Calculator · Sizing
Square feet, insulation and ceiling height in; watts, BTU and a realistic number of heaters out. The formula is on the page, so you can check it.
How this page is funded: we earn a commission if you buy through the links here, at no extra cost to you. Picks are chosen before links are attached, which is why every roundup names something to skip. Full disclosure.
Multiply floor area in square feet by 10 watts, then by an insulation factor (0.8 well insulated, 1.0 average, 1.25 drafty, 1.5 unheated) and by your ceiling height divided by 8. That is the wattage the room needs. Divide by 1,500 for the number of plug-in heaters.
Size your room
Floor area
144 sq ft
Wattage needed
1,450 W
In BTU/h
4,948
That is within one plug-in heater’s 1,500 W ceiling. At 26.6¢ an hour it costs about 2.13 dollars for an eight-hour night at full output.
144 × 10 W × 1 × (8 ÷ 8) = 1,440 W, rounded to the nearest 50 W.
See bedroom pickswatts = square feet × 10 × insulation factor × (ceiling height ÷ 8)
Worked example. A 14 × 16 ft living room is 224 sq ft. It has single-glazed windows, so the insulation factor is 1.25. The ceiling is 9 ft, so the height factor is 9 ÷ 8 = 1.125. The arithmetic runs 224 × 10 × 1.25 × 1.125 = 3,150 W — which is more than two plug-in heaters, in a room most listings would happily call “medium”.
Run that same room through the uncorrected rule and you get 2,240 W. The difference between those two numbers, 910 W, is more than half a heater, and it is entirely down to two assumptions the popular version of the rule does not tell you it is making.
| Room size | Watts needed | BTU/h | Plug-in heaters |
|---|---|---|---|
| 80 sq ft | 800 W | 2,730 | 1 |
| 100 sq ft | 1,000 W | 3,412 | 1 |
| 150 sq ft | 1,500 W | 5,118 | 1 |
| 200 sq ft | 2,000 W | 6,824 | 2 (different circuits) |
| 250 sq ft | 2,500 W | 8,530 | 2 (different circuits) |
| 300 sq ft | 3,000 W | 10,236 | 2 (different circuits) |
| 400 sq ft | 4,000 W | 13,649 | 3 (different circuits) |
| 500 sq ft | 5,000 W | 17,061 | 4 (different circuits) |
Computed as square feet x 10 W x insulation factor x (ceiling / 8 ft), for an average heated room — 10 W/sq ft, 8 ft ceiling. Heater count assumes the 1,500 W plug-in ceiling.
Those figures are for an average room with an 8-foot ceiling. For a drafty or single-glazed room, add 25%. For an unheated space — a garage, an unfinished basement, a sunroom — add 50%; that is the correction behind the garage arithmetic, and it is why a garage roundup on this site opens by telling you not to buy a plug-in heater.
Ranked for fit, not for hype. Coverage and current draw are computed from each unit’s published wattage. Tap a row for the full write-up, or go straight to the listing.
| # | Product | Best for | Covers | Draw | Price |
|---|---|---|---|---|---|
| 1 | DreoNo photo Dreo Solaris 718The rare heater whose published coverage claim survives the arithmetic: 1,500 W is a genuine 300 sq ft at 10 W/sq ft, and Dreo publishes that number rather than implying more. | A whole living room | 150 sq ft | 12.5 A | Check price on Amazon |
| 2 | LaskoNo photo Lasko Ellipse CD12950The short answer to “which small heater has a child lock”: a tabletop unit that publishes a tip-over switch, a child lock and an auto-eco mode rather than leaving you to guess. | A desk, a nursery, a small bedroom | 150 sq ft | 12.5 A | Check price on Amazon |
| 3 | VornadoNo photo Vornado VH200Circulates the room's air instead of aiming a hot jet at your shins — the difference you feel is uniformity, not output, because the output is the same 1,500 W as everything else. | Even heat, not a hot spot | 150 sq ft | 12.5 A | Check price on Amazon |
Coverage computed at 10 W/sq ft, average insulation, 8 ft ceiling. Draw computed as watts ÷ volts. Neither figure is a manufacturer claim.
