Temperature Conversions Project

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Temperature Conversions Guide

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What Is Temperature?

Temperature is a measurement of how hot or cold something is that is of the thermal energy that it contains. It enables the amount of heat to be assessed quantitatively and objectively. Temperature is usually measured using a thermometer, but these thermometers can be calibrated to different scales. The three most commonly used scales of temperature are the Fahrenheit, Celsius and Kelvin scales.

Scales of Temperature

The three main scales of temperature are all defined in different ways. Each has been based around two points. The first is a starting point which determines the temperature that will be considered zero on the scale. The second point will be somewhere above the zero point. The number that is linked with the upper point and the number of degrees that will separate it from zero will determine the size of an individual unit on the scale.

The Fahrenheit Scale

The Fahrenheit scale of temperature was first created in 1724 by a German physicist named Daniel Gabriel Fahrenheit. Fahrenheit created the scale around two measurements. The first, to determine the zero point for the scale, was the temperature measurement that Fahrenheit found when he placed the thermometer in a brine solution. The second measurement, to determine the 100 degrees point for the scale, was obtained by placing the thermometer either in the mouth or under his wife’s armpit. One hundred degrees on the original Fahrenheit scale was, therefore, equivalent to the temperature of the human body.

The Fahrenheit scale was later adjusted into its current form. This modification occurred because it was realized that the melting and boiling points of water were almost exactly 180 degrees apart on the original version of the scale. The scale was, therefore, adjusted in order to make this difference exact. On the current Fahrenheit temperature scale, water freezes at 32 degrees Fahrenheit and boils at 212 degrees Fahrenheit. Making the difference between the melting and boiling points exact made the Fahrenheit scale more useful for scientists. However, the adjustment also meant that the human body temperature was no longer exactly 100 degrees. Instead, the normal human body temperature on the current version of the Fahrenheit scale is 98.6 degrees Fahrenheit.

Fahrenheit had actually based his own original scale on the work of another scientist, Ole Rømer, who had created a temperature scale based on the same zero point as Fahrenheit’s scale, the temperature of brine, and with the boiling point of water being set at 60 degrees. The freezing point of water on the Rømer scale was at 7.5 degrees. Fahrenheit wanted to create a more useful temperature scale in which there would be greater separation between the melting and boiling points of water and so that the temperatures would not be fractions. He did this by multiplying the Rømer scale by 4. He then recalibrated the scale using the brine and human body temperature measurements discussed above in order to create the first version of the Fahrenheit scale.

The Fahrenheit scale is still widely used in some countries. It is the most common scale for everyday use in countries such as the United States, but it is also uses as a secondary scale, alongside Celsius, in the United Kingdom.

Important temperatures on the Fahrenheit scale include absolute zero at -460 degrees Fahrenheit, the freezing point of water at 32 degrees Fahrenheit, human body temperature at 98.6 degrees Fahrenheit and the boiling point of water at 212 degrees Fahrenheit.

On the Fahrenheit temperature scale, the melting and boiling points for water are 180 degrees apart. This means that a single degree on the Fahrenheit scale is 1/180th of the temperature difference between these two points. In comparison, there are 100 degrees Celsius between the melting and boiling points of water, making one degree Celsius 1/100th of this interval. The ratio between the degree units used on the Fahrenheit and Celsius scales is, therefore, 100:180, which is 0.55 or 5/9. This difference in the size of the units on either scale is important for converting temperatures between the two scales.

The Celsius Scale

The Celsius scale of temperature, in a form like we know today, was created independently by a number of different scientists in the 18th century. The famous botanist Carolus Linnaeus was among those who created the Celsius temperature scale. The scale is named after the inventor of the temperature scale from which these 18th century scientists adapted their scale for temperature measurements. The problem with the original scale created by the astronomer Anders Celsius, from Sweden, was that it moved in the opposite direction to other scales, such as the Fahrenheit scale. On Celsius’ original scale, a higher number of degrees referred to a colder temperature. He set his scale with 100 degrees being the freezing point of water and 0 degrees being the boiling point of water. A temperature scale moving in this direction is just as valid as one moving in the direction that we have come to consider normal, but it does not seem as intuitive since we tend to associate heat with rising, increasing, positive words. We expect the numbers to get larger as we get hotter. What Linnaeus and other scientists of the time did was simply to turn the scale around, so the 0 degrees Celsius was the freezing point of water and 100 degrees Celsius was the boiling point of water.

