BTU Calculator for Heating


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When choosing a new heating system for your home, it’s important to choose the correct sized appliance (furnace, fireplace, stove etc.) for your property.

Installing a furnace, gas fireplace or wood stove that is too large can result in a heat output that isn’t adequate for your living space, meaning it won’t provide enough hot air to heat your property. On the other hand, if the heat output is excessive, it will make your living space unbearably warm.

What is BTU?

Heat is measured in a unit of measurement known as British Thermal Units (BTUs) and it is defined as the amount of energy needed to cool or heat one pound of water by one degree Fahrenheit.

Therefore, the BTU formula is a useful metric since it allows you to compare the heating capacity of a variety of different fuels, including wood burning stoves, natural gas fireplaces, propane heaters, and electric space heaters.

An appliances BTU rating gives you information on the amount of heat output it generates, therefore, the greater the BTU rating the greater the heating capacity.

The following video will give you a deeper look into what a BTU is:

BTU Per Square Foot Heating Rule of Thumb

With that said, the standard rule of thumb for BTU per square foot is:

15 x Volume of Space to Heat = Required BTU’s

To get a better idea of what size of BTU output you should be looking for, use the BTU calculator below. You will need the dimensions of the space you intend on heating and the average outdoor temperatures of your local area.

How many BTU’s do I need?

The number of BTUs you require depends on a range of factors, including your local climate, property size, insulation and desired temperature (personal preference). Therefore, even though manufacturers provide guidelines on the number of BTUs you require for a certain area (measured in square feet), it is impacted by a number of other factors too.

BTU Calculator Watts

If you are interested in an electrical appliance such as an electric fireplace or another type of electric heater, you will probably want to know how to compare it to appliances that are rated in BTUs. For example, if you are trying to compare a propane heater rated in BTUs to an electric heater rated in watts.

One BTU per hour amounts to 0.293 watts. Therefore, for a gas fireplace with a heat output of 30,000 BTUs per hour, you would multiply 30,000 by 0.293, which equates to 8790 watts. It’s also common to see electrical appliances rated in terms of kilowatts too, in which case you just need to divide by 100, so 870 watts is equal to 87.9 kilowatts.

If you need to figure out the kilowatt hours, simply multiply kilowatts by the number of hours the heater will be operating. For instance, if you have a 10-kilowatt heater running for 5 hours, that equates to 50 kWh.

Converting Watts to BTUs

If you are looking at an electrical heating appliance and it only provides the heat output in watts, no need to worry it’s very easy to convert watts to BTUs. The thing to remember is that one watt is the same as 3.41 BTUs per hour. Therefore, in the case of a 2,000-watt electric heater, you just multiply 2,000 by 3.41, which works out to be 6820.

Other Factors Affecting Your Required Heat Output 

When looking for a heating appliance with sufficient heat output, these are some of the other important factors that will affect your decision:

Thermal Insulation 

Thermal insulation is designed and installed to reduce the level of thermal transfer between two areas with different temperatures. For example, the walls in modern homes are typically filled with some type of insulating material that reduces the amount of heat loss to the external environment.

Thermal insulation is important when it comes to calculating BTU since a well-insulated home will likely require an appliance with a lower BTU to stay warm. This is due to thermal insulations ability to better handle the movement of heat, ensuring heat stays in your home for longer, so that you use less and require less.

The reason for this is due to the entropic nature of heat, which moves from the warmer to the colder areas until there are no longer any differences in temperature.

As you have learned from our Refrigeration Cycle article, the ability for heat to transfer between mediums is an important aspect in cooling a room (or heating if the cycle is in reverse). You may even want to think about small scale insulation, such as insulating your pipe system if you have a steam-based furnace.

If you live in a modern home, it’s likely that it has some amount of thermal insulation, either in the wall cavities, roof or both. The material used for insulation is given an R-value, which is a measurement of the materials ability to withstand the transfer of heat. The higher the R-value, the more resistant to heat transfer it is.

Do you live in an old property that is poorly insulated, with single pane windows? Or a newer property which has the latest thermal insulation installed? This is important to know when carrying out the BTU calculation since it will greatly influence the heat output your property needs.

Preferred Temperature 

As well as the practical considerations, it’s also important to ask yourself what temperature you would like your home to be? The majority of people like the temperature inside the property to be anywhere between 70 and 80°F, so that’s certainly a good rule of thumb to keep in mind.

If you are not sure, the best way to determine your desired temperature is to use the simple equation of the difference between the outdoor temperature and the indoor temperature that is preferred.

For example,  Kansas winters typically find themselves to be in the range of 40°F with the odd occurrence of it hitting 30°F from time to time. The temperature wanted by the dwellers is 70°F. So, the target temperature would be 70°F – 30°F = 40°F.

Passive Solar Design 

The design of your home and the amount of sun it receives during the day can also have a big impact on the amount of heat your home will require.

For instance, homes with a passive solar design often have very large south-facing windows that allow large amounts of sunlight into the house. This supply of natural thermal energy in the home means that the BTUs required can be greatly reduced.