Mini Split BTU Sizing Calculator
A ductless mini split is sized by the cooling capacity, in BTU per hour, that a room needs. The quick estimate multiplies the room area in square feet by a capacity factor in BTU per hour per square foot, set higher for sunny, poorly insulated, or crowded rooms. Because that factor varies with the building and climate, it is a value you choose from a sizing guide rather than a fixed assumption. This calculator returns capacity in BTU per hour and in cooling tons.
Mini split sizing formula
Room area = length * width
Capacity (BTU/hr) = area * capacity factor
Capacity (tons) = BTU per hour / 12,000
Capacity (kilowatts) = BTU per hour / 3,412.142
One ton of refrigeration is exactly 12,000 BTU per hour, and one kilowatt is 3,412.142 BTU per hour. The capacity factor is your input for the room's exposure and insulation, so the result reflects your actual space.
Mini split sizing notes
- Capacity is rated in BTU per hour; units are often described by ton or kilowatt as well.
- Sizing guides use a base near 20 to 30 BTU per hour per square foot, adjusted for conditions.
- One ton of cooling equals 12,000 BTU per hour by definition.
- An oversized unit short-cycles and leaves humidity high; an undersized unit never reaches setpoint.
- For a final specification use a recognized load calculation or manufacturer guidance.
Mini split sizing: frequently asked questions
What size mini split do I need?
Mini split capacity is rated in BTU per hour. A quick sizing method multiplies the room area in square feet by a capacity factor in BTU per hour per square foot that suits the room's exposure and insulation. This calculator multiplies your area by the factor you choose and reports capacity in BTU per hour and in cooling tons.
What BTU-per-square-foot factor should I use?
The factor depends on insulation, sun exposure, window area, occupancy, and climate, so it is not a single fixed value. Sizing guides commonly use a base around 20 to 30 BTU per hour per square foot, adjusted up for sunny or poorly insulated rooms. Use a value from a reputable sizing reference for your conditions; the calculator takes it as input.
How many BTU are in a ton of cooling?
One ton of refrigeration equals 12,000 BTU per hour, the standard definition used for air-conditioning capacity. The calculator divides the BTU per hour figure by 12,000 to give the equivalent in tons, since indoor and outdoor units are often described by ton rating.
What happens if a mini split is oversized?
An oversized unit cools the space quickly then shuts off, leaving humidity high and cycling on and off frequently, which wastes energy and shortens component life. An undersized unit runs constantly without reaching the setpoint. Matching capacity to the calculated load gives the most efficient, comfortable result.
Is this a substitute for a load calculation?
No. A formal cooling load calculation accounts for each heat gain source individually and is the standard for a final specification. This area-times-factor method is a fast estimate for planning. For purchase and installation, follow the manufacturer guidance or a recognized load calculation procedure.
Official sources
- U.S. Department of Energy: Ductless Mini-Split Heat Pumps.
- U.S. National Institute of Standards and Technology: Unit conversion (BTU, watt, ton of refrigeration).
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.