What size mini split do I need? BTUs, tons, and why guessing fails

A BTU (British Thermal Unit) is a unit of heat, and an air conditioner's capacity is rated in BTUs per hour — how much heat it can move out of a room in an hour. A ton is just shorthand for 12,000 BTU/h. Residential mini split heads commonly come in 6,000, 9,000, 12,000, 18,000, and 24,000 BTU/h sizes.

That's the whole vocabulary. The hard part isn't the units — it's matching the capacity to the actual heat load of your specific room, which is where most sizing goes wrong.

The internet is full of charts that map square footage to BTUs. They fail here for a simple reason: two 300-square-foot rooms in San Antonio can carry wildly different loads. A west-facing bonus room over a garage — unconditioned space below, attic above, afternoon Texas sun on two walls — might need double the capacity of a shaded, well-insulated interior room of the same size.

The variables that actually drive the number: window area and orientation (west glass is brutal here), insulation quality (a 1950s Castle Hills ranch and a 2015 Stone Oak build are different animals), ceiling height, air leakage, what's above and below the room, and how the space is used — a home gym or a kitchen carries more load than a bedroom.

Oversized is the sneaky one, because it sounds safe. An oversized head blasts the room to temperature in minutes and shuts off — over and over. Those short cycles never run long enough to pull moisture out of the air, so the room reads 72° and still feels clammy. In our humidity, dehumidification is half the job. Short-cycling also wears components faster.

Undersized is more obvious: the system runs continuously through August, never quite reaches setpoint, and dies young from the workload. You paid less up front and more every month after.

Modern inverter compressors forgive small sizing errors by ramping output up and down instead of switching on and off — but they can't rescue a fundamentally wrong size. An inverter head oversized by half still bottoms out above the room's true load.

Manual J is the industry-standard load calculation — the actual math on your room's dimensions, windows, orientation, insulation, and use. It replaces the guess with a number: the room's real heat gain in BTU/h, which tells us exactly which head size fits.

We run a load calculation as part of every free in-home estimate. It's twenty minutes of measuring that determines whether the system dehumidifies properly, hits its efficiency rating in practice, and lasts its design life. No reputable ductless install starts from a square-footage chart.

When you compare quotes, look for the load calculation. A quote that names a head size without measuring the room is a guess with a price on it — and it's the most common thing we flag when we review other companies' quotes. If you already have a quote and want the sizing checked, that's exactly what our free second opinion is for.