Heater Size Calculator (Free Tool, 2026)
Not sure how big a heater you need? This free heater size calculator works out the right wattage for any room in seconds. Enter your room size, ceiling height, insulation and climate, and get the recommended baseboard or space heater size, BTU output, amps, baseboard length and monthly running cost.
Heater Size Calculator
Find the right wattage for a baseboard or space heater in any room.
Your heating plan
Estimates use the common 10 watts per square foot rule for an 8 ft ceiling, adjusted for insulation, ceiling height, climate and windows. Cost assumes the heater runs at full power for the hours you set. For whole-home heating or new circuits, have a licensed electrician confirm sizing and wiring.
How to Use the Heater Size Calculator
- Enter the room length and width in feet or meters. For an L-shaped room, split it into two rectangles and add the results.
- Pick the ceiling height. Taller ceilings mean more air to heat, so the wattage goes up about 25% for every 2 feet above 8 feet.
- Set insulation, climate, room position and windows to match your home. A drafty corner room with old windows needs far more heat than an insulated interior bedroom.
- Choose the heater type. A 240 V baseboard can go up to about 2,500 W per unit, while 120 V heaters and plug-in space heaters top out around 1,500 W.
- Add your running hours and electricity rate to see what the heater will cost each month.
The calculator rounds up to the nearest standard heater size you can actually buy and tells you if the room needs more than one heater.
Heater Size Chart by Room Size
These figures assume an 8 ft ceiling and use the common watts-per-square-foot rule. Use the calculator above for your exact room.
| Room size | Good insulation (7.5 W/sq ft) | Average (10 W/sq ft) | Poor (12.5 W/sq ft) |
|---|---|---|---|
| 100 sq ft (10 x 10) | 750 W | 1,000 W | 1,250 W |
| 150 sq ft (12 x 12.5) | 1,125 W | 1,500 W | 1,875 W |
| 200 sq ft (14 x 14) | 1,500 W | 2,000 W | 2,500 W |
| 250 sq ft (15 x 16.5) | 1,875 W | 2,500 W | 3,125 W |
| 300 sq ft (15 x 20) | 2,250 W | 3,000 W | 3,750 W |
| 400 sq ft (20 x 20) | 3,000 W | 4,000 W | 5,000 W |
How the Heater Size Is Calculated
The starting point is a long-standing rule used for electric baseboard heat: about 10 watts per square foot for a room with average insulation and an 8 ft ceiling. Newer, well-insulated rooms can get by with about 7.5 W per square foot, and older, drafty rooms may need 12.5 W or more. The calculator then adjusts that number:
- Ceiling height: +12.5% for 9 ft, +25% for 10 ft and +50% for 12 ft ceilings.
- Climate: about 10% less for mild winters and 15% more for very cold ones.
- Room position: corner rooms with two or more outside walls lose more heat, so they get 10% extra.
- Windows: several windows add 5%, and many or older single-pane windows add 12%.
From the final wattage, it works out heat output in BTU per hour (1 watt = 3.412 BTU/h), current draw in amps (watts divided by volts), and roughly how much baseboard you need, since a standard-density baseboard puts out about 250 W per linear foot.
Baseboard Heater vs Space Heater
| Baseboard heater | Plug-in space heater | |
|---|---|---|
| Power | Up to about 2,500 W per unit at 240 V | Usually 1,500 W max |
| Installation | Hard-wired, dedicated circuit | Plugs into a wall outlet |
| Best for | Primary heat for a room | Spot heating or a small room |
| Control | Wall or line-voltage smart thermostat | Built-in thermostat or smart plug |
| Cost to run | Same per watt | Same per watt |
Electric heat converts all of its electricity into heat, so a 1,500 W space heater and a 1,500 W baseboard cost the same to run. The difference is how much heat each one can deliver and how well it is controlled. If your room needs more than 1,500 W, a single plug-in heater will struggle, and a hard-wired baseboard or a heat pump is the better option.
Circuit and Breaker Sizing
Electric heaters count as a continuous load, so circuits are normally sized at 125% of the heater’s current draw. A 2,000 W heater at 240 V draws about 8.3 A, which fits comfortably on a 15 A, 240 V circuit. Several heaters on one circuit add up quickly, which is why the calculator shows the total amps. New circuits and hard-wired heaters should always be installed by a licensed electrician.
For plug-in heaters, plug directly into a wall outlet, never into an extension cord or power strip, and avoid running two heaters on the same circuit. If you want to see exactly how much power a heater uses, an energy monitoring smart plug rated for 15 A loads will show it in real time.
Tips to Cut Electric Heating Costs
- Turn the temperature down when you sleep or leave. A smart line-voltage thermostat makes this automatic.
- Seal drafts first. Window film, door sweeps and thermal curtains can shrink the heater size you need. See our guide on insulating windows cheaply.
- Heat only the rooms you use. Zoned electric heat lets you keep spare rooms cooler.
- Keep heaters clean and clear. Dust on the fins and furniture pushed against the heater both cut output.
- Track your usage. A home energy monitor shows how much of your bill goes to heating.
For more ideas, read our guide on how to lower your electricity bill at home.
Frequently Asked Questions
What size heater do I need for a 12×12 room?
A 12×12 room is 144 sq ft. With average insulation and an 8 ft ceiling, that is about 1,440 W, so a 1,500 W heater is the right size.
How many square feet will a 1,500 watt heater heat?
About 150 sq ft in a room with average insulation and an 8 ft ceiling. In a well-insulated room it can handle around 200 sq ft, and in a drafty one closer to 120 sq ft.
How many watts per square foot for a baseboard heater?
The common rule is 10 watts per square foot for an 8 ft ceiling and average insulation, 7.5 W for well-insulated rooms and 12.5 W or more for older, drafty rooms.
Is it better to oversize an electric heater?
Slightly oversizing an electric heater is fine because the thermostat simply shuts it off sooner, and electric heaters do not lose efficiency when they cycle. Going far oversized just costs more upfront and can cause uneven heat.
How much does it cost to run a 1,500 W heater?
A 1,500 W heater uses 1.5 kWh per hour at full power. At $0.18 per kWh, that is about $0.27 per hour, or about $65 a month if it runs 8 hours a day.
Should I use 120 V or 240 V baseboard heaters?
240 V is the usual choice for whole-room heat. It draws half the current for the same wattage, so you can run larger heaters or more of them on one circuit.
More Free Smart Home Tools
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- Thermostat Savings Calculator: estimate yearly savings from a smart thermostat
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- Generator Size Calculator: running and starting watts for backup power
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