BTU Calculator | Radiator Heating Calculator - Just Radiators

BTU Calculator - Find your perfect radiator with our heating calculator

BTU & Watt Calculator

Find your perfect radiator

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What is a BTU Calculator?

Our BTU (British Thermal Units) calculator accurately works out the heat energy required for your room, per hour – so you can ensure that each room stays at a comfortable temperature, without wasting energy, or money.

How to choose the right radiator?

Before selecting your radiator, you need to know what the heat output requirements of the room are. Not sure what they are? Don’t panic.

We have a simple BTU calculator to help you!

All you need is the room dimensions and the number and type of windows you have.

The calculator will then tell you what size radiator you need to make the room just the right level of cozy!


Unit of measurement for power, used by the international system. Watts are used to measure the power of appliances and calculate energy consumption. The most interesting value is the watt hour i.e. the consumption of an appliance per hour of use (the most frequently used is the kilowatt hour equal to 1000 Watts/h).

BTU (British Thermal Unit):

This is a unit of measurement for energy, adopted in the United Stated and United Kingdom, and currently used instead of Watts (W). A BTU is defined by the quantity of heat required to raise the temperature of 454 grams of water from 60 to 61 degrees Fahrenheit.  BTUs/h are normally used in the definition of the cooling power of air conditioning systems as well as of the heating power of fuel.

For W to BTU/h conversions, 1 W is equal to 3.41 BTU/h. Therefore to convert BTU/h into Watts you need to divide by 3.41; to convert Watts into BTU/h you need to multiply by 3.41.

How is BTU measured?

A single BTU value is determined by the amount of energy needed to raise 1 pound of water above sea level by 1oF, and is a calculation recognised around the world. By considering the dimensions, the windows and the type of room – this measurement can be used to work out how much energy is required to heat the room to a standard temperature (ranging between 18oc to 23oc).

How to use this BTU Calculator?

Simply answer the questions below, then receive your BTU calculation for the room, and explore our personalised recommendations on the radiators that will suit your requirements.

Remember that you can either combine multiple radiators, or stick to just one, to reach the BTU requirement.