What size space heater do I need for my room?

How to size a space heater

The power you need is the room volume times a heat need per unit of volume: 40 W per cubic meter for a poorly insulated room, 30 W/m³ for a well insulated one. These come from EDF, the French electricity supplier, which gives about 100 W per m² of floor for a poorly insulated home and 75 W/m² for a well insulated one; we divide them by a 2.5 m (8 ft 2 in) reference ceiling. In US units, 40 W/m³ is 1.13 W per cubic foot, about 9 W per square foot with an 8 ft ceiling, and 30 W/m³ is about 6.8 W per square foot.

That matches what Consumer Reports found on the shelf: most space heaters use 1,500 watts on high and are claimed to heat about 150 to 200 square feet, which is 7.5 to 10 W per square foot. You can change the heat need in the advanced options. The calculator shows a ±10% range around the estimate, because the rule ignores windows, outside walls and climate, then picks the smallest size on sale that covers it (with a 5% tolerance).

Worked example: a 12 × 12 ft bedroom

These are the values the calculator opens with in US units: a 12 × 12 ft room with an 8 ft ceiling, poorly insulated, an electric heater as the only heat.

• Volume: 12 × 12 × 8 = 1,152 cubic feet (32.6 m³)
• Power: 32.6 × 40 = 1,305 W
• Range: about 1,170 to 1,440 W

The calculator suggests one 1,500 W space heater. At 120 V it draws 12.5 A. Well insulated, the room needs 979 W: still a 1,500 W heater, because a 750 W model is too small.

In the UK, with metres: a bedroom of 4 × 3.5 m with a 2.4 m ceiling is 33.6 m³. Poorly insulated it needs 1,344 W, so a 1,500 W heater; well insulated, 1,008 W, and a 1,000 W heater is enough.

How many square feet does a 1,500 W heater heat

With 8 ft ceilings, here is the power for common room sizes and the heaters to buy:

Room	Poorly insulated	Well insulated
100 sq ft	910 W: 1 × 1,500 W	680 W: 1 × 750 W
150 sq ft	1,360 W: 1 × 1,500 W	1,020 W: 1 × 1,500 W
200 sq ft	1,810 W: 2 × 1,500 W	1,360 W: 1 × 1,500 W
250 sq ft	2,270 W: 2 × 1,500 W	1,700 W: 2 × 1,500 W
300 sq ft	2,720 W: 2 × 1,500 W	2,040 W: 2 × 1,500 W
400 sq ft	3,630 W: 3 × 1,500 W	2,720 W: 2 × 1,500 W

So one 1,500 W heater covers up to about 170 square feet in a poorly insulated room and about 230 square feet in a well insulated one.

Plug-in heaters in watts, stoves in BTU

Most plug-in space heaters use 1,500 W on high (Consumer Reports), and smaller 750 W models are common for small rooms. In Europe electric heaters come in 1,000, 1,500, 2,000 and 2,500 W. Wood and pellet stoves are sold by heat output: in BTU/h in the US, in kW in Europe. One watt is 3.412 BTU/h, so a 1,500 W heater gives about 5,100 BTU/h.

The calculator's stove sizes start at 20,000 BTU/h (5 kW in Europe), which is more than a single bedroom needs; the receipt flags it. A stove makes sense for a large room: a poorly insulated 30 × 20 ft great room with an 8 ft ceiling needs about 18,600 BTU/h, and a 20,000 BTU/h stove fits.

Common mistakes

• Counting square feet only. A 10 ft ceiling holds 25% more air than an 8 ft one, and needs 25% more heat.
• Two heaters on one circuit. Two 1,500 W heaters at 120 V draw 25 A, more than a 15 A or 20 A circuit carries.
• Mixing up watts and kWh. Watts tell you how much heat the heater gives, kWh how much energy it uses: 1,500 W on high for one hour is 1.5 kWh.
• An oversized wood stove for one room. Even turned down, it can give more heat than the room needs.