When is comes to temperature conversion, it is easy to convert Celsius to Kelvin, however is you want to convert Celsius to Fahrenheit, it becomes more tricky... Unless you use our temperature conversion tool! With our tool you do temperature unit conversion, for example, from Fahrenheit to Celsius or between virtually any other two temperature units in a moment. There is no need for you to remember any temperature conversion formula, or perform any mathematical operation; just introduce the temperature in your preferred units and watch the result appear. And if you want to use the time we saved you to learn more about temperature scales, feel free to read below!

What is temperature

It is always hard to come up with a good definition for everyday terms, but with temperature, it is notoriously difficult. We all know what hot is or what cold is, but temperature? Temperature is much harder to define without getting technical, which is why we will get technical in just a moment. To do this we have to turn to physics, in particular to thermodynamics and statistical physics, which is like thermodynamics meets quantum physics.

But there is no need to worry, because when we dig deeper into what is temperature, the answer is as simple as confusing: temperature is speed, or rather, momentum of the atoms and molecules that make up the material. What this means is that the higher the temperature of something, the higher the particles velocity faster the molecules that make up that something vibrate. This is very closely related to the concept of thermal energy and means that heat is just another expression of kinetic energy.

Image of a thermometer

As always, there's much more to these concepts than what we've mentioned here, like for example the fact that speed is related to the kinetic energy by means of a square root. Or the fact that when we mention speed we should technically speak of the average of all the speeds of all the particles... But this would, then, become a large essay on the physical concept of temperature and there would not be space for a conversion tool.

We have space and time, however, to point out some interesting facts, like how much more obvious it is that any force of friction will end up creating heat (increasing the temperature). And it is also interesting to note that once we think of temperature as a form of energy it becomes fairly clear that our body burns more calories when we try to burn calories by running vigorously, which incidentally creates much more heat inside out bodies than light exercising.

Thinking about temperature and heat in terms of energy is very useful and helps us understand much better many of the things that happen around us. Nevertheless, that is not the only thing we will talk about. Since this is a temperature conversion tool first, it is only fair that we will talk about units of measuring temperature. We also need to acknowledge the different ways of measuring temperature. We're not talking only about different types of thermometers, but also about different scales.

We will talk about different ways of measuring temperature both in terms of temperature scales, from Celsius to Fahrenheit to Kelvin and back to Celsius; and also in terms of what properties we can use to accurately make measurements in any scale we want.

Temperature and relation with other magnitudes

Everything is connected is something we hear a lot, but we barely ever consider to what extent that is true. In physics it is an unavoidable truth, one thing affects the other; luckily for us we know almost always how one thing affects another and we can predict those changes and use them to our advantage. We have already seen how a change in temperature involves a change in momentum by the particles, but there are many more connections than just speed-temperature.

Gas particles moving as consequence of non-zero temperature

Just by simply looking at the weather you can already have an intuition that temperature and pressure are closely related. And if you're into your cycling numbers you probably also know that temperatures affect air air density and, hence, drag at a given speed. This is due to the thermal expansion of the gases in the air, with the corresponding increase in volume which translates to a drop in density, since the weight remains constant.

This whole relationship between pressure, temperature, volume (and density for that matter) is best explain by the ideal gas law equation, at least in the perfect case. We won't go into details but by using such calculator together with the air density and drag calculators you could easily see how a change in temperature can significantly change the results of an experiment like the free fall with air resistance. This is easy to correct for, but if you don't do, you might end up with strange values for the gravitational force of the Earth. In this case, the problem would be due to the difference in buoyancy force due to different density of the air.

All these relations between temperature and volume, or pressure, or speed... Depend on the properties of the material you are dealing with and it generally parametrized by physicist by quantities such as specific heat, latent heat, thermal conductivity... And then, they are integrated into the theories by means of rules and laws such as Newton's law of cooling, Joule heating...

Pressure as a result of tempreature

Before we can dive any deeper into the consequences or practical applications of this connections, we probably should talk about this temperature conversion calculator, how temperature is measured, and why do we even need different units to measure temperature. Because, spoiler alert, having measurements in Celsius, and temperature in Fahrenheit or using Lord Kelvin's temperature scale is very useful, even if that means we need to convert temperature from Fahrenheit to Celsius to even understand each other.

