What size space heater do you actually need?

The rule, and why it is only a starting point

Ten watts per square foot is the number every publisher repeats, and it is a reasonable first pass. What almost nobody publishes is what it assumes: average insulation, an 8-foot ceiling, and a room that is already part of a heated house. Break any one of those assumptions and the rule under-sizes, sometimes badly.

That matters more than it sounds, because under-sizing does not present as a heater that fails. It presents as a heater that runs continuously, never reaches the thermostat setpoint, and costs you full output every hour it is on. The unit looks like it is working. Your bill agrees that it is working. The room is still cold.

The two corrections are simple multiplications and they are on every page in this section. Ceiling height is a straight ratio: a 10-foot ceiling is 10/8, so multiply by 1.25. Insulation is a judgment call with a published range: 0.8 for a well-sealed modern room, 1.0 for average, 1.25 for single-glazed or drafty, and 1.5 for a space that is not part of the heated envelope at all — a garage, an unfinished basement, a sunroom.

The number that ends most of these conversations

A standard U.S. wall outlet sits on a 120 V, 15 A branch circuit. Electrical code treats anything running three hours or more as a continuous load and caps it at 80% of the breaker rating, which is 12 A, which is 1,440 W. That is why every plug-in heater in the country is built at 1,500 W and none is built bigger: 1,500 W is the ceiling, and no brand can sell its way past it.

So the sizing question resolves into a much blunter one. Under about 150 sq ft, one heater is comfortably enough. Between 150 and 300, one heater is working hard and you should expect it to run most of the time. Above 300 sq ft, one heater is not a sizing choice — it is a compromise you should make knowingly, and usually the better answer is two units on two different circuits, or a heater that is not plug-in at all.

Sizing gives you a wattage. Then it gives you a bill

The reason this section exists next to the running-cost pages rather than in a different corner of the site is that they are the same calculation from opposite ends. Sizing tells you the wattage the room needs; wattage times hours times your electricity rate tells you what that costs. A correctly sized heater in a small room is cheap. The same heater fighting a room twice its size costs exactly the same per hour and never wins.

That is the whole argument for zone heating, and it is also its limit. Heating one 150 sq ft room at 1,500 W is genuinely cheaper than heating a whole house — the comparison is worked through here. Heating four rooms with four space heaters is not.

Where to go from here

• Know your square footage? Use the sizing calculator — it applies both corrections and routes you to the matching picks.
• Want the underlying rule and the BTU conversion? Watts per square foot.
• Have a specific room size in mind? The 150, 200, 500 and 1,000 sq ft pages each work the arithmetic and name the picks that fit.
• Heating an unheated space? Read the garage roundup before you buy anything — the answer there is not a plug-in heater.