Room air conditioners and heaters are sold by capacity in BTU per hour. Too small and the room never gets comfortable on the hottest or coldest days. Too big and an AC short-cycles, cooling the air fast but switching off before it removes the humidity, which leaves the room cold and clammy. Getting the size roughly right matters more than buying the biggest unit you can afford.
Cooling rule of thumb
Start at about 20 BTU per square foot of floor area. Then adjust:
- Add 10% for a very sunny room, and take 10% off for a heavily shaded one.
- Add 600 BTU for each regular occupant beyond two.
- Add about 4,000 BTU for a kitchen.
- Scale up for ceilings higher than 8 ft.
12,000 BTU per hour is one ton of cooling. A 300 sq ft bedroom typically needs around 6,000 BTU; a 500 sq ft living room around 10,000.
Heating rule of thumb
Heating needs in Canada depend heavily on climate: roughly 35–45 BTU per sq ft on the BC coast, 45–55 in southern Ontario, southern Quebec and the Maritimes, and 60 or more on the Prairies and in the North, adjusted for insulation and windows. An old, drafty room needs much more.
Whole-house systems
For furnaces, central air and heat pumps, Part 9 of the building code requires the equipment serving a house to be sized with a CSA F280 heat loss and heat gain calculation, with the ducts designed to suit. Rules of thumb routinely oversize whole-house equipment, which costs more to buy, runs less efficiently and controls humidity badly.
What affects the load
- Insulation levels in walls, attic and floor
- Window area, orientation, shading and glazing type
- Air leakage
- Number of occupants and appliances
- Local design temperatures
Heat pumps
Heat pumps provide both heating and cooling, so size them for the larger of the two loads in your climate, and check their capacity at your design low temperature, since output falls as it gets colder. Cold-climate models hold their capacity much better.
Ventilation
Bathrooms and kitchens need exhaust fans sized for the room: at least 50 CFM (25 L/s) from a bathroom and 100 CFM (50 L/s) from a kitchen under Part 9 of the building code. The exhaust fan calculator works it out.
The BTU calculator gives a quick size for any room, with the adjustments above built in.
Measuring a room for heating and cooling
Measure the floor length and width, and the ceiling height. Note the number of external walls, the size and type of each window, and which way they face. Check the insulation where you can: the attic, and wall cavities at outlets. Ask the client which rooms feel cold or hot now. That's often the best clue to where the heat goes.
Common mistakes
- Sizing by floor area alone, ignoring ceiling height, windows and insulation
- Oversizing "to be safe", which costs more and works worse
- Ignoring the room's use: kitchens and sunrooms behave very differently from bedrooms
- Choosing a unit by cooling capacity without checking its heating capacity at your winter temperature
- Forgetting ventilation, which affects both comfort and heat loss
When to call a specialist
Rules of thumb are fine for single rooms and replacements. For whole-house systems, new builds, extensions with large glazed areas, and anything that needs a grant or certification, use a qualified HVAC designer with a full CSA F280 calculation. The cost of the calculation is small compared with the cost of the wrong equipment.
Common questions
Can one unit cool several rooms?
A single room unit struggles to cool rooms it can't blow air into directly. For several rooms, a ducted system or a multi-zone mini-split with a head in each room works much better.
Does better insulation change the size?
Yes, a lot. Insulating a attic, replacing single glazing or sealing drafts can cut a room's load by a third or more. When a job includes both insulation and new equipment, size the equipment for the insulated building, not the old one.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.