Temperature Scales and conversions

When it comes to measuring temperature, you could say that it is measured in degrees and it's just that different people use different degrees. But that just delays the problem without providing a real answer, and it also makes scientist very angry since the preferred nomenclature is without the "degree" part of it. Any which way, there are many different units to measure temperature in, the most common ranging from Kelvin to Celsius to Fahrenheit, depending on the country or the type of conversation.

However, there are many more units of temperature, some arguably as useless as unknown. In this temperature unit conversion calculator we have them all and we will let you play with weird temperature conversions, and even show you how to make your own temperature conversion table for unknown units, if you're into that sort of stuff. For now, let's just mention some of them.

Fahrenheit Scale compares to Celsius scale

Among the weirdest and obscure we can think of are units of temperature such as Rankine, Delisle, Newton, Réaumur, Rømer... If you're an avid reader and follower of science you've probably noticed that they are all names of scientists. In fact, from (Lord) Kelvin to Fahrenheit to Celsius all the units of temperature are names of those who (allegedly) created such temperature scales.

The only exception would be the centigrade scale, which we would mention briefly later, but it's practically the same as the Celsius. The reason there are so many units is that temperature is hard to measure and there is not an obvious way to establish proper scale. In fact, for thousands of years nobody even knew if there was even an upper or lower limit to temperatures, so it was very hard to establish points of reference.

We will see in more detail some of the most important temperature scales, and the temperature conversion between this commonly used scales. We will also see how natural phenomenons such as freezing point, dew point or boiling points were crucial in the development of a widespread temperature scale.

Temperature conversion using the calculator

Before we dive deeper into the temperature scales themselves we should probably take a look at how to use the tool that allows us to convert from Fahrenheit to Celsius or basically from any temperature units into any other. When faced with such a problem, your options are always twofold: either you DIY it the help of Google, or you use our easy calculator and save time. To those who chose the first option: -We're sorry to see you go, please come back another day-. To the rest: let's see how to use this calculator, or any other converstion calculators from Omni, such as the length conversion, or the Unix time converter.

In the particular case of the temperature conversion calculator, we provide all available units at a glance. By default, the order in which they appear is:

  • Celsius
  • Fahrenheit
  • Kelvin
  • Rankine
  • Delisle
  • Newton
  • Réaumur
  • Rømer

However, you can change it to whatever order better suits your needs. It is very easy to change the units of a particular field, and it only takes a couple clicks. Let's say that you want to change the first text box from Celsius to Fahrenheit, the second one from Fahrenheit to Kelvin and the third one from Kelvin to Celsius, all you need to do is:

  1. Click over ºC, use the drop-down menu to change units from Celsius to Fahrenheit by selecting Fahrenheit (ºF).
  2. Click over ºF on the second text box, and select Kelvin (K). You now have changed the units from Fahrenheit to Kelvin on the second field.
  3. Click over K, select Celsius (ºC) from the drop-down menu.
  4. Contemplate your masterpiece, and maybe use it to convert temperature units.

Once you have all the units in your desired position, just proceed by entering the value of temperature in your reference units and you will see it appear in all other units automatically. Just like that, no need to even know the temperature conversion formula. Don't you love technology, progress and the magic of the Internet?

Celsius temperature scale

The Celsius temperature scale is probably the most widely use of the 'non-scientific' units of temperature. It's origin dates to the 18th century when Anders Celsius invented it. The principles behind it are as simple and easy to create as things can get. The first assumption that Anders made was that ice water melts always at the same temperature, and it also boils at the same temperature. So with a very well constructed thermometer (probably just a blank glass tube filled with alcohol), he put a mark for each of these points. Only he put 0 and 100

He then created 100 divisions between these two marks and called each of them a "degree". This is the reason why this scale is also called the centigrade scale: because it has a hundred (centi) degrees (grade). Since it's creation, this unit has been pretty much the de facto standard in most countries (not the USA) and was even the SI unit of temperature for many years, until Lord Kelvin came and created a more scientifically founded scale.

