Understanding your result
The headline gives the number of heads and the outdoor unit class, and the sub-line gives the total cooling load. The bars show each zone’s load against the others and the head class it rounds up to, so you can see at a glance which rooms drive the system and which are being served by the smallest head available. The table has a load and a head row for every zone, then the total of the heads and the outdoor unit size.
The connected load row is the one to look at twice. It is the heads total divided by the outdoor unit’s capacity. Below 100% every head can run flat out at once; between 100 and 130% the system relies on rooms peaking at different times, which most manufacturers allow and which is realistic for bedrooms and living rooms; above 130% the calculator says to go up a size. A head in a very small room gets a note, because even a 6k head is oversized for a room that needs 2,500 BTU/hr, and the zoning planner helps decide which rooms to group.
The loads use the same climate-zone figures as the central AC size calculator, so a whole-house mini-split design can be compared directly with a ducted one. For a single room where a mini-split seems like a lot, the window AC size calculator gives the room-unit alternative, and the mini-split installation cost calculator prices the system this calculator sizes.
How we calculate this
The load per square foot is the cooling figure for the climate zone on the BTU per square foot table, for an average-insulated room with 8 ft ceilings. The use factor is 0.95 for a bedroom or office, 1.0 for a living area, 1.0 plus the 4,000 BTU/hr kitchen allowance for a kitchen, and 1.3 for a sunroom, garage or bonus room over a garage, which have more exposed surface and less insulation than an interior room. Sun and ceiling factors are the same ±10% and 10%-per-foot adjustments used throughout the site. The diversity factor of 0.9 reflects manufacturers’ allowance for connected heads to exceed outdoor capacity; it is not applied to a single-zone system, where the head and condenser are sold as a matched pair.
Head and outdoor classes are the sizes sold in North America; elsewhere the same heads are labelled 2.0, 2.5, 3.5, 5.0 and 7.0 kW. In metric mode the inputs are converted and the loads shown in kilowatts; the unit conversions page has the factors.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Cooling load per sq ft by zone | 24 (zone 1) to 12 (zone 7) BTU/hr per sq ft | Rule-of-thumb climate-zone sizing charts; see the BTU per square foot table |
| Use factors | 0.95 bedroom, 1.0 living, kitchen +4,000 BTU/hr, 1.3 sunroom or garage | ENERGY STAR kitchen allowance; judgement for sunrooms and garages, which have more exposed surface |
| Sun and ceiling | ±10%; +10% per foot above 8 ft | ENERGY STAR room AC guidance; air volume |
| Head classes | 6k, 9k, 12k, 18k, 24k, 36k BTU/hr | Ductless indoor units sold in the US |
| Outdoor classes | 12k to 60k BTU/hr | Ductless multi-zone condensers sold in the US |
| Diversity | 0.9 for two or more zones | Manufacturers typically allow connected heads of 100 to 130% of outdoor capacity |
Assumptions last reviewed October 7, 2026.
The calculator sizes for cooling. In heating-dominated climates the heating load at the design temperature decides the head size and whether a cold-climate model is needed, and a head’s heating output falls with outdoor temperature in a way that differs by model. It also does not model line-set length, which reduces capacity on long runs, or the drop in a multi-zone condenser’s efficiency when only one small head is running. The guide to mini-split zoning and sizing covers those cases.
Two worked examples
A living room and a bedroom in a mixed climate
Two zones in climate zone 4: a 320 sq ft living room with west-facing windows, and a 180 sq ft bedroom with average sun, both with 8 ft ceilings.
- Zone 1: 320 × 18 × 1.0 use × 1.1 sun = 6,336 BTU/hr → 9k head
- Zone 2: 180 × 18 × 0.95 use = 3,078 BTU/hr → 6k head
- Heads total: 15,000 BTU/hr; × 0.9 diversity = 13,500 BTU/hr
- Outdoor unit: 18k, connected load 83%
- Total load: 9,414 BTU/hr
The bedroom gets a note: it needs 3,078 BTU/hr and the smallest head is 6,000, so the head will modulate down to its floor of about 2,000 and cycle a little on mild days. That is acceptable; the alternative of leaving the bedroom door open to the living room’s 9k head is the only other option in a two-room system.
Three zones in a hot climate
Climate zone 2: a 400 sq ft living room with average sun, a 200 sq ft kitchen, and a 140 sq ft bedroom, all with 8 ft ceilings.
