Space Heater Size Calculator — What Size Heater Does Your Room Need?
The short answer: you need about 10 watts of heater for every square foot of room — so a 120 square foot bedroom needs about 1,200 watts, and a standard 1,500-watt heater covers any room up to about 150 square feet. Bigger than that, one plug-in heater is not enough, because US wall outlets cap out at 1,500 watts. This free calculator does the exact math for your room, including ceiling height and insulation, and tells you whether one heater does it or you need two.
Enter your room size, pick your insulation, and get a straight answer: the watts you need, the BTU per hour, and the heater size to buy. The rule below is the standard heating-industry rule of thumb — 10 watts per square foot for an 8-foot ceiling with average insulation — adjusted for your actual room. No prices here, because prices move daily — always check the live price before you buy.
What Size Space Heater Does Your Room Need?
Measure your room, tell us about the insulation, and get the watts, the BTU per hour, and the heater size to buy — plus the honest answer when one plug-in heater is not enough.
Rule used: 10 watts per square foot for an 8-foot ceiling with average insulation — 8 watts if well insulated, 15 watts if drafty — adjusted for your ceiling height by room volume. See the FAQ below for where those numbers come from.
Your result
| Your room | |
| Room volume | |
| Recommended watts | |
| In BTU per hour | |
| Heater size to buy | |
| Outlet |
As an Amazon Associate we earn from qualifying purchases. Wattage figures use the standard 10 watts per square foot rule, checked 8 November 2026 — always confirm the heater’s rated wattage on the product page before you buy.
How many watts per square foot does a space heater need?
The heating industry's standard rule of thumb is 10 watts per square foot, for a room with an 8-foot ceiling and average insulation. It comes from the volume of air you are heating: an 8-foot ceiling means each square foot of floor is 8 cubic feet of air, and it takes about 1.25 watts to heat each cubic foot. That is why this calculator works from volume, not floor area alone — a 12 by 12 room with 10-foot ceilings holds 25 percent more air than the same room with 8-foot ceilings, so it needs about 1,800 watts instead of 1,440.
Insulation moves the number more than anything else:
- Well insulated — 8 watts per square foot: modern double glazing, sealed weatherstripping, insulated walls. A 150 square foot room needs about 1,200 watts.
- Average — 10 watts per square foot: normal windows, some drafts, typical older home. A 150 square foot room needs about 1,500 watts.
- Drafty — 12 to 15 watts per square foot: single glazing, uninsulated walls, gaps around doors. This calculator uses 15 watts, the safe end, so the heater can hold temperature on the coldest nights. A 150 square foot drafty room needs about 2,250 watts — more than one plug-in heater can give.
One honest warning about maker coverage claims: if a 1,500-watt heater claims to heat 300 or 400 square feet, that is supplemental heating — it assumes your central heating is already holding the house at 62 to 65 degrees and the heater is topping up one room. As the main heat source, 1,500 watts covers about 150 square feet. This calculator sizes for the main-heat case, so the answer is the size that actually warms the room from cold.
Room size to heater size — the quick table
For 8-foot ceilings and average insulation. If your ceilings are higher or the room is drafty, use the calculator above — the table does not adjust for either.
| Room size | Watts needed | BTU per hour | Heater to buy |
| 75 sq ft (small office, bathroom) | 750 W | 2,559 | 750 W |
| 100 sq ft (10 by 10 room) | 1,000 W | 3,412 | 1,000 W |
| 120 sq ft (10 by 12 bedroom) | 1,200 W | 4,094 | 1,500 W |
| 150 sq ft (12 by 12 bedroom) | 1,500 W | 5,118 | 1,500 W |
| 200 sq ft (large bedroom) | 2,000 W | 6,824 | 1,500 W + 500 W (two heaters) |
| 250 sq ft (living room) | 2,500 W | 8,530 | 1,500 W + 1,000 W (two heaters) |
| 300 sq ft (15 by 20 living room) | 3,000 W | 10,236 | 1,500 W + 1,500 W (two heaters) |
What if your room needs more than 1,500 watts?
Standard US wall outlets run 120 volts on a 15-amp circuit, and electrical codes say a continuous load should not pass 80 percent of the circuit — 1,500 watts. That is why almost every plug-in space heater tops out at 1,500 watts: it is the most a normal outlet can safely sustain. So a room that needs 2,000 or 3,000 watts cannot be heated by one plug-in heater, however expensive it is.