The key achievement of this scale is to asign easy to manipulate numbers for normal temperatures that we find on a daily basis. Numbers between 0 and 100 are easy numbers to imagine, easy to manipulate and understand. It is rare that on a normal day we would encounter temperatures that would be higher than 100ºC and even more rare to find anything lower than -100ºC.

However, there is a problem with the Celsius scale: it's not universal. The biggest flaw of this temperature scale is the fact that water changes its boiling point at altitude, or rather with pressure changes. It also changes it's melting point. So setting a standardized 0ºC point or 100ºC point becomes really tricky.

This doesn't bother scientists too much, though. They would just simply measure all the other variables, correct for them and be done with it. However, what concerns scientists more is the lack of a universal motif behind the decisions. Why chose water and not oil? What oil? What if I would be on a different planet with different conditions, could I reproduce the scale? And hence the Kelvin scale of temperatures was born, but that's a story for another section.

Fahrenheit temperature scale

Much in the fashion of the Celsius scale, the Fahrenheit scale was created by a man of science that wanted to make the best temperature scale with the best temperature units in the universe; and he also failed. Daniel Gabriel Fahrenheit was a contemporary Dutch of Anders Celsius, that thought he could do better than the centigrade scale. The whole story of how he ended up creating the current Fahrenheit units is much better explained in the video, so we would recommend you to watch it.

The Fahrenheit is as acceptable as any other scale but because of how it was conceived seems a bit less intuitive to use for those not used to it. Temperature unit conversion from F to C it's also more complicated than it is to convert Celsius to Kelvin. The temperature conversion formulas for going from Fahrenheit to Celsius and from Celsius to Fahrenheit are not very complicated. Let's see now how we can take temperature in Fahrenheit and turn that into a temperature expressed in the metric unit of temperature, the Celsius.

For those of you that want to convert Celsius to Fahrenheit, here is how. Take the metric unit of temperature and apply the following formula: (C * 9/5) + 32 = F where we use C for degrees Celsius and F for degrees Fahrenheit. If you wish to do a temperature unit conversion from F to C you need to reverse the formula. The result should look something like: (F-32) * 5/9 = C. And this way you can do the temperature unit conversion F to C as you desired.

Kelvin temperature scale

And now onto the Temperature scale attributed to Lord Kelvin. The Kelvin scale of temperature is currently the most widely used in science, and for good reason. It has everything a truly scientific measuring unit should have: it's based on universal principles, it's independent of "outside" factors, and, as an extra bonus, it's defined in a way that makes it easy to convert Celsius to Kelvin and vice versa. The size of a Kelvin was set to be the same as the Celsius degree, for convenience. This means that the difference between the SI unit of temperature (Kelvin) and the metric unit of temperature is none, and the scale differ only in the offset.

The universality of the Kelvin comes from the fact that 0K is exactly the coldest temperature physically possible, the absolute zero. It's not easy to understand how can one be sure of a temperature being the coldest ever possible, period; more so if we realize that it is also not possible to reach in reality. One of the tricks used to arrive at this conclusion with scientific certainty is in the relationship between pressure, temperature, and volume.

Extrapolation to find absolute zero (0 Kelvin) temperature

If you were to plot how either pressure or volume relate to temperature, you would find something fairly obvious: pressure decreases with temperature, and so does volume; at least in an isolated system in which all other variables are kept constant. Furthermore, you could do some analysis on this relation by means of a chi square, or linear interpolation to obtain the trend line. If you then extrapolate until you can find where it intercepts the axis (just like you can do with our slope intercept calculator), you will find that the temperature at which the volume and pressure would be 0 will be the same for most material: 0K = -273.15ºC = 459.67ºF.

This value is what we call absolute zero because it would be impossible to go below it, since it would mean reaching negative pressures or negative volumes, which are physically absurd. This is by no means the only way to find the value of absolute zero, and more modern experiments of different nature have backed up this result through other paths. It is very surprising to think that while there is no upper limit to temperatures in the universe, the coldest temperature is something very clearly defined and independent of the system.

