BTU calculators can be found on almost every website with even the most tenuous connection to heating, but what exactly do they do and how is it useful in real terms? Let’s take a closer look at the term BTU.

In simple terms, BTU is the accepted abbreviation for British Thermal Unit; despite the name the BTU is a standard unit of energy all over the world, not just in Britain.

One BTU is defined as the amount of heat required to raise the temperature of one pound of liquid water by 1 degree Fahrenheit at its maximum density.

BTU would be more correctly referred to as BTU per hour, but for convenience’s sake the term has come to be understood to mean BTU/hr.

The relationship between BTU and the other measurements of energy

1 BTU = 251.9 calories

1 BTU = 1055 joules

1 Watt = 3.413 BTU

1kWatt = 3413 BTU

Electrical heating equipment is by no means the sole generator of BTU. Even human bodies generate a not-inconsiderable amount of BTU. This amount of BTU varies depending on the activity the person is engaged in:

State of Activity

BTU/hr generated

Asleep

250

At rest

350

Office work (seated)

420

Office work (standing)

640

Ten-Pin Bowling

960

Walking (3mph)

1040

Factory work (heavy)

1600

Exercise (heavy)

1800

As far as we’re concerned though, a BTU calculator works by taking some or all of the following measurements and factors into consideration and then using them to calculate the BTU total your heating systems will need to put out:

• Length of the Room

• width of the Room

• Height of the Room

• Dining Room/Lounge

• Bedroom

• Kitchen/Common Area

• North Facing Wall

• French Windows

• Double Glazing

Divide the resulting BTU total by the number of radiators you intend to install. Your BTU calculator will give the amount of BTU each radiator will need to output.

However it is important to remember that the figure your BTU calculator provides is what the radiator will put out in an average room with no external factors. If you live in a warm climate, on top of an hill completely exposed to the elements or in a region where snow is common, you will have to compensate for these factors.

Remember, it is always better to overestimate your needs than underestimate. You can lower a radiator's maximum heat output but you can't force it to generate more heat than it's physically capable of!