Mini-Split BTU Calculator for Single & Multi-Zone Systems

Add each room you want to condition, choose your climate and insulation, and get the indoor head size for every zone, the outdoor unit capacity, the heating load for winter and an estimate of what the system will cost to run each year.

1. Home & climate

These settings apply to all rooms.

2. Rooms / zones

One indoor head per room. Up to 6 zones.

3. Running cost (optional)

How this mini-split calculator works

A ductless mini-split is sized room by room. Every indoor unit (the “head”) has to match the load of the space it serves, and the outdoor condenser has to handle the combined load of all heads that run at the same time. Buying one big head for an open floor plan that is really three separate rooms, or one oversized outdoor unit for a few small bedrooms, are the two most common and most expensive sizing mistakes.

The calculator starts from a cooling factor in BTU per square foot that depends on your climate: about 30 BTU/sq ft in hot and humid regions, 25 in mixed climates and 20 in cold northern climates where summers are mild. That base is then corrected for ceiling height (relative to an 8-foot ceiling), the quality of your insulation and windows, how much direct sun the room gets, and the number of people who regularly use it. Each person beyond two adds roughly 600 BTU, and a kitchen adds about 4,000 BTU for cooking heat.

For heating, the factor rises as the climate gets colder, because the temperature difference between indoors and outdoors in January is much larger than in July. In a cold climate the heating load is often 1.5 to 2 times the cooling load, so the heating number usually decides the size of the head. The calculator picks the larger of the two loads when you choose “cool & heat”.

Typical mini-split head sizes

Head sizeTonsTypical room (mixed climate, average insulation)
6,000 BTU0.5Up to ~250 sq ft – small bedroom, office, nursery
9,000 BTU0.75250–375 sq ft – bedroom, home office
12,000 BTU1.0375–500 sq ft – master bedroom, studio
18,000 BTU1.5500–750 sq ft – living room, open kitchen
24,000 BTU2.0750–1,000 sq ft – large great room, garage workshop
30,000–36,000 BTU2.5–3.01,000–1,500 sq ft – open floor plans, small homes

Sizing the outdoor unit for a multi-zone system

Multi-zone outdoor units are rated by their total capacity, for example 18k, 24k, 36k or 48k BTU. Most manufacturers allow you to connect indoor heads whose sizes add up to more than the outdoor rating – often up to 130% – because not every room needs full power at the same moment. A 36,000 BTU outdoor unit can usually carry a 12k + 12k + 9k + 9k combination (42k of heads). The calculator therefore sizes the outdoor unit from the real combined load, not simply by adding up head sizes, and warns you when the connected heads exceed about 130% of the outdoor capacity.

If the combined load of your rooms is larger than 48,000 BTU, or the rooms are far apart, two smaller outdoor units are usually better than one large one: line-set lengths stay short and you keep heating if one unit fails.

Why oversizing a mini-split is a problem

Modern mini-splits use inverter compressors that slow down to match the load, which makes them more forgiving than old on/off systems. Still, every inverter has a minimum speed. A head that is far too big for its room will reach that minimum quickly and start cycling on and off. The result is uneven temperature, poor humidity removal in summer (a cool but clammy room), more wear and a lower real-world efficiency. Undersizing has the opposite effect: the system runs at full speed for long periods and may not hold temperature on the hottest or coldest days. Choosing the head closest to your load – slightly above it rather than far above – is the goal.

Understanding SEER2 and HSPF2

Since 2023, US efficiency ratings use the updated SEER2 (cooling) and HSPF2 (heating) test procedures, which reflect higher external static pressure and give slightly lower numbers than the old SEER and HSPF. The federal minimum for a split system is around 13.4–14.3 SEER2 depending on region, while good mini-splits reach 20–30 SEER2 for single-zone models. A higher SEER2 directly cuts cooling energy: going from 14 to 22 SEER2 reduces cooling electricity by roughly 36%. In cold regions, look for HSPF2 of 8.5 or higher and a model listed as “cold-climate” or “hyper-heat”, which keeps rated capacity well below 5 °F (−15 °C).

Tips for an accurate result

Mini-split sizing FAQ

How many BTU do I need per square foot for a mini-split?

Plan for about 20 BTU per sq ft in cold climates, 25 in mixed climates and up to 30–35 in hot, humid climates, then adjust for ceiling height, insulation, sun and occupancy. A 400 sq ft bedroom in a mixed climate needs roughly 10,000 BTU, so a 12,000 BTU head is the usual choice.

Is it better to oversize or undersize a mini-split?

Slightly oversizing (10–20%) is acceptable because inverter compressors modulate. Large oversizing causes short cycling and poor dehumidification. Undersizing is a bigger risk for heating in cold climates, where the system may not keep up on the coldest nights.

Can one mini-split head cool several rooms?

Only if the rooms are open to each other. Air from a wall-mounted head does not travel well through doorways and hallways. Closed bedrooms each need their own head, or a ducted/ceiling-cassette indoor unit that distributes air through short ducts.

What size outdoor unit do I need for 3 or 4 zones?

Add up the real loads of the rooms (not the head sizes) and choose the next outdoor size above that number. Then check that the combined head sizes are within the manufacturer’s connection limit, commonly 100–130% of outdoor capacity.

How much does it cost to run a mini-split?

A 12,000 BTU head at 20 SEER2 running 8 hours a day uses about 4.8 kWh per day at full load – around $0.80 per day at $0.17/kWh. Real costs are usually lower because inverters run below full capacity most of the time.

Does a mini-split work for heating in winter?

Yes. Mini-splits are heat pumps. Standard models heat efficiently down to about 15–20 °F; cold-climate models deliver most of their capacity at 5 °F and keep working below −13 °F. In very cold regions keep a backup heat source.

About MiniSplit Sizer

MiniSplit Sizer is an independent, free tool that helps homeowners understand what size of ductless mini-split they need before talking to installers or shopping online. The formulas are based on widely used sizing rules from manufacturers, the US ENERGY STAR program and HVAC trade guidance, combined into one transparent calculation you can adjust yourself. We do not sell equipment and are not paid by any brand.