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Mini Split Sizing Calculator

Mini split head size for a room, matched to real unit sizes.

Your measurementsEvery assumption editable01
Head sizeCalculated live02

Result

9000BTU

A 9k head — 8750 BTU/hr calculated

Base load350 sq ft × 25 BTU/sq ft
8750 BTU/hr
Calculated load
8750 BTU/hr
Nearest head sizeheads come in 9k, 12k, 18k, 24k and 36k
9000 BTU

Mini-splits punish oversizing harder than ducted systems. An inverter compressor modulates down, but only to a floor — below that it cycles, and an oversized head in a small bedroom will leave the room damp while barely running.

For a multi-zone system, the outdoor unit is not simply the sum of the heads. Manufacturers publish combination ratios, and most allow the heads to total more than the condenser because the rooms do not all peak at once.

Line-set length affects capacity. Long runs and significant height differences between the head and the condenser derate the system, and the manufacturer's tables give the correction.

Enter the room area, your climate and what the room is used for. The result matches to the head sizes ductless systems actually come in — 9k, 12k, 18k, 24k and 36k — because a mini split punishes oversizing harder than a ducted system does.

Why use this tool?What it does differently03

Why use this tool?

Real head sizes

Matched to the 9k / 12k / 18k / 24k / 36k steps ductless equipment is sold in.

Room type adjustment

A bedroom, a living room, a kitchen and a sunroom of identical area need meaningfully different capacity.

Oversizing warning

Inverters modulate down but only to a floor. Below it they cycle, and an oversized head leaves a small room damp.

Multi-zone guidance

Explains why the outdoor unit is not simply the sum of the heads.

How this worksThe method04

How this mini split sizing calculator works

Room area times a climate load gives a base figure, adjusted by a percentage for room type — down ten per cent for a bedroom, up twenty for a kitchen, up thirty-five for a sunroom or conservatory with significant glazing.

The result is matched to the nearest standard head size rather than rounded up. Mini splits are less forgiving of oversizing than ducted systems: an inverter compressor modulates down, but only to a floor, and below that it cycles like a fixed-speed unit.

For a multi-zone system, the outdoor unit is not the sum of the heads. Manufacturers publish combination ratios, and most permit the heads to total more than the condenser precisely because the rooms do not all peak at once.

How to use itStep by step05

How to use it

  1. Step 1: Measure the room

    Floor area of the single room the head will serve.

  2. Step 2: Pick the climate load

    Mild, moderate, hot or very humid.

  3. Step 3: Choose the room type

    Bedroom, living room, kitchen or sunroom — this adjusts for internal gains and glazing.

  4. Step 4: Check line-set limits

    Long runs and large height differences derate capacity. The manufacturer's tables give the correction.

Example usageWorked figures06

Example usage

A 350 sq ft living room
At 25 BTU per square foot with no adjustment, 8,750 BTU/hr — a 9,000 BTU head.
A 350 sq ft sunroom
The same area at plus thirty-five per cent is 11,813 BTU/hr, taking it to a 12,000 BTU head. Glazing is the single biggest driver in a room like this.
A small bedroom
A 150 sq ft bedroom at minus ten per cent needs 3,375 BTU/hr — well below the smallest head made. A 9,000 unit is the only option and will cycle; that is a known limitation of ductless in small rooms.
Frequently asked questionsCommon questions07

Frequently asked questions

What size mini split do I need for a 400 square foot room?

Around 10,000 BTU/hr in a moderate climate, so a 12,000 BTU head. A bedroom of that size could take 9,000; a sunroom would want 12,000 or more.

Can a mini split be too big?

Yes, and it matters more than with ducted systems. An inverter modulates down only to a floor — below that it cycles, and an oversized head in a small room leaves it cold and damp rather than comfortable.

How do I size a multi-zone system?

Size each head for its room, then check the manufacturer's combination ratio for the outdoor unit. Most allow the heads to total more than the condenser because rooms peak at different times.

Does line-set length affect capacity?

Yes. Long runs and significant height differences between the head and condenser derate the system, and manufacturers publish correction factors for both.

Are mini splits good for heating?

Modern cold-climate heat pumps hold useful capacity well below freezing, but capacity does fall as it gets colder. Heating sizing should use the manufacturer's rated output at your design temperature, not the nominal figure.

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