The name Celsius was attached to the scale in recognition of the scientist’s work in determining the effect of atmospheric pressure on temperature. Celsius had shown through experimentation that while atmospheric pressure had little effect on the melting point of water, it did have a significant effect on its boiling point. This is why mountaineers who are high on a mountain, where the atmospheric pressure is lower, find that water boils at a lower temperature than it does when they are at sea level. The boiling point of a liquid is the point at which it turns to steam and when the atmospheric pressure is lower it is easier for the liquid to evaporate so it does not take as much heat to boil it. Celsius therefore suggested that it was important to consider pressure when working with temperature scales and that the temperature scale should be based on conditions of mean barometric pressure at sea level. This has come to be known as standard pressure or one standard atmosphere.

Although this temperature scale is today known after the Swedish astronomer who played a role in its development, for over two centuries the scale was actually known as the Centigrade temperature scale. The term Centigrade is still sometimes used to refer to this scale today, but use of the older term has been discouraged since the 1960s. This is because the term Centigrade is also used in Spanish and French to refer to the degrees used to measure angles. Using the term for both angles and temperature could be confusing, so the name Celsius was chosen instead in order to keep the two scales of measurement clear and separate.

The original scale created by Celsius and the adapted version with its reversed direction were both based around the zero point of the freezing point of water and the 100 degree point of the boiling point of water. The scale has more recently been redefined for scientific use around two different temperatures, absolute zero, which is at -273.15 degrees Celsius and the triple point of a special form of water know as VSMOW, which is at 0.01 degrees Celsius. The triple point is the point at which all three phases of a substance (solid, liquid and gas) coexist in a thermodynamic equilibrium. This redefinition does not significantly affect the points on the scale at which water melts and boils, which remain at 0 and 100 degrees Celsius. It just means that scientists define the Celsius scale in a different way. As science develops the ability to measure temperature more precisely it has become clear that the original definitions of the melting and boiling points did not match precisely with the original 0 and 100 degree points, but the difference is so slight that it does not matter in everyday life.

Important temperatures on the Celsius scale include absolute zero at -273 degrees Celsius, the freezing point of water at 0 degrees Celsius, human body temperature at 37 degrees Celsius and the boiling point of water at 100 degrees Celsius.

On the Celsius temperature scale, the melting and boiling points for water are 100 degrees apart. This means that a single degree on the Celsius scale is 1/100th of the temperature difference between these two points. In comparison, there are 180 degrees Fahrenheit between the melting and boiling points of water, making one degree Fahrenheit 1/180th of this interval. The ratio between the degree units used on the Celsius and Fahrenheit scales is, therefore, 180:100, which is 1.8 or 9/5. This difference in the size of the units on either scale is important for converting temperatures between the two scales.

The Kelvin Scale

The Kelvin scale of temperature is the most recent and it is also the scale that is most commonly used in science, particularly in the physical sciences. The Celsius scale is still used a lot in many areas of science, but it is the Kelvin that is the main SI unit for temperature. Measurements may be referred to as Kelvin or using the symbol K.

This scale was named after the physicist William Thomson, who was the 1st Baron Kelvin. His title derives from the name of a river near the university of Glasgow, where he worked. It was Lord Kelvin who first wrote of the need for an absolute scale of temperature. He pursued some important work in thermodynamics and he recognized the fact that there must be a lower limit beyond which the temperature could not drop. Thus, he created the idea of absolute zero. The Kelvin scale was named in his honor.

