WarmRoom

Free heating calculator

How big a space heater do you need?

The “10 watts per square foot” rule ignores insulation, ceiling height and how cold it gets outside. This calculator estimates your room’s real heat loss and turns it into a heater size, a safe plug-in setup and an honest running cost.

Room
Temperatures
Use a typical cold night for your area, not a record low.
Power & cost

How the heater size is calculated

Every room constantly loses heat through its walls, windows, floor, ceiling and through drafts. To stay warm, a heater has to put that heat back at the same rate. The calculator estimates the heat loss with a standard volume-based method:

Heat loss (W) = Room volume (m³) × Temperature difference (°C) × Insulation factor

The insulation factor ranges from about 0.6 W per cubic meter per degree for a well-insulated modern room to more than 2 for an uninsulated garage. The result is then adjusted for the number of exterior walls, unheated spaces above or below, and the number and type of windows. Watts convert to BTU per hour by multiplying by 3.412.

Example: a 14 × 12 ft living room with an 8 ft ceiling is about 38 m³. With average insulation, 70 °F inside and 20 °F outside (a 28 °C difference), the base loss is 38 × 28 × 1.2 ≈ 1,280 W. With two exterior walls, an attic above and two double-pane windows it rises to roughly 1,450–1,500 W, so a single 1,500 W heater is just enough.

Why not just 10 W per ft²?

The common rule of thumb – 10 watts per square foot – works for an average room with an 8 ft ceiling in a mild winter. It underestimates drafty rooms, tall ceilings and cold climates, and oversizes well-insulated rooms. Using volume and temperature difference is more accurate and still simple.

The 1,500 W limit

In North America almost all plug-in heaters top out at 1,500 W, because a standard 120 V, 15 A circuit can safely supply about 1,440–1,800 W of continuous load. If a room needs more, use two heaters on separate circuits, or a hard-wired 240 V heater installed by an electrician.

Which type of space heater?

TypeBest forProsCons
Oil-filled radiatorBedrooms, nurseries, all-night useSilent, gentle even heat, stays warm after switching offSlow to warm up, heavy
Ceramic fan heaterOffices, quick warm-upCompact, cheap, heats fastFan noise, dries the air, heats unevenly
Convection panelLiving rooms, permanent installationQuiet, slim, can be wall-mounted, good thermostatsSlower than fan heaters
Infrared / radiantGarages, workshops, drafty spaces, desksWarms people and objects directly, works in draftsOnly warms what it “sees”
240 V forced-airLarge garages and workshopsHigh output (3–7.5 kW)Needs a dedicated circuit and an electrician

What it really costs to run

All electric resistance heaters convert 100% of their electricity into heat, so a 1,500 W heater of any type uses 1.5 kWh per hour on full power. Claims that one heater is “more efficient” usually mean it heats people more directly (infrared) or holds its temperature better (oil-filled). The real saving comes from a good thermostat: once the room is warm, the heater cycles on and off. The calculator assumes the heater runs at full power roughly half the time once the room is up to temperature, which is typical on cold days.

Space heaters are cheapest when they let you keep the rest of the home cooler. Heating one room to 70 °F while turning the central thermostat down a few degrees can lower your total heating bill. Heating the whole house with portable heaters is almost always more expensive than a heat pump or gas furnace.

Safety checklist

Questions people ask

How many square feet will a 1,500 W heater heat?

About 150 ft² as a main heat source in an average room, or up to 250–300 ft² in a well-insulated room in a mild climate. As a supplement to central heating it can cover a larger area.

Is it cheaper to run a space heater or central heat?

Heating a single room with a space heater while lowering the central thermostat is usually cheaper. Heating the whole home with space heaters is usually more expensive, especially compared with a heat pump.

How many amps does a space heater use?

Amps = watts ÷ volts. A 1,500 W heater draws about 12.5 A at 120 V, or about 6.5 A at 230 V. That is why you should not plug anything else heavy into the same circuit.

What size heater do I need for a garage?

Uninsulated garages lose heat very quickly, so a 1,500 W plug-in heater only takes the chill off. For a two-car garage you typically need 4,000–7,500 W from a 240 V heater, or an infrared heater aimed at your work area.

Can I leave a space heater on overnight?

Only modern heaters with a thermostat, tip-over switch and overheat protection, kept well away from bedding. Oil-filled radiators are the most common choice for bedrooms because they have no exposed hot elements and no fan noise.

Does a bigger heater use more electricity?

Not necessarily. With a thermostat, an oversized heater runs for shorter periods. The total energy depends mainly on how much heat the room loses, not on the heater’s maximum wattage.