Space Heater Size Calculator

The short answer

Most rooms need about 10 W per sq ft of floor. A 1,500 W heater, the most a plug-in heater usually gives, covers about 150 sq ft with average insulation, or about 250 sq ft in a newer, well-insulated building. Older and drafty rooms, very cold climates, rooms with lots of windows and high ceilings all need more.

The calculator applies those adjustments to your room, tells you whether one heater is enough and estimates what it costs to run.

How to use the calculator

1. Measure the room. Measure the length and width from wall to wall, and the height from the floor to the ceiling.
2. Pick your building. Newer means mostly built from the 1980s on, with insulated walls and double-pane windows. Older means built before 1980, often with single-pane windows. Not sure? Use the general rule.
3. Tick what applies. Check the box for a very cold climate or high elevation, and the one for a room with more windows than most.
4. Add the running cost details. Enter the heater’s wattage (it’s on the label), how many hours a day you’ll run it and your electricity price per kWh from your bill.

The price starts at the US residential average of 18.19¢ per kWh for January to August 2026, from the U.S. Energy Information Administration. Prices vary a lot by state and by utility, so your bill is the better number.

Switch to metric with the toggle at the top. Use Copy link to save your result.

Space heater size by room size

How many watts common room sizes need, with a standard ceiling and no other adjustments:

Anything above 1,500 W is more heat than one plug-in heater gives. In that case, use a heater to warm the part of the room you’re in rather than the whole room.

How we calculate

The calculator uses rules of thumb from heater makers, not a full heat loss calculation. Dimplex itself recommends a heat loss calculation when you need an exact number.

• Watts per square foot. The general rule is 10 W per sq ft, the figure Dimplex gives for a standard insulated room. King Electric recommends about 6 W per sq ft for newer homes and 10 W per sq ft for older homes, plus 2 W per sq ft at high elevations or in extreme cold.
• Windows. King Electric suggests 50% more wattage for rooms with more window space than average.
• Ceiling height. The general rule assumes 8 ft ceilings. For taller rooms, Stelpro uses 1.25 W per cubic foot, which is the same as scaling the watts by the extra height: a ceiling a quarter taller gets a quarter more watts. We scale King Electric’s figures the same way, from the ceiling each one assumes: 8 ft for newer buildings and 9 ft for older ones.
• BTU per hour. One watt is about 3.412 BTU per hour, using the NIST conversion factor.
• Heater size. Plug-in heaters usually top out at 1,500 W (Dimplex notes that heaters for a regular household outlet are usually limited to that), and Virginia Tech Extension lists portable heaters at 750 W to 1,500 W. If you need 750 W or less, a small heater or a bigger one on its low setting will do.
• Running cost. Kilowatts × hours a day × price per kWh, the appliance energy formula Virginia Tech Extension uses. A month is the average month. We assume the heater runs at full power the whole time, so the result is the most it should cost.

Worked examples

Example: A bedroom, building age unknown

What we know

• Room: 12 × 12 ft, 8 ft ceiling
• Building: not sure
• Heater: 1,500 W, 8 hours a day at $0.18 per kWh

How it works out

1. Floor area: 12 × 12 ft = 144 sq ft.
2. Watts per sq ft: 10 (general rule).
3. Heat needed: 144 sq ft × 10 W = 1,440 W (about 4,913 BTU/h).
4. Running cost: 1.5 kW × 8 hours × $0.18 = $2.16 a day, about $65.75 a month.

One 1,500 W heater covers it.

Example: A small home office in a newer building

What we know

• Room: 10 × 10 ft, 8 ft ceiling
• Building: newer, well insulated
• Heater: 750 W, 6 hours a day at $0.18 per kWh

How it works out

1. Floor area: 10 × 10 ft = 100 sq ft.
2. Watts per sq ft: 6 (newer building).
3. Heat needed: 100 sq ft × 6 W = 600 W (about 2,047 BTU/h).
4. Running cost: 0.75 kW × 6 hours × $0.18 = $0.81 a day, about $24.65 a month.

A 750 W heater, or a 1,500 W heater on its low setting.

Example: An older living room with a high ceiling

What we know

• Room: 15 × 13 ft, 10 ft ceiling
• Building: older or drafty
• Heater: 1,500 W, 4 hours a day at $0.18 per kWh

How it works out

1. Floor area: 15 × 13 ft = 195 sq ft.
2. Watts per sq ft: 10 (older building).
3. Ceiling: the rule assumes 9 ft, so a 10 ft ceiling scales the watts by 10 ft ÷ 9 ft.
4. Heat needed: 195 sq ft × 10 W × (10 ft ÷ 9 ft) = 2,167 W (about 7,393 BTU/h).
5. Running cost: 1.5 kW × 4 hours × $0.18 = $1.08 a day, about $32.87 a month.

That's more heat than one plug-in heater gives. Use one to warm the spot where you sit.

Reading a space heater’s label

Once you know how many watts your room needs, the label and product page tell you whether a heater fits:

• Watts. Most plug-in heaters list a high setting of 1,500 W. Many also have a low setting, so check its wattage too: Virginia Tech Extension puts portable heaters anywhere from 750 W to 1,500 W.
• Room size. Many product pages list a maximum room size or coverage area. Compare it with your own room, and remember it assumes an average room, not a drafty one with a high ceiling.
• Thermostat. A built-in thermostat turns the heat down once the room is warm, which saves electricity.
• Safety features and marks. Look for tip-over and overheat protection and the mark of a testing laboratory (more on that below).

The wattage is the number that matters for running cost. A heater with a higher room size claim but the same wattage won’t put out more heat.

Ways to cut the running cost

Running cost is simple math: watts, hours and your price per kWh. Change any one of them and the cost changes with it.

• Use the low setting when it’s enough. At the default price, a heater on 750 W for 8 hours a day costs about $32.87 a month, compared with $65.75 at 1,500 W.
• Run it for fewer hours. Half the hours means half the cost. A timer helps if your heater has one.
• Let the thermostat do the work. Set it to a comfortable temperature instead of the highest one, so the heater cycles off more often.
• Heat only the room you’re in. A space heater makes the most sense when you’re warming one room, not the whole home.

Space heater safety

A heater that’s the right size still has to be used safely. Fire safety groups agree on the basics:

• Give it space. Keep anything that can burn at least 3 feet away, people included (U.S. Fire Administration and NFPA).
• Plug it straight into the wall. Never use an extension cord or a power strip (NFPA and the Consumer Product Safety Commission).
• Turn it off when you leave or go to sleep. NFPA also says to unplug it.
• Buy one with safety features. Look for an automatic shut-off if it tips over, a thermostat and overheat protection.
• Check for a safety mark. NFPA says to buy a heater with the seal of a qualified testing laboratory.

Renters: heaters, leases and your heating

A space heater is extra heat for one room, not a replacement for the building’s heating. If your main heating isn’t keeping your home warm, tell your landlord or building manager in writing and keep a copy.

Check your lease before you buy, since it may have rules about portable heaters. And if your rent includes heat but you pay for your own electricity, a space heater moves part of the heating cost onto your bill. The running cost estimate above shows how much.

Pick a heater with a thermostat over a bigger one without. A thermostat holds the room at the temperature you set instead of running flat out, which is easier on your electric bill.