OwnerSpec

Space heater size calculator: watts per square foot, BTU/h and how many 1,500 W heaters a room needs

Watts = length x width x 10 W per sq ft at an 8 ft ceiling (Dimplex, Stelpro), 12.5 at 10 ft. A 10 x 14 ft room needs 1,400 W: one 1,500 W heater.

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Short answer

Space heater watts = room length x width x watts per square foot. Dimplex’s technical support page and Stelpro’s sizing guide both print 10 W per square foot for an 8 ft ceiling, and Stelpro’s retail brochure raises that to 11.25 W at 9 ft, 12.5 W at 10 ft and 15 W under a cathedral ceiling. A 10 x 14 ft bedroom at 8 ft is 140 sq ft x 10 = 1,400 W, which EIA’s 3,412 Btu per kWh makes 4,777 BTU/h: one 1,500 W plug-in heater covers it. A 12 x 14 ft room at 9 ft is 168 x 11.25 = 1,890 W, and two heaters. Cadet’s sizing article runs lower: 1,500 W for a 250 sq ft room with central heat, 2,000 W with no other heat, which is 6 and 8 W per square foot. One 1,500 W heater covers 150 sq ft at the makers’ 10 W rule and draws 12.5 A, so a second heater needs a second circuit.

The picks

  1. Room up to 150 sq ft and wants a digital thermostat

    Dreo Atom One (DR-HSH004) ceramic fan heater

    Dreo product page: 1000-1500 W, thermostat 41-95 F in 1 F steps, 1-12 h timer, 37.5 dB, 70-degree oscillation, tip-over and overheat protection, 12-month warranty

    Check price on Amazon Why this one

  2. Room up to 150 sq ft and wants a tip-over switch and a lab listing

    Vornado MVH (2026) whole room heater

    Vornado product page: 3 heat settings, mechanical thermostat, tip-over switch, cool-touch case, overheat shut-off, ETL Listed, 5-year limited warranty; the MVH owner's guide prints 1500 and 1125 W and 12.5 A on high

    Check price on Amazon Why this one

  3. Wants an oscillating tower with a timer and accepts no tip-over switch

    Lasko 751320 23 in ceramic tower heater

    Lasko product page: 900-1500 W, 2 heat settings, adjustable thermostat, 7-hour auto-off timer, overheat protection, cool-touch housing, ETL Listed, 3-year warranty; the spec list prints no tip-over switch

    Check price on Amazon Why this one

  4. Bedroom under 150 sq ft and wants no fan at all

    De'Longhi EW7707CM oil-filled radiator heater

    De'Longhi product page: 1500 W, 3 power settings, room thermostat plus safety thermostat, thermal cut-off, no fan; the page prints no tip-over switch and no lab listing

    Check price on Amazon Why this one

If the result below is over 1,500 W, this page’s job is to stop you buying a second plug-in: that reader needs the baseboard heater review or the garage heater review, not another cord.

Space heater size: watts, BTU/h and heaters needed

Enter your numbers.

Formula: watts = length x width x W per sq ft; BTU/h = watts x 3.412 (EIA: 1 kWh = 3,412 Btu); heaters = watts / 1,500, rounded up. The 10 W rule is Dimplex's and Stelpro's for a standard insulated room with an 8 ft ceiling; the 11.25, 12.5 and 15 W steps are Stelpro's brochure table. The 6 and 8 W rates are read off Cadet's 250 sq ft example, and Cadet's only ceiling advice is to move up to the next wattage, so the ceiling menu is ignored on those two. Metres convert at 0.3048 m per foot (NIST). No maker prints an insulation multiplier, so there is no insulation input.

What is the formula?

Watts per square foot is the heat a maker says a standard room needs for each square foot of floor. Dimplex’s technical support page prints the rule as “10 watts of heat per square foot of space”, with a 1,000 W heater for a 100 sq ft room “in a standard insulated room”. Stelpro’s sizing guide prints the same: “Multiply the area in feet by 10”, valid for 8 ft ceilings.

StepFormulaWho prints the figure
Floor arealength x width, in feet (metres x 1 / 0.3048)NIST SP 811 Appendix B.9 for the foot
Wattsarea x 10 at 8 ft; x 11.25 at 9 ft; x 12.5 at 10 ft; x 15 cathedralDimplex (10 W); Stelpro brochure table (all four); Stelpro guide (1.25 W per cubic foot, which gives the same steps)
Heaterswatts / 1,500, rounded upESFI lists a portable heater at 1,500 W; the Vornado MVH guide prints 1,500 W on high
BTU/hwatts x 3.412EIA: 1 kilowatthour = 3,412 Btu

Stelpro’s brochure says to multiply, then “round up to the higher power”. The 1,500 W per heater line is the plug-in ceiling, not a rule for the room: a 1,500 W heater draws 12.5 A at 120 V, which the tripping-breaker guide sets against ESFI’s 1,440 W recommendation for a 15 A circuit.

What does a worked example look like?

