Mini Split Size Calculator

Sizing a ductless system is three questions, not one: which head goes in each room, how much connected head capacity the outdoor unit will allow, and what happens when the head is larger than the room needs. Enter the room area, ceiling height, sun exposure and occupancy and you get a screening capacity, the nominal head size that covers it and the oversizing ratio. A second mode adds up the heads against an outdoor unit and shows the combination ratio and what each head actually gets when every zone calls at once, and a third checks a head against a design load you already have. The room figure comes from the published room air conditioner sizing table with its standard adjustments, which is a screening number rather than a Manual J.

Calculator Type
Measure the Room In
Sets the floor area and the ceiling height you type. Capacities are quoted in BTU per hour either way, because that is how heads are catalogued, with the kilowatt figure printed beside them when you choose metric.

One room, one indoor head. The screening capacity, then the nominal size that covers it.

The table assumes 8 ft, which is 2.44 m, and an empty box uses it. A taller room holds more air to condition, so the capacity is scaled by the height over 8 ft.
Two is the baseline the table assumes. Each person beyond two adds 600 BTU per hour.
The lowest capacity the compressor can hold without cycling off, as a percentage of nominal. Manufacturers publish it in the engineering data; a single zone inverter head is commonly 30 to 40 percent, and a head on a multi zone outdoor unit is usually worse.
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Several heads on one condenser: what may be connected, and what each head actually gets when every zone calls at once.

The highest connected head capacity the outdoor unit allows, as a percentage of its own nominal. Most multi zone units publish something between 100 and 150 percent. Check the submittal for the model you are buying.
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Already have a design load from a proper calculation? This is the oversizing penalty on the head you are considering.

From a Manual J or an envelope calculation, at your design outdoor temperature.
The lowest capacity the compressor can hold without cycling off, as a percentage of nominal. Manufacturers publish it in the engineering data; a single zone inverter head is commonly 30 to 40 percent, and a head on a multi zone outdoor unit is usually worse.
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