Space Heater Size Calculator: How Many Watts Do You Need?
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Most space heaters sold in the U.S. top out at 1,500 watts, because that is about all a standard household outlet can supply. The real question is whether 1,500 W is enough for your room, or whether you need two heaters or a 240-volt unit. Enter your space below and the calculator works it out.
Space heater size calculator
How the calculator works
The starting point is the rule heater makers commonly use: about 10 watts per square foot for a room with an 8-foot ceiling inside a heated home. From there it adjusts for:
- Ceiling height. A 10-foot ceiling holds 25% more air than an 8-foot one, so it needs about 25% more heat.
- Insulation. A drafty room loses heat faster, so it gets more watts per square foot (12 instead of 10). A tight, well-insulated room gets fewer (8).
- Basements and garages. These start much colder than a room inside a heated house, so they get a higher base figure, plus an adjustment for how cold your winters are.
The answer is an estimate for picking a heater size, not an engineering heat-loss calculation. If you are heating a whole garage that you use every day, a heating contractor can do a proper load calculation.
Quick sizing chart
| Space (8 ft ceiling, average insulation) | Heat needed | What fits |
|---|---|---|
| Bedroom or office, up to 150 sq ft | Up to 1,500 W | One plug-in heater |
| Living room, 150–300 sq ft | 1,500–3,000 W | One heater for the area you use, or two on separate circuits |
| One-car garage, about 250 sq ft | About 3,100 W in a cold climate | 240 V heater, 3,000–4,000 W |
| Two-car garage, about 500 sq ft | About 6,300 W in a cold climate | 240 V heater, 6,000–7,500 W |
Watts and BTUs
Electric heaters are rated in watts and furnaces in BTUs. To convert, multiply watts by 3.412. A 1,500 W heater puts out about 5,100 BTU per hour, and a 7,500 W garage heater about 25,600. Every electric heater turns essentially all of its power into heat, so a 1,500 W heater from one brand gives the same heat as a 1,500 W heater from another. What differs is how it spreads the heat, how quiet it is, and how well its thermostat holds a steady temperature.
What a space heater costs to run
Multiply the heater's kilowatts by the hours it runs and your price per kWh. At 17 cents per kWh, close to the U.S. average, a 1,500 W heater on full power costs about 26 cents an hour, or about $2 for an 8-hour night. With a thermostat it cycles off once the room is warm, so the real figure is usually lower. A 7,500 W garage heater costs about $1.28 an hour at full power. You can put your own electricity price into the calculator above.
Which type of heater?
- Ceramic (fan) heaters warm you up fast and are light enough to carry around. The fan makes some noise. Good for offices and short sessions.
- Oil-filled radiators have no fan, so they are silent. They take 15–20 minutes to warm up but give steady heat and stay warm after they switch off. Good for bedrooms.
- Infrared heaters warm people and objects directly, which works well in larger rooms and spaces with drafts.
- 240 V garage heaters mount on a wall or ceiling and put out 3,000–7,500 W. They need a dedicated circuit.
Space heater safety basics
- Plug the heater straight into a wall outlet. Heater makers and the U.S. Consumer Product Safety Commission both warn against extension cords and power strips.
- A 1,500 W heater draws about 12.5 amps. Don't run two on the same circuit, or with a microwave or hair dryer on that circuit.
- Keep 3 feet of clear space around it: beds, curtains, paper and clothes.
- Look for tip-over and overheat protection and a safety certification mark (UL, ETL or CSA).
- Turn it off when you leave the room. Safety groups also advise turning heaters off when you go to sleep, so a timer or auto-off setting is useful in a bedroom.
- 240 V garage heaters are hardwired. Have an electrician run the circuit and follow the wire and breaker sizes in the manual.
More heater guides
Questions people ask
How many square feet does a 1,500 watt heater heat?
About 150 square feet in a room with an 8-foot ceiling and average insulation, using the common 10 watts per square foot rule. In a well-insulated room it can stretch to about 190 sq ft; in a drafty one, about 125.
Can a space heater heat a 500 square foot room?
Not on its own. 500 sq ft needs about 5,000 W, and a plug-in heater gives 1,500 W at most. A space heater will warm the area near you, but heating the whole room is a job for central heat.
What size heater do I need for a 2-car garage?
About 6,000–7,500 W for a 500 sq ft garage with a 9-foot ceiling in a cold climate. That means a 240 V hardwired garage heater, not a plug-in model.