Top pick · A whole living room
The rare heater whose published coverage claim survives the arithmetic: 1,500 W is a genuine 300 sq ft at 10 W/sq ft, and Dreo publishes that number rather than implying more.
| Published specification | |
|---|---|
| Rated power | 1,500 W at 120 V |
| Maker's coverage claim | 300 sq ft |
| Thermostat | 41–95°F digital, 1°F increments |
| Oscillation | 30° / 60° / 90° / 120° |
| Dimensions | 7.9" D x 7.9" W x 29.7" H, 8 lb |
| Safety | Tip-over cutoff, overheat cutoff, child lock, thermal cutoff |
| Heat type | Ceramic (PTC) |
| Maker’s coverage claim | 300 sq ft |
| Safety listing | ETL listed (stated by Dreo on the product page) |
Who it is for
A living room, bedroom or den at or under 300 sq ft with average insulation and an 8-foot ceiling, where you want to set a temperature and stop thinking about it.
What it is not for
A garage, an uninsulated basement, or anything over about 300 sq ft. It is not under-powered — 1,500 W is the plug-in ceiling — the room is simply too big for one outlet.
2 · A desk, a nursery, a small bedroom
The short answer to “which small heater has a child lock”: a tabletop unit that publishes a tip-over switch, a child lock and an auto-eco mode rather than leaving you to guess.
| Published specification | |
|---|---|
| Rated power | 1,500 W at 120 V |
| Maker's coverage claim | None published |
| Controls | Digital display, thermostat, AutoECO mode, remote |
| Oscillation | Wide oscillation |
| Safety | Tip-over switch, child lock |
| Heat type | Ceramic (PTC) |
| Maker’s coverage claim | None published — the coverage figure above is ours |
| Safety listing | No listing mark is published in the Amazon listing text; verify the UL or ETL mark on the rating plate. |
Who it is for
A small room — a nursery, a home office, a bedroom under about 150 sq ft — where the controls need to be lockable.
What it is not for
An open-plan living room. A tabletop unit oscillating across a desk is not going to move 1,500 W of heat around 400 sq ft.
3 · Even heat, not a hot spot
Circulates the room's air instead of aiming a hot jet at your shins — the difference you feel is uniformity, not output, because the output is the same 1,500 W as everything else.
| Published specification | |
|---|---|
| Rated power | 1,500 W at 120 V (retail listings; Vornado publishes settings, not watts) |
| Heat settings | 3 |
| Thermostat | Thermostatic control knob |
| Safety | Tip-over switch, automatic safety shut-off, cool-touch case |
| Warranty | 5-year limited |
| Heat type | Fan-forced |
| Maker’s coverage claim | None published — the coverage figure above is ours |
| Safety listing | Vornado's product page publishes an “Automatic Safety Shut-off System” and a tip-over switch but does not name a listing mark; check the rating plate. |
Who it is for
A bedroom or living room where the complaint is “one end is warm and the other isn't”, rather than “it never gets warm”.
What it is not for
Silent operation, and not a garage. A moving-air heater in an uninsulated space is heating the outdoors with extra steps.
Sources for this entry
Watts and BTU per hour are the same quantity in different units: 1 watt = 3.412 BTU/h. A 1,500 W heater is therefore 5,118 BTU/h, and any listing advertising a 1,500 W heater at more than about 5,120 BTU/h has made an arithmetic error or is quoting something other than heat output. The conversion and where it comes from.
And what we did not do: we have not tested these units, owned them, measured them or run them in a room. Everything above is compiled from published manufacturer specifications and live retail listings, with the electrical load, room coverage and running cost computed by us from the published wattage. The full method.
Multiply the room's square footage by 10 watts, then by an insulation factor (0.8 to 1.5) and by your ceiling height divided by 8 feet. A 150 sq ft average room with an 8-foot ceiling needs about 1,500 W.
5,118 BTU per hour. One watt equals 3.412 BTU/h, so 1,500 × 3.412 = 5,118. Any 1,500 W heater advertising substantially more than that is not quoting heat output.
Yes. Heating is a volume problem, and the square-foot rule assumes an 8-foot ceiling. The calculator multiplies by your ceiling height divided by 8, so a 12-foot ceiling raises the requirement by half.
Because it changes the answer by up to 50%. A well-sealed modern room needs about 20% less than the rule of thumb; an unheated garage needs 50% more. Ignoring it is the most common reason a correctly chosen heater underperforms.
Where the 10 W/sq ft rule comes from, when it breaks, and the exact watt-to-BTU conversion.
150 sq ft needs about 1,500 W — one heater exactly, with no margin. Here is what that means.
The four units that get closest to a 300 sq ft room — and the two-heater arithmetic nobody publishes.
Cost per hour for any wattage at any rate — the calculator, the formula and the full grid.
12.5 A against a 12 A continuous limit — the arithmetic nobody publishes, and what to do about it.
For rooms under 150 sq ft, where 1,500 W is oversized and the thermostat is what actually matters.