Heater watts for 400 sq ft, by insulation and ceiling height
A 400 sq ft space is a big family room, a finished basement or a two-car garage. The rule of thumb is about 10 W per square foot for an average room, 8 W for a well-insulated one and 12 W for a drafty one, scaled up for ceilings over 8 ft.
| Insulation | 8 ft ceiling | 9 ft ceiling | 10 ft ceiling | 1,500 W heaters (8 ft) |
|---|---|---|---|---|
| Well insulated | 3,200 W | 3,600 W | 4,000 W | 3 |
| Average | 4,000 W | 4,500 W | 5,000 W | 3 |
| Poor or drafty | 4,800 W | 5,400 W | 6,000 W | 4 |
What it costs to run, at $0.17 per kWh
The cost of 4,000 W for 8 hours a day, 30 days a month. Change the price to your own rate in the electricity cost calculator.
| How it runs | Per hour | Per 8-hour day | Per month |
|---|---|---|---|
| Flat out (no thermostat) | $0.68 | $5.44 | $163.20 |
| Thermostat, cold room | $0.51 | $4.08 | $122.40 |
| Thermostat, cycling about half the time | $0.34 | $2.72 | $81.60 |
One heater or more for 400 sq ft?
400 sq ft needs 4,000 W, more than one 1,500 W heater can give, so you would need 3 heaters, each on its own circuit, since one 1,500 W heater already draws 12.5 A of a 15 A circuit. For a room this size that you heat often, a permanent 240 V baseboard heater (about 16.0 ft of standard baseboard on a 25 A breaker) or a mini-split heat pump is usually the better and cheaper answer.
Choose a heater with a thermostat, tip-over and overheat protection, and keep it 3 ft from bedding and curtains. The space heater sizing guide covers the types of heater, and the running cost guide compares prices.
Want to change anything else? The calculator above is the full Space Heater Size Calculator: edit any number and the results update instantly.
Tools & materials for this project
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Frequently asked questions
What size space heater do I need for 400 square feet?
About 4,000 W (13,648 BTU per hour) for a room with average insulation and an 8 ft ceiling. A well-insulated room needs 3,200 W, and a drafty one 4,800 W.
How many 1500 watt heaters for 400 sq ft?
3 at average insulation, each plugged into a different circuit. In a drafty room with 10 ft ceilings it is 4.
How much does it cost to heat 400 sq ft with a space heater?
Running 4,000 W flat out at $0.17 per kWh costs $0.68 an hour, $5.44 for an 8-hour day and $163.20 a month. With a thermostat cycling about half the time it is closer to $81.60 a month.
Related sizes and guides
Related sizes
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.