- Zone 1: 400 × 22 = 8,800 BTU/hr → 9k head
- Zone 2: 200 × 22 + 4,000 kitchen = 8,400 BTU/hr → 9k head
- Zone 3: 140 × 22 × 0.95 = 2,926 BTU/hr → 6k head
- Heads total: 24,000 BTU/hr; × 0.9 = 21,600 BTU/hr
- Outdoor unit: 24k, connected load 100%
The kitchen allowance nearly doubles the kitchen’s load and puts it in the same class as a living room twice its size. The 24k condenser can run every head at full output at once, which is right for a hot climate where all three rooms are in use on summer afternoons; in a milder climate a 21k or 18k unit at 115 to 130% connected load would be a reasonable saving.
Where to find your inputs
Zone areas. Measure each room a head will serve. If a head is meant to cool two rooms through an open doorway, enter their combined area as one zone and expect uneven temperatures.
Use. “Sunroom, garage or bonus room” is for spaces with three or more exposed walls, a large glass area or a thin roof; a finished attic room belongs here too.
Climate zone. The DOE map zone for your county; example cities are in the dropdown and on the BTU per square foot table.
Ceiling. Measure vaulted ceilings at their average height.
Common mistakes
- One head for a house with closed doors. Bedrooms behind closed doors get almost nothing from a head in the living room.
- A 12k head for every room. Most bedrooms need 6k; oversized heads short-cycle and leave rooms humid, and multi-zone condensers run worst with small loads on big heads.
- Sizing the condenser to the sum of the heads in a house where rooms are used at different times. 110 to 120% connected load is normal and cheaper.
- Forgetting heating. In zones 5 to 7 the heating load, not the cooling load, usually sets the size; check it with the heat pump size calculator.
- Ignoring line-set length. Runs over about 50 ft lose capacity; the installer should de-rate the head or shorten the run.
- Skipping the room-by-room Manual J. Mini-split sizing is forgiving because the heads modulate, but a load calculation is still the standard before buying.
Questions people ask
- What size mini-split do I need for a 300 square foot room?
- In a mixed climate (zone 4) a 300 sq ft living room with average sun and 8 ft ceilings has a cooling load of about 5,400 BTU/hr, which a 6k head covers; the same room with west-facing windows is about 5,900 and a 6k head still covers it. In hot zones 1 and 2 the load rises to 6,600 to 7,900 BTU/hr and a 9k head is the usual choice. A 12k head for a 300 sq ft room is oversized in every climate.
- Can one mini-split cool a whole house?
- A single head cools the room it is in and whatever air mixes with it through open doorways. In an open-plan house of 800 to 1,200 sq ft a single 18k or 24k head can work; in a house with bedrooms behind doors it cannot, because the bedrooms get little of the cooled air. The realistic options are a head per room group, or a ducted mini-split cassette in a hallway ceiling feeding short ducts to several rooms.
- How big should the outdoor unit be for a multi-zone system?
- Add up the heads, then allow for the fact that not all rooms peak at once. Manufacturers permit the connected heads to total 100 to 130% of the outdoor unit's capacity, so a 9k + 9k + 12k system (30k connected) can run on a 24k or 27k condenser if the rooms are used at different times, and should have a 30k or 36k if they are all occupied on summer afternoons. The calculator applies 90% diversity and shows the connected-load percentage.
- Is a multi-zone system or several single-zone systems better?
- Several single-zone systems are usually more efficient and more resilient, because each outdoor unit modulates to its own room, a fault takes out one room rather than all, and single-zone pairs are rated higher on SEER2. One multi-zone outdoor unit saves wall space, line sets and often installation cost. For two or three rooms, price both; for a bedroom that needs cooling only at night next to a living room used all day, two singles work better than one multi.
- Why does the calculator warn that a 6k head is oversized?
- Because the smallest head sold is 6,000 BTU/hr and a 120 sq ft bedroom in a mixed climate needs about 2,000. Mini-splits modulate down to roughly a third of their rating, so a 6k head can run as low as about 2,000 BTU/hr, but no lower; in a very small room it will cycle on and off like a fixed-speed unit and dehumidify poorly. Serving two small rooms from one head through a transfer grille, or using a ducted cassette, avoids the problem.
- Do mini-splits need the same sizing for heating?
- In cold climates, heating decides the size. A head that is right for cooling in a zone 5 or 6 house may be well short of the heating load on a design night, and a heat pump's capacity falls as the outdoor temperature drops. Size for cooling with this calculator, then check the heating load with the heat pump size calculator and choose a cold-climate model if the two disagree.