The Kelvin scale is an absolute thermodynamic scale of the type that Kelvin had stressed was needed in science. This type of scale is independent of the physical properties of any substance. A single unit of temperature on this scale would produce the same mechanical effect whatever the number of the temperature. When a unit of heat passes from one substance, which has a temperature of T, to another cooler substance with a temperature of T-1 that unit of heat will produce the same effect, whatever temperature is represented by T. The point at which no more heat can be transferred from a substance is absolute zero.

Lord Kelvin first wrote of the need for a scale based on the starting point of absolute zero in 1848, at which point he also calculated that absolute zero would be equivalent to about -273 degrees Celsius. The second defining point for the scale, the triple point of water, was set in 1954.

The Kelvin scale is based around the two points of absolute zero and the triple point of water. Absolute zero is the starting point of the Kelvin scale, being set at 0 Kelvin. This means that there are no negative numbers on the Kelvin scale since it is not possible to have a temperature below absolute zero, which is the point at which molecules would have no thermal energy or motion. The triple point of water is set at 273.16 K, with one kelvin being equal to 1/263.16 of the difference between these two points.

The new definition for the Celsius scale is based around the same two points as the Kelvin scale, which ensures that a single Kelvin is the same size as a single degree Celsius.

The Kelvin scale is based around the idea of absolute zero. This is the temperature at which any molecule would have no kinetic energy. It would not be possible to have a temperature that was lower than this. Absolute zero is selected as the zero point on the Kelvin scale, so 0 Kelvin is absolute zero. There are no minus numbers on the Kelvin scale since it is not possible to have a temperature that is less than absolute zero.

Important temperatures on the Kelvin scale include absolute zero at 0 Kelvin, the freezing point of water at 273 Kelvin, human body temperature at 310 Kelvin and the boiling point of water at 373 Kelvin.

The Kelvin scale uses units that are the same size as the degrees on the Celsius scale. In the same way that there are 100 degrees Celsius between the melting and boiling points of water, there are 100 Kelvin between these two points. One Kelvin is the same size as one degree Celsius, and both are equal to 1/100th of the interval between the melting and boiling points of water.

Using Temperature Scales and Measurements

It is important to measure temperature in a number of different settings. Many people use temperature measurements in their everyday lives. In most of the world, people use the Celsius scale for everyday temperature measurements, although in some countries such as the United States, the Fahrenheit scale is still in common use.

Outside of science and medicine, the most common uses of temperature measurements are for weather reports and for cooking. Most people are most familiar with one sort of temperature scale, so when they encounter a measurement on another scale they will need to convert it into a more familiar form in order to understand its significance. It is also important to understand how to convert between different scales of temperature when cooking if the temperature listed in a recipe is on a different scale to that shown on the oven.


Scientific Measurements

Accuracy is essential when temperature is being measured for scientific or medical reasons. The main scale that is used internationally for temperature measurement is the Kelvin, although the Celsius scale is also used by scientists. These two scales are closely related, so it is relatively easy for scientists to use them alongside one another. The size of a single unit is the same on both scales, so a temperature difference of one Kelvin will be the same size as a difference of one degree Celsius. However, the two scales have different starting points, with 0 degrees Celsius being equivalent to -273.15 Kelvin.


Converting Between Temperature Scales

In order to convert between two different scales of temperature, it is necessary to account for both the differences in the starting points of the scales and any differences in the relative sizes of the units.

Each of the three temperature scales starts at a different point, so zero means a different thing on each scale. A temperature of 0 Kelvin is colder than a temperature of 0 degrees Celsius, which is colder than a temperature of 0 degrees Fahrenheit. The starting point of the Kelvin scale is absolute zero. The starting point of the Celsius scale is the freezing point of water. The starting point of the Fahrenheit scale is the temperature of brine.

The Kelvin and Celsius scales use units of the same size, but the Fahrenheit scale is based on smaller degree units. A temperature difference of one degree is, therefore, greater when it is a difference on the Celsius scale rather than on the Fahrenheit scale.

The differences between the different temperature scales stem from the ways in which the scales have been defined. They determine how a temperature measurement on one scale can be converted into a measurement on another scale.


