Room Heat Load
Load level: — | Calculated load: — BTU/hr (— BTU/hr per sq ft) | Daily cost: —
Recommendation Summary
| Recommended heater | — |
| Acceptable range | — |
| Room load | — |
| Operating cost | — |
| Heat-up time | — |
Heater Sizing Chart
| Room size | Typical heater |
|---|---|
| 100 sq ft | 1,500 – 3,000 BTU |
| 200 sq ft | 3,000 – 5,000 BTU |
| 400 sq ft | 6,000 – 10,000 BTU |
| 600 sq ft | 10,000 – 15,000 BTU |
| 800 sq ft | 14,000 – 20,000 BTU |
BTU vs Watts
| Watts | BTU/hr |
|---|---|
| 750 W | ~2,560 |
| 1,000 W | ~3,412 |
| 1,500 W | ~5,120 |
| 2,000 W | ~6,824 |
| 5,000 W (240V) | ~17,060 |
1 watt is about 3.41 BTU/hr. Electric heaters convert nearly all power to heat.
What Size Heater Do I Need?
You are standing in a cold room and you want to know which heater to buy, not a physics lecture. This calculator turns your room into a heater recommendation: enter the room size, ceiling height, insulation, and climate, pick the heater type, and it returns a recommended BTU and wattage range, whether that heater type is suitable, what it will cost to run, and roughly how long the room takes to warm up. As a rule of thumb a room needs about 20 BTU per square foot of supplemental heat, adjusted up for cold climates, tall ceilings, and poor insulation.
BTU vs Watts
Electric heaters are usually rated in watts, gas and propane heaters in BTU per hour, and the two convert directly: one watt equals about 3.41 BTU per hour. So a 1,500-watt space heater puts out roughly 5,120 BTU per hour, and a 5,000-watt 240-volt garage heater about 17,060. Because electric resistance heaters turn nearly all their power into heat, their wattage is also their heat output, which makes operating cost easy to estimate from your electricity rate.
Garage Heater Examples
Garages are a top search for a reason: they are big, poorly insulated, and have a huge uninsulated door. A typical insulated two-car garage of about 400 to 500 square feet often needs 7,500 to 10,000 BTU, while a three-car garage or an uninsulated shop can need 15,000 to 30,000 BTU or more. Select the garage or shop room type with poor insulation above to see the adjusted size. For larger spaces a 240-volt hardwired electric heater or a propane forced-air unit is usually required, since a plug-in space heater tops out at 1,500 watts.
Why Oversizing Backfires
It is tempting to buy the biggest heater available, but an oversized unit heats the air in quick blasts and then shuts off, so the room swings between too warm and too cool and the heater short-cycles. For electric heaters, an oversized 240-volt unit may also trip breakers or need wiring you do not have. Matching the heater to the calculated load gives steadier comfort and lower running cost, which is why the calculator gives a range rather than just the largest possible number.
How Long Until the Room Warms Up?
Heat-up time depends on the room volume, how much you want to raise the temperature, and the heater output, plus the thermal mass of furniture and walls that must warm along with the air. A right-sized heater typically brings a room up to comfort in fifteen to forty-five minutes; a colder start, a bigger room, or a weaker heater takes longer. The estimate above assumes the heater runs at full output and includes a factor for furnishings.