BTU Heating Calculator: What Size Heater Do You Need?

How this calculator works

We multiply the room's floor area by a heating factor for your climate, then adjust for ceiling height, insulation, and exposure:

BTU/hr = square feet × climate factor × (ceiling height ÷ 8) × insulation factor × exposure factor

Climate	Examples	Factor
Warm	Gulf Coast, Florida, Southern California, low desert	30 BTU/sq ft
Mild	Southeast, Texas, Southwest, coastal California	35 BTU/sq ft
Moderate	Mid-Atlantic, lower Midwest, Pacific Northwest	40 BTU/sq ft
Cold	Northeast, Great Lakes, Plains, Mountain West	50 BTU/sq ft
Very cold	Northern MN/WI/ND, Maine, Alaska, high elevations	60 BTU/sq ft

Insulation multiplies the result by 0.85 (good), 1.0 (average), or 1.3 (poor). Lots of windows or outside walls adds 10%. To convert to electric heat we divide by 3.412, since one watt of electric resistance heat equals 3.412 BTU per hour.

What the result means

Up to about 5,100 BTU: a single 1,500-watt plug-in space heater can heat the room on its own.
5,100–10,000 BTU: one heater helps but won't keep up on the coldest days. Use two heaters on separate circuits, a hardwired 240-volt heater, or a mini-split.
Over 10,000 BTU: beyond what plug-in heaters can do economically. A ductless mini-split is usually the best long-term answer; for garages and workshops, a 240-volt or propane heater also works.

Plug-in heater safety: a 1,500-watt heater draws about 12.5 amps — nearly everything a standard 15-amp circuit can safely carry. Never run two on the same circuit, and plug heaters directly into the wall, not an extension cord or power strip.

Frequently Asked Questions

How many BTUs do I need per square foot?

As a rule of thumb, rooms need roughly 30 to 60 BTU per hour of heat for each square foot, depending on how cold your winters are. Higher ceilings, poor insulation, and lots of windows push that number up.

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

A 1,500-watt heater puts out about 5,100 BTU per hour. As the main heat source, that covers roughly 100 to 170 square feet depending on climate and insulation. It can take the chill off a larger room if your furnace is already doing most of the work.

How do I convert watts to BTUs?

Multiply watts by 3.412 to get BTU per hour. A 1,500-watt electric heater produces about 5,118 BTU per hour, and every electric resistance heater of the same wattage puts out the same amount of heat.

Is this calculator accurate enough to size a furnace or heat pump?

No. It gives a quick estimate for a single room. Whole-home equipment should be sized by a contractor using an ACCA Manual J load calculation, which accounts for your home's actual walls, windows, air leakage, and local design temperatures.

BTU Heating Calculator

Tell us about the room

Estimated heat needed