Stelpro’s brochure Example 1 is the calculator’s default, a 10 x 14 ft bedroom with an 8 ft ceiling.

  1. Area: 10 x 14 = 140 sq ft.
  2. Watts: 140 x 10 = 1,400 W. Stelpro rounds up to a 1,500 W unit.
  3. BTU/h: 1,400 x 3.412 = 4,777 BTU/h.
  4. Heaters: 1,400 / 1,500 = 0.93, so one 1,500 W plug-in on high.
  5. Same bedroom at 9 ft: 140 x 11.25 = 1,575 W, just past one heater, so two.

Stelpro’s Example 2 is a 14 x 12 ft kitchen at 9 ft: 168 x 11.25 = 1,890 W, which Stelpro rounds to 2,000 W. For a plug-in owner that is 1,890 / 1,500 = 1.26, so two heaters, and two circuits. The same 168 sq ft at 8 ft is 1,680 W and still two heaters; at 10 ft it is 2,100 W.

Why do the makers disagree?

They print different rules because the rule stands in for a heat loss figure nobody has measured for your room.

SourceRule150 sq ft room, 8 ft ceiling
Dimplex technical support10 W per sq ft, standard insulated room1,500 W
Stelpro guide and brochure10 W at 8 ft, 11.25 at 9 ft, 12.5 at 10 ft, 15 cathedral1,500 W
Cadet sizing article1,500 W for 250 sq ft with central heat, 2,000 W with none (6 and 8 W per sq ft)900 W as a supplement, 1,200 W alone

Two caveats the sources print themselves. Dimplex says “to get an accurate measurement we recommend using a heat loss calculator”. Stelpro says the formulas carry a margin of error and that for suspected insulation deficiencies you should “add a few percentage points”, with no number, which is why the calculator has no insulation menu. Cadet’s only ceiling advice is to “move up to the next wattage”.

One more transfer to admit: all three makers are sizing baseboard and wall heaters. No maker page sizes a portable plug-in by the square foot, so this page applies the same rule to a 1,500 W plug-in and takes 150 sq ft (1,500 / 10) as its practical room limit.

Which heater fits the number?

Any 1,500 W plug-in delivers the same 5,118 BTU/h; the space heater review separates the four above on thermostat, tip-over switch, timer and noise. What this page adds is the stop line: a result over 1,500 W is two circuits or a hardwired unit, and the running cost calculator will show what two heaters cost to run.

CPSC’s 2026 winter release says to plug electric space heaters directly into a wall outlet, never an extension cord or power strip, and to keep them three feet from anything that burns. A room that needs the heater across the floor from the only outlet is a room that needs a different heater.

Measure the room before you pick a heater

Do not buy on the “heats up to 150 sq ft” line on the box: every 1,500 W heater makes the same heat, and the box figure is the makers’ 10 W rule run backwards. Measure length, width and ceiling height, run the calculator, and buy a Vornado MVH or Dreo Atom One only if the result is 1,500 W or under. If it is over, skip the plug-in aisle and read the baseboard heater review.

Diagnose the problem, size the unit, match the part, then compare the shortlist. These pages continue the chain.

Frequently asked questions

How many square feet does a 1,500 watt space heater heat?

At the 10 W per square foot that Dimplex and Stelpro print for an 8 ft ceiling, 1,500 W covers 150 sq ft, about 10 x 15 ft. At Stelpro's 12.5 W for a 10 ft ceiling it drops to 120 sq ft. Cadet's own example runs the other way: 1,500 W for a 250 sq ft room that also has central heat.

Can I just buy two 1,500 W heaters for a bigger room?

Only on two different circuits. The Vornado MVH owner's guide says any 1,500 W heater draws up to 12.5 A, and ESFI's booklet puts a 15 A, 120 V circuit at 1,800 W with a recommended ceiling of 80 percent, 1,440 W. Two heaters on one circuit is 3,000 W. Above about 150 sq ft a hardwired 240 V baseboard or garage heater is the honest answer.

Does the calculator adjust for poor insulation or old windows?

No, because no maker page prints a number for it. Stelpro's sizing advice says only to add a few percentage points when you suspect insulation deficiencies, and Dimplex recommends a heat loss calculator for an accurate figure. Treat the result as a starting estimate for a standard insulated room, and round up rather than down in a drafty one.

How do I convert space heater watts to BTU per hour?

Multiply watts by 3.412. EIA's unit page prints 1 kilowatthour = 3,412 Btu, so 1,500 W is 5,118 BTU/h, 1,000 W is 3,412 BTU/h and 750 W is 2,559 BTU/h. Stelpro prints the same conversion the other way, kW = BTU/h divided by 3,412.14. The BTU figure only matters for comparing with a furnace or heat pump rating.

Why do heater makers disagree on watts per square foot?

Because the rule is a shortcut for heat loss, which depends on climate, windows and whether other heat exists. Dimplex and Stelpro print 10 W per sq ft for a standard insulated room at 8 ft. Cadet's example gives 6 W per sq ft when the room has central heat and 8 W when it has none. The calculator offers all three, labelled by who says it.

