WeekendCalc

Space Heater Size Calculator

Enter the size of the room and how well it is insulated. You get the wattage of heater to look for, the equivalent in BTU, and what it costs to run.

Your project

$

Example only. Check your bill.

Results

Heater size needed
1,440 watts
In BTU per hour
4,913 BTU/hr
1,500 W heaters needed
1 heaters
Room area
144 ft²

Running cost at full output

Cost per hour
$0.24
Cost per 8 hours
$1.96
  • A thermostat cycles the heater on and off, so real running cost is usually lower than full output. Plug 1,500 W heaters straight into a wall outlet, not an extension cord, and keep them clear of anything flammable.

How this calculator works

The common rule of thumb is 10 watts of heat per square foot for an average room, which is why a 1,500 W heater is rated for about 150 ft². A drafty or poorly insulated room needs closer to 12 W per ft², and a well-insulated one closer to 8. Ceilings taller than 8 ft increase the volume to heat, so we scale up for them. One watt is 3.412 BTU per hour.

watts = area × watts per ft² × (ceiling height ÷ 8)

Worked example: a 12 × 12 ft room (144 ft²) with 8 ft ceilings and average insulation needs 1,440 W, so a single 1,500 W heater is about right. Running it flat out at $0.17 per kWh costs about 24 cents per hour.

Tips before you buy

  • A space heater is meant to heat the room you are in, not the whole house. Turn down the central heating in empty rooms and warm only the one you are using.
  • Most home outlets handle a single 1,500 W heater. Do not run two high-draw appliances on the same circuit.
  • Choose a heater with tip-over and overheat protection, and a thermostat. Never leave one running unattended or near bedding or curtains.
  • For a room you heat often, a mini-split heat pump is far cheaper to run than electric resistance heaters.

Tools & materials for this project

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Ceramic space heaterOil-filled radiatorDraft stopperWindow insulation filmSmart thermostat

Frequently asked questions

How many watts do I need to heat a room?

About 10 watts per square foot for an average room. A 150 ft² room needs 1,500 W, which is a standard portable heater.

How many square feet will a 1500 watt heater heat?

Around 150 ft² in a typical insulated room. In a poorly insulated one it may only handle 120 ft² or less.

Are space heaters expensive to run?

A 1,500 W heater costs roughly 25 cents an hour at average US electricity prices, if it runs flat out. That adds up: about $60 a month at 8 hours a day, which is why they are best used sparingly.

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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.