In terms of defining a temperature scale, however, we still need to get more data, since we have only defined one point of reference and we need at least two. The second point of reference was chosen by scientists to be the triple point of water. One reason behind these decisions is that water is a fairly simple, widely available liquid, that also happens to be a requirement for life as we know it. The triple point of water is a state in which water coexists in solid, liquid and gaseous forms at the same time, and it is only possible to achieve at a certain pressure, and, crucially for us, temperature.

Color-temperature correspondence in Kelvin units

Armed with two points of reference and our desired size of the unit (same as degree Celsius), we can start setting values for common events. Under this premises, water melts at 273.15K, and boils at 373.15K; with the normal human body temperature sit around 306K. This scale has the problem of dealing with asigning fairly big number for normal temperatures in pursuit of a no-negative-numbers scale, which is the reason it has not become the standard unit of measurement for everyone everywhere. It is, however, the si unit of temperature, and by far the most used of all temperature units.

Just a final remark before we move into other topics: we have been very careful to precisely say there are no "colder" temperatures than 0k but never say "there are no negative absolute temperatures" since that would've been false. For those, the moment all you need to know is that negative absolute temperatures do exist, they are hotter than any other positive temperature possible and only occur in very special systems with certain properties.

temperature sensors, how to measure temperature

A very interesting question many of you might have is "how can you actually measure temperature?" or conversely "how do temperature sensors work?" and the key to answering that is in the first section: through the effects on other variables like pressure, volume, electrical resistance of a wire... Since temperature is really average velocity of molecules and atoms, measuring it directly is practically impossible, but we can very easily measure the effects of temperature in other properties of a material.

One of the most common ways to measure it, or at least one of the oldest is through thermal expansion. Since most materials will enlarge as temperature rises, one can carefully design a container or a table of calibrated markings that can translate from size (typically volume of length) into temperature. This is how mercury and alcohol thermometers work. this method has some limitations like the temperature range, and the need to be in contact with the heat source to determine its temperature and, most notably in our day and age, it is really hard to get any kind of electrical or digital reading our of it.

infrarred thermometer gun

This problem can be solved by using other different properties of materials, which are also affected by temperature. If we are really interested in obtaining a digital/electrical output from out measuring device the intuitive thing to do is take a look at electrical properties that are dependent on temperature. Fortunately enough, most of the electrical properties and effects have a strong dependence on temperature either directly or indirectly.

Just to name a few of the most widely used effects there is the hall effect, the voltage drop within a material (connected to electrical resistance via ohm's law), or Seebeck effect (which is how thermocouple temperature sensors work). These are effects that can be track down to the mobility of electrons in a material, or conversely their drift velocity, which is a fundamental property of a material and depends strongly and directly on temperature being the vibration of atoms and molecules.

thermocouple diagram
diagram of a thermocouple

Another way to measure temperature using its effects on other phenomena is how starts outer temperature is measured: using the Stefan-Boltzmann law and Wien's law. These law relates the energy of a photon (and corresponding de Broglie's wavelength, that is the color of light, emitted by an object an its temperature. And is why the color of light bulbs is often measured using the kelvin temperature scale. This technique is how scientists measure the temperature of a black hole and other celestial objects, provided that there is not redshift effect occurring on the way, or provided that we correct for them. This last method is very closely related to how infra-red gun-shaped thermometers work, too.

Certainly there are other thermodynamic processes such as Carnot efficiency that could be used for this purpose. We could even use the Doppler effect and the speed of sound; however, it is not a very efficient or effective way to do it, which is why generally only electrical or quantum effects are used to measure normal temperatures that we encounter on a regular day.

black body spectrum

In any case, the temperature is a very relevant quantity for us and we have pretty much mastered the 'art' of measuring it. Even if there is still a long way to go before all the world agrees on which is the best unit to use, at least for our everyday life.

Piotr Małek and Álvaro Díez, PhD student

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Temperature Conversion | Celsius, Fahrenheit, Kelvin and more