The dilemma that assails us when we have to buy a radiator, a stove, or any device that develops heat, is how to calculate the the thermal requirement (expressed in Watts) of our environment, and choose the right radiator suitable for heating it comfortably, making your stay pleasant and functional.

With the advent of the online market, it has never been so complicated to choose a radiator given the variety of products, shapes and sizes that we are offered: radiators, heated towel rails, towel warmers, stoves and fireplaces. It is necessary to be able to independently calculate the room’s thermal requirement and to relate it to the output in Watts of the chosen radiator.

Below, we will examine the simplest and most immediate way to best choose our new radiator, relying on a simple mathematical formula. This formula is able to give us the exact indication of the Watts that our environment needs to be properly heated.

The formula for the heat requirement expressed in Watts

The formula that follows is the one commonly used by the plant engineer, or (by the seller), who quickly have to advise us on the choice best suited to our needs.

In order not to make a mistake in purchasing, risking over-sizing or under-sizing our radiator, we will perform a calculation that is as simple as it is reliable.

To do this we need two parameters: the volume of the room expressed in cubic meters and the thermal coefficient of the area of residence. Multiplying the two factors we will obtain the heat demand expressed in Watts.

MC x CT = Watt

setting-furniture-radiator-design radiator-thermal-requirement

Let’s analyze the formula

CM – Cubic meters

Let’s proceed with the calculation of the first parameter “CM”. This is an expression of the volume of the room to be heated, called cubature, and deriving from the formula: ” L x L x H = MC “

For example, if our room has a wall Long M. 5,00 and the other M. 4,00 and a height, between floor and ceiling, of M. 3,00 we will calculate it with a simple multiplication:

Side 5,00 x Side 4,00 x Height 3,00 = 60,00 cubic meters

And here we have acquired our first value with this simple and intuitive formula, CM = 60,00. We will then go on to perform the same calculation in any environment where there is a need to install a radiator.

TC – Thermal coefficient

The second value, which we need to calculate the heat requirement, must be acquired: it represents the energy power needed to heat every cubic meter of our room.

The TC is expressed in Watts x CM, and on the Italian territory it is a number that varies between 27 and 42 (where 27 represents the minimum value, attributed to the hottest areas, while 42 represents the maximum value, attributed to very cold areas).

the CT is shown to us in a table, drawn up by the ENEA(Agency for New Technologies, Energy and the Environment of the Ministry of Economic Development) and shown below.

set-Florian-Black-thermal-requirement-deisgn radiator-watt

Distribution of climatic zones on the Italian territory


To trace the Thermal Coefficient, all we need to do is identify our area of belonging and here we will find our value here, highlighted in yellow.

For example, if we live in Ancona, our climatic zone is ZONE D and we assign it a thermal coefficient of 34.

Now, taking the formula as expressed at the beginning of the article TC x CM = Watt and, using the data collected so far, we make a practical example on how to develop our needs:

If we lived in Ancona and the volume of our room was 60.00 MC, then the formula would be:

TC 34 x CM 60 = 2.040,00 Watt

Here is the fundamental data for the choice of our radiator if we were residents in the surroundings of Ancona,2,040 Watts.

How to use the data acquired for the calculation of the heat requirement

Once we understand how to calculate the heat requirement necessary to heat our room, the need arises to relate this value to the parameters of the item we intend to purchase.

As an example, consider the purchase of a radiator for our bedroom. The calculation, previously made, showed us that to heat it properly we need about 2,040.00 watts.

With the help of the individual technical data sheet (associated with each product) we can choose the radiator with attention and knowledge of the cause which, by model and size, best suits our needs.

The technical drawing contains all the information and parameters necessary for an efficient and weighted choice of your radiator.

Technical sheet

Once all the necessary calculations have been made, we finally move on to choosing the radiator that best suits the customer’s needs. In this case, it is of fundamental importance to consult the product’s technical data sheet, which contains all the information and specifications that best characterize and describe the aforementioned article.

The last step is to identify the line in which the value of the thermal development in relation to the size of the product is reported and, finally, choose the radiator that best meets our needs.

PLEASE NOTE Some companies, in the technical data sheets report the thermal value of the radiator expressed in Kcal and not in Watt. Don’t worry, even in this case we can go back to the value in question, simply by converting the Watts into Kcal (or vice versa).

All this is possible with a simple division. Assuming that the ratio between Kcal and Watt is equal to 0.86 (1 kWh = 0.86 Kcal), our formula becomes:

48,00 MC x 34 CT = 1632,00 WATT

1632,00 WATT x 0,86 = 1.403,52 Kcal

Thanks to these two simple formulas, we can calculate our heat requirement in both Kcal and Watt!

Generally the product data sheets show the yields in Watts; see example below:



Buying a radiator is never a simple choice and it is very important to pay attention to all the parameters involved. Rely on the advice of an expert when you are not able to establish your thermal needs yourself, it will guide you in a thoughtful choice and in line with your thermal and furnishing needs.

If you have doubts about it or need more assistance in choosing and buying a heated towel rail, a heated towel rail or a radiator in general, do not hesitate to leave a comment at the end of the article!

The heat requirement is also essential for the installation of underfloor heating or pellet stoves, in order to perfectly calculate the quantity needed to heat the environment and to choose the type of product appropriately.

Go to the online portal CasaOmnia.it and discover our variety of radiators, suitable for any kind or need of furniture.


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