You have two honest options. First, two heaters on separate circuits — for example a 1,500-watt heater at one end of the room and a 1,000-watt heater at the other, each plugged directly into its own wall outlet on its own circuit. Never plug two big heaters into the same circuit, and never use a power strip: a 1,500-watt heater draws 12.5 amps on its own, and two on one 15-amp circuit will trip the breaker — or worse. Second, a 240-volt heater on a dedicated circuit, the kind of outlet a dryer uses. That is a job for an electrician, not a plug-in purchase, but it is the right answer for a large room you heat every day.
Frequently asked questions
What size space heater do I need?
Multiply your room's square footage by 10 watts. A 120 square foot room needs about 1,200 watts, so buy a 1,500-watt heater. Well-insulated rooms need about 8 watts per square foot; drafty rooms need 12 to 15.
How many watts to heat a 12x12 room?
A 12 by 12 room is 144 square feet, so about 1,440 watts with average insulation and 8-foot ceilings — a standard 1,500-watt heater covers it.
How many watts to heat a 10x10 room?
A 10 by 10 room is 100 square feet, so about 1,000 watts. A 1,000-watt heater is the right size; a 1,500-watt heater works too, on its thermostat.
Can one space heater heat a large room?
Only up to about 150 square feet as the main heat source — the 1,500-watt cap on standard US outlets is the limit. Bigger rooms need two heaters on separate circuits, or a 240-volt heater.
How many BTU is a 1,500-watt space heater?
About 5,118 BTU per hour — 1 watt equals 3.412 BTU per hour. Gas and propane heaters are rated in BTU instead of watts; the same room needs about 40 BTU per square foot.
Do high ceilings change the size of space heater I need?
Yes — the rule is really about volume. For every foot of ceiling above 8 feet, add about 10 to 15 percent. A 12 by 12 room with 10-foot ceilings needs about 1,800 watts instead of 1,440.
How many BTU per square foot does a space heater need?
About 40 BTU per square foot. A 1,500-watt heater is about 5,118 BTU per hour (1 watt equals 3.412 BTU per hour), so a 150 square foot room needs about 5,118 BTU per hour — a standard 1,500-watt heater. Gas and propane heaters are rated in BTU instead of watts.
What size space heater do I need for a 200 square foot room?
About 2,000 watts (6,824 BTU per hour) with average insulation and 8-foot ceilings. One plug-in heater cannot do it — the 1,500-watt cap on standard US outlets is the limit — so use a 1,500-watt heater plus a 500-watt heater on a separate circuit.
Can I plug two space heaters into the same outlet?
No. Never plug two big heaters into the same circuit, and never use a power strip. A 1,500-watt heater draws 12.5 amps on its own, and two on one 15-amp circuit will trip the breaker — or worse. Put each heater on its own wall outlet on its own circuit.
What size space heater do I need for a small 75 square foot room?
About 750 watts (2,559 BTU per hour) with average insulation and 8-foot ceilings. A 750-watt heater is the right size; a 1,000-watt heater works too, and a 1,500-watt heater on its thermostat covers it from cold.
Real-world examples
These examples use the calculator's rule: room volume times 1.25 watts per cubic foot, adjusted for insulation (0.8 well insulated, 1.0 average, 1.5 drafty).
- Bedroom, 10 by 12 feet, 8-foot ceilings, average insulation: 120 sq ft, 960 cu ft — needs 1,200 watts (4,094 BTU per hour). Buy a 1,500-watt heater; it runs on its thermostat and covers the room from cold.
- Living room, 15 by 20 feet, 8-foot ceilings, average insulation: 300 sq ft, 2,400 cu ft — needs 3,000 watts (10,236 BTU per hour). One plug-in heater cannot do it: use two 1,500-watt heaters on separate circuits, or a 240-volt heater.
- Home office, 8 by 10 feet, 8-foot ceilings, average insulation: 80 sq ft, 640 cu ft — needs 800 watts (2,730 BTU per hour). Buy a 1,000-watt heater, or run a 1,500-watt heater on low.
- Drafty bedroom, 10 by 12 feet, 8-foot ceilings: 120 sq ft — needs 1,800 watts (6,142 BTU per hour). More than one plug-in heater: a 1,500-watt heater plus a 500-watt heater on a separate circuit, and fix the drafts if you can — sealing them beats buying watts.