Converting between Fahrenheit and Celsius

In order to convert a temperature from Fahrenheit into Celsius it is necessary to account for both the difference in the size of the degrees used in each scale and for the different starting points that these two scales use.

In order to convert a temperature from Fahrenheit into Celsius it is necessary to subtract 32 and then to multiply by 5/9.

In order to convert a temperature from Celsius into Fahrenheit, the opposite calculation must be performed. It is necessary to multiply by 9/5 and to add 32.

Multiplying by 5/9 or 9/5 accounts for the difference in the size of the degrees on the two scales, while adding or subtracting 32 accounts for the different starting points of the scales.

Converting between Celsius and Kelvin

The units that are used on the Celsius and Kelvin scales are the same size, so it is only necessary to account for the different starting points of these scales when you are converting between them. This makes the conversion much easier. It also means that when discussing temperature differences rather than actual temperatures, a difference in degrees Celsius will have the same meaning as a difference in Kelvin.

The Kelvin scale starts at a colder temperature than the Celsius scale. Zero degrees Celsius is equivalent to 273.15 Kelvin. Zero Kelvin is equivalent to -273.15 degrees Celsius.

In order to convert between Celsius and Kelvin it is, therefore, necessary to add or subtract 273.15, depending on the direction of the conversion.

To convert from Celsius to Kelvin, you subtract 273.15. To convert from Kelvin to Celsius, you add 273.15.

Converting between Fahrenheit and Kelvin

The conversion between the Fahrenheit and Kelvin scales, like the conversion between Fahrenheit and Celsius, must account for both the differences in the starting points of the scales and for the differences in the sizes of the individual units that are used on these scales.

In order to convert a temperature from Fahrenheit into Kelvin, it is necessary to add 459.67 and multiply by 5/9.

In order to perform the reverse conversion, from Kelvin into Fahrenheit, it is necessary to multiply by 9/5 and to subtract 459.67.

Example Conversions

(C=temperature in degrees Celsius, F=temperature in degrees Fahrenheit, K=temperature in Kelvin)


To convert 100 degrees Fahrenheit to Celsius:

C = (F -32) x 5/9

C= (100-32) x 5/9

C= 37.78


To convert 100 degrees Fahrenheit to Kelvin:

K = (F + 459.67) x 5/9

K = (100 + 459.67) x 5/9

K = 310.93

To convert 40 degrees Celsius to Fahrenheit:

F = C x 9/5 + 32

F = 40 x 9/5 + 32

F = 104


To convert 40 degrees Celsius to Kelvin:

K = C + 273.15

K = 40 + 273.15

K = 313.15


To convert 150 Kelvin to Celsius:

C = K – 273.15

C = 150 – 273.15

C = -123.15

To convert 150 Kelvin to Fahrenheit:

F = K x 9/5 – 459.67

F = 150 x 9/5 – 459.67

F = -189.67


Estimating the Conversion

It is possible to estimate the conversion of a temperature from one scale to another. This can be a quicker option than calculating the conversion exactly so it can be a good way to quickly work out the correct temperature when you are busy, for example if you are in the middle of cooking a meal.

As a general rule, a temperature in Fahrenheit is about twice the temperature in Celsius. If you are rough-estimating temperature conversion from Fahrenheit to Celsius, all you need to do is halve it. If you are estimating a temperature conversion from Celsius to Fahrenheit, then all you need to do is double it.

For example, if you are following a recipe and it asks you to turn your oven on to 350 degrees Fahrenheit, but your oven uses Celsius temperature measurements, then you would simply halve the measurement to 175 degrees Celsius.

Similarly, if you were watching a weather report on the television and it said that the temperature was expected to be 25 degrees Celsius, but you are more familiar with the Fahrenheit scale of temperature, then you may want to convert it by doubling it to 50 degrees Fahrenheit.

Converting temperatures measurements in this way can produce useful estimations, but if you need an accurate temperature measurement then you will need to perform an exact calculation.