out in ACCA Manual S.
Sources
How the estimate works
Step 1: Whole-house load
ZIP, size, year built and insulation give an estimated heating and cooling load for the house at local design temperatures.
Step 2: Share by area and exposure
Each zone gets a share of the load by floor area, weighted down for interior rooms and up for exposed rooms with more wall, glass or roof.
Step 3: Heating capacity at design
Each zone's heating share is divided by the capacity a cold-climate or standard unit keeps at your design temperature.
Step 4: Pick the head
The suggested head is the smallest common size that covers the larger of the cooling load and the rated heating capacity needed.
Questions
What size mini split do I need for one room?
It depends on that room's heating and cooling load, which depends on the climate, insulation, windows and how many outside walls it has. Use single-zone mode for one open area, or list rooms as zones. The result is an estimate; confirm with a room-by-room Manual J.
How many BTU per square foot for a mini split?
There is no fixed figure. A room in a tight new house can need a small fraction of what a drafty room with big windows needs. The calculator works from the load instead of a per-square-foot rule.
Should every room get its own head?
Not always. Small, well-insulated bedrooms can be far below the smallest head size, which leads to cycling. Grouping rooms on a ducted or slim-duct air handler often fits small loads better than one wall head per room.
Can I oversize the heads and let the inverter turn down?
Only within the unit's minimum output. If the room needs less than the head's lowest output, it will cycle on and off no matter how good the inverter is. Check the minimum capacity in the manufacturer's data.
How do I size the outdoor unit?
From the total load of the zones it serves and the manufacturer's capacity data at your design temperature, not by adding up head nameplates. Many multi-zone outdoor units are allowed to connect more head capacity than they can deliver at once.
Do mini splits work in cold climates?
Cold-climate models are designed to keep much of their capacity at low outdoor temperatures. Check the manufacturer's data at your design temperature, or the NEEP cold-climate product list, before you rely on a unit as the main heat source.
Is this calculator a Manual J?
No. It splits a whole-house estimate by area and exposure. A Manual J calculates each room from its own walls, windows, orientation and air leakage.
Related calculators and guides
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