Use this mini split size calculator to find the right BTU capacity for a ductless heat pump based on your room's dimensions, insulation, sun exposure, and climate. Oversized units short-cycle and waste energy; undersized units can't keep up on the hottest or coldest days.
Estimates only. By using this calculator you agree to our Terms.
Enter your room's length and width, ceiling height, and select the insulation quality, climate zone, and sun exposure that best match your space. Add the number of occupants and check the kitchen box if it applies. Click "Calculate" to get a recommended BTU capacity and the closest standard mini split unit size.
Sizing a mini split correctly is one of the most overlooked steps in a ductless install. Contractors sometimes default to "one ton per 400-600 sq ft" without accounting for ceiling height, sun exposure, or insulation quality. A bedroom with 9-foot ceilings and west-facing windows in Phoenix needs meaningfully more capacity than the same room in Seattle with north-facing windows. Oversizing wastes money upfront and causes short-cycling, which leads to poor humidity control and a clammy feeling even when the temperature reads "comfortable." Undersizing means the compressor runs constantly on the hottest afternoons and still can't hit setpoint. This calculator is useful for homeowners adding a mini split to a sunroom, garage conversion, home office, or bonus room, and for anyone comparing quotes from HVAC installers who gave wildly different BTU recommendations for the same space.
The calculator starts from the standard rule-of-thumb baseline of 20 BTU per square foot for an 8-foot ceiling in a moderate climate, then applies multipliers:
BTU = Area × 20 × Ceiling Factor × Insulation Factor × Climate Factor × Sun Factor + Occupant Load + Kitchen Load
Where Area = Length × Width, Ceiling Factor = Ceiling Height ÷ 8 (so a 6 ft ceiling reduces the requirement and a 10 ft ceiling increases it proportionally), occupant load adds 600 BTU for each person beyond the first two, and kitchen load adds a flat 4,000 BTU for cooking heat and appliance load. The result is then matched to the nearest standard mini split unit size that meets or exceeds the requirement. Most major manufacturers (Mitsubishi, Fujitsu, Daikin, LG) begin their indoor head lineup at 9,000 BTU, so that is used as the smallest available size here rather than the older, less common 6,000 BTU units.
A 15 ft × 12 ft bedroom (180 sq ft) with an 8 ft ceiling, average insulation, mild climate, moderate sun, and 2 occupants:
Area = 180 sq ft. Base BTU = 180 × 20 = 3,600. Ceiling factor = 8/8 = 1.0. Insulation = 1.0. Climate = 1.0. Sun = 1.0. Occupant load = 0 (only 2 people). No kitchen.
Adjusted BTU = 3,600 × 1.0 × 1.0 × 1.0 × 1.0 = 3,600 BTU. Since the smallest standard mini split sold today is 9,000 BTU, that becomes the recommended unit — this matches real-world guidance that small bedrooms typically only need the smallest available head unit, with capacity to spare.
A 12×12 room is 144 sq ft. At the standard 20 BTU per sq ft baseline, that's roughly 2,880 BTU before adjustments — the smallest standard unit most manufacturers sell, 9,000 BTU, comfortably covers it unless the room has poor insulation or heavy sun exposure.
A 12,000 BTU unit generally covers 500–600 sq ft under average conditions, but that drops with high ceilings, poor insulation, or intense sun exposure, and increases in well-insulated, shaded rooms.
Yes. An oversized unit reaches temperature quickly and shuts off, which prevents it from properly removing humidity from the air, leaving the room feeling cool but clammy. It also causes more wear from frequent cycling.
Yes. Standard BTU-per-square-foot rules assume an 8-foot ceiling. This calculator scales capacity proportionally with ceiling height, so taller ceilings raise the requirement and shorter ceilings lower it.
Yes. Cooking appliances and general kitchen heat add meaningful load, so this calculator adds a flat 4,000 BTU when the kitchen checkbox is selected.
Yes, multi-zone systems connect one outdoor condenser to several indoor heads. Size each room separately with this calculator, then choose a condenser rated for the combined total BTU of all connected zones.