Quick answer
Most single rooms need a 9,000 or 12,000 BTU mini-split: about 9,000 BTU for up to 400 square feet, 12,000 for 400 to 550, 18,000 for 550 to 850 and 24,000 for open areas up to about 1,200. Go up a size for strong sun, high ceilings or a kitchen. If it will heat through an Ontario winter, size it by its output at -15 °C or colder.
What size mini-split you need depends on the room's heat gain in summer and, if the unit will also heat, its heat loss in winter. Floor area gets you to the right class. Sun, ceiling height, cooking and insulation move you up or down one step. Mini-splits come in fixed sizes, so the decision is usually between two neighbours on the list.
Mini-split BTU classes by room area
For cooling a reasonably insulated Ontario room with 8-foot ceilings, 20 to 25 BTU per square foot is the working rule. It gives these starting points:
- 9,000 BTU: up to about 400 sq ft. A bedroom, a home office or a small den.
- 12,000 BTU: about 400 to 550 sq ft. A primary bedroom with an ensuite, a living room or a garage workshop.
- 18,000 BTU: about 550 to 850 sq ft. An open living and dining area or a finished basement.
- 24,000 BTU: about 850 to 1,200 sq ft. An open main floor, or a small bungalow served from one central spot.
Treat the table as a first pass, not a design. For a system that will carry a room's heating, or for any multi-zone layout, a room-by-room load calculation replaces the rule of thumb.
Corrections for sun, ceiling height, kitchens and insulation
- Sun exposure: add about 10% for large south or west windows, and take off about 10% for a heavily shaded room.
- Ceiling height: the rule assumes 8 feet. A 10-foot ceiling holds 25% more air, so scale the number up in proportion; a vaulted ceiling needs the same treatment.
- Kitchens: add about 4,000 BTU for cooking heat when the head serves the kitchen.
- People: add about 600 BTU for each regular occupant beyond two.
- Insulation and exposure: a sunroom, a room over a garage, an attic conversion or an uninsulated pre-war brick wall can need a full size up. A basement with earth on three sides often needs a size down for cooling.
Sizing for heating in Ontario: capacity at -15 °C to -25 °C, not the nominal rating
If the mini-split will heat the room in winter, the number on the box is the wrong one to size by. Nominal heating capacity is rated at about 8 °C outdoors, and every heat pump produces less as the temperature falls. A standard model gives up a large part of its output by -15 °C, while a cold-climate model is built to hold most of it.
The figure to ask for is the maximum heating capacity at -15 °C, and at -25 °C where the listing gives it. Both are published model by model on the NEEP cold-climate heat pump list. Compare them with the room's heat loss at the local winter design temperature, which sits in the high minus-teens around Burlington, Hamilton and Toronto and colder toward Caledon.
In practice the heating load often picks the size, and the unit ends up slightly large for cooling, which an inverter handles by turning itself down. Cold-climate models add roughly $500–$1,500 per zone over standard ones.
Why oversizing hurts: short cycling and poor dehumidification
An oversized mini-split cools the air fast, reaches the setpoint and backs off before it has run long enough to pull moisture out. Inverter compressors modulate, but each has a minimum output, and a unit whose minimum is above the room's load cycles on and off. On a humid day near Lake Ontario the result is a room at 22 °C that still feels damp, plus extra wear from the repeated starts.
Multi-zone systems make this worse when the matching is careless. Each indoor head is sized to its own room, then the outdoor unit is chosen so the total of the heads falls inside the manufacturer's connected-capacity range. A multi-zone outdoor unit has a higher minimum output than a single-zone one, so a small bedroom head on a large outdoor unit tends to overshoot. Sometimes two single-zone systems serve a house better than one multi-zone, and the trade-offs are set out in single-zone versus multi-zone.
What each size of system costs installed
- Single zone, one indoor head: $4,000–$7,000 installed.
- Dual zone: $7,500–$11,000 installed.
- Three to four zones: $10,000–$16,000+ installed.
Inside a band, the BTU class matters less than line-set length, electrical work and whether the model is cold-climate rated; more figures are on the pricing page. ZK Mechanical sizes each ductless mini-split from a site visit that covers room measurements, window area and exposure, insulation and the electrical panel. The quote is free and in writing, and it names the model and its low-temperature capacity.
Call (647) 801-1252 or book a sizing visit anywhere from Burlington and Oakville to Hamilton, Mississauga and Caledon. Other sizing questions are in the cooling answers.