Choose · Sizing inputs
Mini-split sizing guide
The short answer is a square-foot range, and every capacity we list publishes one. The longer answer is that area alone never finishes the job — here is what changes it.
DecisionDecide what to measure before you choose a size
Length8 sections · 4 min read
ReviewedAugust 15, 2026
01Choose
What size mini split do I need?
Every single-zone system we list publishes a coverage range. Nine thousand BTU covers up to 350 square feet, 12,000 BTU covers 350 to 500, 18,000 BTU covers 500 to 750, 24,000 BTU covers 750 to 1,000, and 36,000 BTU covers 1,000 to 1,500.
Read those as a starting bracket for one conditioned space, not a verdict. Nominal capacity and rated capacity are also not always the same number, and the difference is not the same across every series. Each product page carries the figures that apply to that model.
02Choose
Size by climate zone
Where the home is changes how much cooling each square foot needs. The chart splits the country into five zones, from zone 1 in red, the hottest, to zone 5 in blue, the coldest. Find your zone's column, follow it down to the room's floor area, and read the size on the left.
Zone 3 matches the coverage MRCOOL publishes for each single-zone size. Each step hotter needs about 5% more capacity per square foot, and each step colder about 5% less. Every range assumes an 8 ft ceiling, average sun, windows and insulation, and up to two people; the calculator on the sizing page adjusts for your own room.
| CapacityFloor area, sq ft | Zone 1Hot | Zone 2Warm | Zone 3Temperate | Zone 4Cool | Zone 5Cold |
|---|---|---|---|---|---|
| 9,000 BTU0.75 ton | Up to 340 sq ft | Up to 350 sq ft | Up to 370 sq ft | Up to 390 sq ft | Up to 410 sq ft |
| 12,000 BTU1 ton | 341–450 sq ft | 351–470 sq ft | 371–500 sq ft | 391–520 sq ft | 411–550 sq ft |
| 18,000 BTU1.5 tons | 451–680 sq ft | 471–710 sq ft | 501–750 sq ft | 521–780 sq ft | 551–830 sq ft |
| 24,000 BTU2 tons | 681–900 sq ft | 711–950 sq ft | 751–1,000 sq ft | 781–1,050 sq ft | 831–1,110 sq ft |
| 36,000 BTU3 tons | 901–1,360 sq ft | 951–1,420 sq ft | 1,001–1,500 sq ft | 1,051–1,570 sq ft | 1,111–1,660 sq ft |
03Choose
Sizing a central ducted system
A central system is sized for the whole conditioned floor area rather than one room. It averages sunny rooms with shaded ones, so it covers more area per BTU than a room's own unit: MRCOOL rates its 2 ton packages to 1,200 square feet, 2.5 tons to 1,500, 3 tons to 1,800 and 5 tons to 3,000.
Read the chart the same way: your zone's column, your conditioned floor area, and the tonnage on the left. Count every floor that is heated and cooled, and leave out a basement unless it is finished and conditioned.
| System sizeFloor area, sq ft | Zone 1Hot | Zone 2Warm | Zone 3Temperate | Zone 4Cool | Zone 5Cold |
|---|---|---|---|---|---|
| 1.5 tons18,000 BTU | Up to 810 sq ft | Up to 850 sq ft | Up to 900 sq ft | Up to 940 sq ft | Up to 1,000 sq ft |
| 2 tons24,000 BTU | 811–1,090 sq ft | 851–1,140 sq ft | 901–1,200 sq ft | 941–1,260 sq ft | 1,001–1,330 sq ft |
| 2.5 tons30,000 BTU | 1,091–1,360 sq ft | 1,141–1,420 sq ft | 1,201–1,500 sq ft | 1,261–1,570 sq ft | 1,331–1,660 sq ft |
| 3 tons36,000 BTU | 1,361–1,630 sq ft | 1,421–1,710 sq ft | 1,501–1,800 sq ft | 1,571–1,890 sq ft | 1,661–2,000 sq ft |
| 3.5 tons42,000 BTU | 1,631–1,900 sq ft | 1,711–2,000 sq ft | 1,801–2,100 sq ft | 1,891–2,210 sq ft | 2,001–2,330 sq ft |
| 4 tons48,000 BTU | 1,901–2,180 sq ft | 2,001–2,280 sq ft | 2,101–2,400 sq ft | 2,211–2,520 sq ft | 2,331–2,660 sq ft |
| 5 tons60,000 BTU | 2,181–2,720 sq ft | 2,281–2,850 sq ft | 2,401–3,000 sq ft | 2,521–3,150 sq ft | 2,661–3,330 sq ft |
04Choose
Read the size off your current system
Replacing a system? Its capacity is usually coded into the outdoor unit's model number, not its serial number. On MRCOOL DIY units the number after DIY- is thousands of BTU, so a DIY-12 model is a 12,000 BTU system.
Central equipment uses the same code in thousands of BTU: 18 is 1.5 tons, 24 is 2, 30 is 2.5, 36 is 3, 42 is 3.5, 48 is 4, and 60 is 5.
05Choose
What moves you up or down a size
Square footage is one input. Ceiling height changes the volume being conditioned. Sun exposure, window area and glazing, insulation level, air leakage, how many people use the room, and whether it holds heat-producing equipment all push the requirement up or down from the bracket.
A room that is one number on a floor plan and another number in practice — a west-facing sunroom, a finished attic, a garage with an uninsulated door — is exactly the case where the bracket stops being reliable.
Planning checklist
- Measure length, width, and ceiling height
- Note window count, size, and which direction they face
- Note insulation, and any wall or ceiling that faces outside
- Note typical occupancy and any heat-producing equipment
06Choose
Keep one record per room
Record each room separately, even when the rooms sit next to each other. A multi-zone configuration assigns a specific indoor unit to a specific room, so a merged note cannot be resolved back into the right head later.
For a multi-room project, that per-room record is what the configurator turns into one exact bundle, and what you hand to an installer if you get help with the work.
07Choose
One room or several
One conditioned space is a single-zone project: one indoor unit paired to one outdoor unit, chosen by capacity. Two to six rooms is a multi-zone project: one outdoor unit feeding a head in each room, chosen as one exact bundle.
If you may add rooms later, decide that now. Adding a zone later means a different outdoor unit, not an extra indoor unit on the existing one.
08Choose
When you need a Manual J
Manual J is the industry load calculation. It works from your actual construction, orientation, insulation, and local design temperatures rather than a square-foot bracket, and it is what settles the size when a room sits near a boundary or behaves unusually.
A qualified HVAC professional performs it. Have one done before you finalize capacity, then reconcile the result against the documents for the exact product you intend to buy.