The BTU of a space heater is important because it tells you if the unit is powerful enough to heat the space you plan to use it in.
A 1500-watt heater, the most common space heater size, puts out about 5118 BTU of heat. This is enough to heat about 170 square feet. But this number is based on average conditions.
Below, we’ll look at how to calculate BTU for a range of different-sized space heaters. We’ll also look at how BTU relates to room size coverage so you can find the right size heater for your room based on your needs and climate.
Watts to BTU Conversion Calculator
A BTU calculator is the easiest way to figure out how many BTU your heater puts out based on the wattage of the unit.
To use this electric heater BTU calculator, simply input the wattage of your space heater in the box below. The calculator will automatically calculate the BTU (British Thermal Units) of the unit.
Technically, this calculator gives you the BTU per hour (BTUh) rate for your unit. This is because wattage is a measure of rate while BTU is a measure of heat. To convert from one to the other, we must transform BTU into a rate measurement.
But for simplicity’s sake, we can look at this calculation as simply telling us the BTU output of the heater.
Watts to BTU Conversion Chart
Another easy way to figure out how many BTU your unit puts out is to reference our BTU conversion chart.
To make this chart even more helpful, we’ve included information on room coverage associated with each BTU output.
How much heat it takes to warm a room of a given square footage depends on how cold it is outside, how well the home is insulated, airflow, and many other factors. In general, you will need between 30 and 60 BTU per square foot to warm any given space.
To use this chart:
- simply locate the wattage of your heater and reference the BTU output
- The associated room coverage calculations tell you what size room the unit will work in for both average conditions and very cold conditions.
| Wattage | BTU | Room Coverage - Average Conditions | Room Coverage - Very Cold Condition |
|---|---|---|---|
| 100 | 341 | 11 sq ft | 6 sq ft |
| 200 | 682 | 23 sq ft | 11 sq ft |
| 300 | 1024 | 34 sq ft | 17 sq ft |
| 400 | 1365 | 45 sq ft | 23 sq ft |
| 500 | 1706 | 57 sq ft | 28 sq ft |
| 600 | 2047 | 68 sq ft | 34 sq ft |
| 700 | 2388 | 80 sq ft | 40 sq ft |
| 800 | 2730 | 91 sq ft | 45 sq ft |
| 900 | 3071 | 102 sq ft | 51 sq ft |
| 1000 | 3412 | 114 sq ft | 57 sq ft |
| 1100 | 3753 | 125 sq ft | 63 sq ft |
| 1200 | 4094 | 136 sq ft | 68 sq ft |
| 1300 | 4436 | 148 sq ft | 74 sq ft |
| 1400 | 4777 | 159 sq ft | 80 sq ft |
| 1500 | 5118 | 171 sq ft | 85 sq ft |
| 2000 | 6824 | 227 sq ft | 114 sq ft |
| 3000 | 10236 | 341 sq ft | 171 sq ft |
| 4000 | 13648 | 455 sq ft | 227 sq ft |
| 5000 | 17060 | 569 sq ft | 284 sq ft |
| 6000 | 20472 | 682 sq ft | 341 sq ft |
| 7000 | 23884 | 796 sq ft | 398 sq ft |
| 8000 | 27296 | 910 sq ft | 455 sq ft |
| 9000 | 30708 | 1024 sq ft | 512 sq ft |
| 10000 | 34120 | 1137 sq ft | 569 sq ft |
| 11000 | 37532 | 1251 sq ft | 626 sq ft |
| 12000 | 40944 | 1365 sq ft | 682 sq ft |
| 13000 | 44356 | 1479 sq ft | 739 sq ft |
| 14000 | 47768 | 1592 sq ft | 796 sq ft |
| 15000 | 51180 | 1706 sq ft | 853 sq ft |
| 16000 | 54592 | 1820 sq ft | 910 sq ft |
| 17000 | 58004 | 1933 sq ft | 967 sq ft |
| 18000 | 61416 | 2047 sq ft | 1024 sq ft |
| 19000 | 64828 | 2161 sq ft | 1080 sq ft |
| 2000 | 68240 | 2275 sq ft | 1137 sq ft |
Manual Watts to BTU Calculation
Electric heaters are nearly 100% efficient in turning BTU input into BTU output. This makes manually calculating BTU for electric heaters quite simple. All you need to do is multiply the wattage of the unit by 3.412, as demonstrated by this equation:
Wattage * 3.412 = BTU
For example, a 1,500-watt heater multiplied by 3.412 gives us a BTU of 5,118.
Alternatively, you can find the wattage of the heater you need based on the BTU required to heat the space using this equation:
BTU/3.412 = Wattage
How many BTU you need to heat a given space depends on how cold it is outside and how well-insulated the room is.
In general:
- Moderate temperatures outside require 30 BTU per square foot for adequate heating
- Cold temperatures outside require 45 BTU per square foot for adequate heating
- Very cold temperatures outside require 60 BTU per square foot for adequate heating
If your home is poorly insulated you will need more BTU per square foot for adequate heating.
Once you know how many BTU you need to heat your room, you can use this calculation to find out the optimal wattage for your new heater:
(Optimal BTU * Room Square Footage)/3.412 = Wattage
As we discussed above, wattage is a rate measurement and BTU is a simple heat measurement. This means that technically you need to convert BTU to a rate measurement to move between the two units.
The number 3.412 allows us to do this as it represents the number of BTU a single watt puts out every hour.
In the above equations, we have simplified BTU somewhat. To get a full understanding of what this measurement represents and how it is used to calculate different heat and air conditioning needs, check out this article.
