Temperature is a variable that intervenes in almost all natural processes. It is one of the most measured variables in fields ranging from scientific research to engineering and space science. This property of matter can be expressed in different units, among which are those of the Fahrenheit scale and the Kelvin scale. The first is the most widely used scale in engineering and in countries like the United States and England, where the imperial system of units is still used.
On the other hand, the absolute temperature in Kelvin is essential in many branches of physics and chemistry, especially with regard to calculations with ideal gases and thermodynamics. For this reason, knowing how to convert Fahrenheit (°F) to Kelvin (K) is a very useful skill, and this article teaches you how to do it in different ways.
But before learning how to convert these two important units of temperature, it is necessary to have an idea of what both scales are.
Fahrenheit scale
The Fahrenheit scale is a relative scale of temperature. This means that all temperatures expressed on this scale only indicate whether a temperature is lower or higher than a reference temperature, but does not actually indicate the temperature of a particular point.
Like any relative temperature scale, the Fahrenheit scale was established using two reference points: a low temperature that would represent the “zero” of the scale, and a second temperature that defines the size of each degree or unit.
In relation to the 0 of the scale, many researchers seem to agree that Fahrenheit decided to establish as the zero of its scale the lowest temperature that it could register, in order to avoid the existence of negative temperatures. This corresponded to a mixture of water, ice and ammonium chloride. This mixture has the particularity that it reaches a quite stable and quite low equilibrium temperature, which is why Fahrenheit assigned it to zero on its scale at this point.
The next point of reference, according to some authors, was the freezing point of water, to which he assigned a temperature of 32 °F in order to divide the scale between 0 and 32 into 4 groups of 8 units each, something that is very common in the imperial unit system. Using this scale, the normal boiling temperature of water occurs at 212°F.
The absolute temperature scale and the Kelvin (K)
Unlike the Fahrenheit scale, the Kelvin scale is an absolute temperature scale , which does not depend on any reference point. Zero on the Kelvin scale actually indicates the absence of thermal energy in a body. It was created by William Thompson (also known as Lord Kelvin, hence the name) in 1848, who assigned each unit on his scale the same size as the degree on the Celsius scale. This makes converting from Kelvin to Celsius much easier, but makes converting from Fahrenheit to Kelvin a little less easy.
It should be noted that the temperature unit on the Kelvin scale is called Kelvin and its symbol is K ; It is not called a Kelvin degree nor is its symbol °K. This is precisely because it is an absolute and not a relative scale. For example, 273 K is read two hundred and seventy-three Kelvin and not two hundred and seventy-three “degrees” Kelvin.
Formula to convert degrees Fahrenheit to Kelvin
The transformation from Fahrenheit to Kelvin is usually done indirectly, first transforming to Celsius and then to Kelvin. However, here we present a faster and more direct way that allows you to skip a step.
Using the reference temperatures shown above and knowing that the freezing point of water is at a temperature of 273.15 K (corresponding to 32 °F) and that the boiling point is at 373.15 K (corresponding to 212 °F), it can be deduced that each Kelvin unit is equal to 1.8 or 9/5 of a degree Fahrenheit. These observations are summarized in the following formula for converting Fahrenheit to Kelvin:
This formula is very simple, but it can be further simplified by dividing 9 by 5 to get a decimal number in the denominator. In that case, the equation remains:
In both formulas, °F represents the temperature in Fahrenheit (the one we want to convert) and K represents the temperature on the Kelvin scale (the one we want to determine).
So converting °F to K is a simple two-step process:
Step 1: Add the temperature in Fahrenheit with 459.67
Step 2: The result of the sum is divided by 1.8
Fahrenheit to Kelvin conversion examples
Example 1:
You want to transform the average body temperature, which is 98.6 °F , to Kelvin.
Step 1: Substitute 98.6 where °F appears in the formula.
Step 2: the addition is made in the numerator, then the result is divided by 1.8 and finally the unit (K) is placed.
Finally, 98.6°F equals 310.15 K.
Example 2:
Now, suppose we want to convert 0 Fahrenheit to K. As before, we start by plugging this temperature into the formula:
And the two operations are carried out:
Finally, 0 °F equals 255.37 K.
Reverse conversion, from Kelvin to Fahrenheit
The reverse process is equally simple. To get the formula to transform degrees Fahrenheit to Kelvin, all you need to do is clear or solve the equation for °F. The result is:
You can also avoid using the fraction in a similar way as before. The result is:
expert tip
When using these formulas, it is not advisable to include units when replacing the values, as it can create confusion with the two temperatures. They should only be placed after the conversion is complete.
References
Brown, T. (2021). Chemistry: The Central Science (11th ed.). London, England: Pearson Education.
The mathematical treatment of the measurement results. (n.d.). Retrieved from https://espanol.libretexts.org/@go/page/1798
The measures. (2020, October 30). Retrieved from https://espanol.libretexts.org/@go/page/1796
The Editors of Encyclopaedia Britannica. (2018, October 25). Fahrenheit temperature scale | Definition, Formula, & Facts. Retrieved from https://www.britannica.com/science/Fahrenheit-temperature-scale
