What size heater do I need for my room?

Start with floor area

Measure the length and width of the room in metres and multiply them. A 3 m by 3.5 m bedroom is 10.5 m², comfortably inside the range of a single plug-in ceramic heater.

As a working rule for a normally insulated Canadian home with standard ceilings, a plug-in 120 V heater handles up to about 12 m² (130 sq ft). Beyond that you need either a second unit or a different type of heating.

Then adjust for the room itself

• Ceilings above 2.7 m: treat the room as one size larger
• Below grade or above an unheated garage: treat it as one size larger
• Two or more exterior walls, or large windows: one size larger
• Poor or no insulation: a plug-in heater is the wrong tool entirely
• Open to a hallway or another room: expect to lose much of the heat

Quick reference

• Up to 12 m² (bedroom, office, nursery, bathroom): one unit
• 12–20 m² insulated: two units placed apart, or accept a slower warm-up
• Open-plan living area: improve the central system; plug-in heaters will disappoint
• Uninsulated garage or shop: dedicated 240 V or gas unit heater

Why more than one small unit beats one big one

A single 120 V outlet limits how much heat any plug-in appliance can deliver, so 'bigger' plug-in heaters mostly are not. Two small units in two rooms let you heat exactly the spaces you occupy, and you can run only one when that is all you need.

That is why our two- and three-packs exist and why most Canadian households choose them.

The calculation, with the arithmetic shown

Start with the floor area. Multiply length by width in feet: a 10 ft by 12 ft bedroom is 120 sq ft. The common rule of thumb is 10 watts per square foot for a well-insulated room, so 120 sq ft needs about 1,200 W — comfortably inside a 1,500 W heater's range.

In metric, 12 m² is about 130 sq ft and lands in the same place. If your room is above about 150 sq ft, one 1,500 W plug-in unit will struggle to hold temperature on a cold night.

Adjusting for real Canadian rooms

• Ceilings over 8 ft — add roughly 20%, because you are heating more air
• A room over an unheated garage or crawl space — add roughly 20%
• Large or single-glazed windows — add 10 to 20%
• An exterior corner room with two cold walls — add 10 to 20%
• An open doorway to another room — treat the two rooms as one space
• A well-insulated interior room — no adjustment needed

When the answer is two heaters, or none

If the adjusted figure comes out above roughly 1,500 W, you have two honest options: two units in the two areas you actually use, or a permanently installed system. Buying one heater and hoping is how people end up disappointed.

For anything over about 20 m², or any uninsulated space such as a garage or workshop, a plug-in 120 V heater is not the right product at all — you need a hard-wired 240 V unit or a gas heater.