Fact sheet and how to cite this page

Answer
Dimplex and Stelpro both print 10 W per square foot for an 8 ft ceiling; Stelpro’s brochure prints 11.25 W at 9 ft, 12.5 W at 10 ft and 15 W cathedral. A 10 x 14 ft room at 8 ft needs 140 x 10 = 1,400 W, or 4,777 BTU/h, so one 1,500 W plug-in heater; 12 x 14 ft at 9 ft needs 1,890 W, two heaters on two circuits.
Formula
Watts = length (ft) x width (ft) x W per sq ft (10 at 8 ft, 11.25 at 9 ft, 12.5 at 10 ft, 15 cathedral, Stelpro); heaters = watts / 1,500, rounded up; BTU/h = watts x 3.412 (EIA: 1 kWh = 3,412 Btu); metres x 0.3048 per foot (NIST)
Inputs
  • Room length: 10 ft
  • Room width: 14 ft
  • Ceiling height: 8 ft (10 W per sq ft)
  • Sizing rule: makers’ 10 W per sq ft (Dimplex, Stelpro); or Cadet’s example at 8 W per sq ft with no other heat and 6 W as a supplement to central heat
Output
140 sq ft x 10 = 1,400 W, 4,777 BTU/h, 1,400 / 1,500 = 0.93 so one 1,500 W heater; at 9 ft the same room is 1,575 W and two heaters
Assumptions
  • Dimplex and Stelpro print the 10 W rule for electric baseboard and wall heaters; this page transfers it to portable plug-in heaters, which no maker page sizes by the square foot
  • The ceiling multiplier is Stelpro’s alone (1.25 W per cubic foot); Cadet says only to move up to the next wattage for high or vaulted ceilings
  • No maker prints an insulation multiplier. Stelpro says to add a few percentage points for suspected insulation deficiencies, so the calculator offers no insulation input
  • A 1,500 W plug-in heater draws 12.5 A at 120 V (Vornado owner’s guide), above the 1,440 W that ESFI recommends as the 80 percent ceiling of a 15 A circuit, so a second heater needs a second circuit
Method
The calculator multiplies floor area by the watts per square foot the cited maker prints for the chosen ceiling height, divides by 1,500 W per plug-in heater and rounds up, and converts watts to BTU/h with EIA’s 3,412 Btu per kWh. Every rule is quoted from the Dimplex, Stelpro and Cadet pages as read on 2026-10-31. OwnerSpec ran no heater and measured no room; the worked examples were checked by arithmetic only.
Primary source
Stelpro, Retail Brochure (PDF): recommended watts per square foot by ceiling height (manufacturer or vendor document)
Facts verified
Last updated
Canonical URL
https://ownerspec.com/air/calculators/space-heater-size-calculator-watts-per-square-foot/
Cite as
OwnerSpec, "Space heater size calculator: watts per square foot, BTU/h and how many 1,500 W heaters a room needs", https://ownerspec.com/air/calculators/space-heater-size-calculator-watts-per-square-foot/, facts verified 31 October 2026.

Every figure above also appears, with its source, in the article. All sources ยท How OwnerSpec verifies numbers

Sources

  1. Dimplex, Electric Heater Technical Support: 10 watts of heat per square foot; 1000 watt heater heats 100 square feet in a standard insulated room; use a heat loss calculator for accuracy (manufacturer or vendor document)
  2. Stelpro, Calculating the right wattage for a room: area x 10 for 8 ft ceilings, 1.25 watts per cubic foot for taller rooms, 20 x 20 ft example at 4,000 and 4,500 W, add a few percentage points for insulation deficiencies, kW = BTU/h / 3412.14 (manufacturer or vendor document)
  3. Stelpro, Retail Brochure (PDF): recommended watts per square foot by ceiling height (manufacturer or vendor document)
  4. Cadet, How to Pick the Right Wattage to Heat Your Room: 1,500 W for a 250 sq ft room with central heat, 2,000 W with no other heat; move up to the next wattage for high or vaulted ceilings (manufacturer or vendor document)
  5. ESFI, Indoor Electrical Safety Check booklet (PDF): portable heater 1500 watts; a 15-amp, 120-volt circuit carries 1800 watts, do not exceed 80 percent, 1,440 watts (manufacturer or vendor document)
  6. Vornado, MVH Owner's Guide (PDF): High 1500 watts, Medium 1125 watts; any 1500-watt electric heater draws up to 12.5 amps (retailer listing or retailer-hosted document)
  7. CPSC, Keep Warm and Safe This Winter (2026): plug electric space heaters directly into a wall outlet, never an extension cord or power strip; keep three feet from combustibles (government or regulatory source)
  8. EIA, Energy Units and Calculators Explained: 1 kilowatthour = 3,412 Btu (government or regulatory source)
  9. NIST Guide to the SI, Appendix B.9: foot to meter 0.3048 (exact) (government or regulatory source)