Measurements of temperature are often used in everyday life, most commonly in reference to the weather or when following a recipe. However, temperature can also be important in other fields, such as science and medicine. If you need to use measurements of temperature for any reason, then you are likely to need to convert from one form of measurement into another at some point. For example, you may be cooking using an oven, which is set using one form of measurement, but with a recipe that provides temperatures in another form. Alternatively, you may be watching a weather report that uses a form of measurement with which you are not familiar, so if you want to know how warm it will be you will need to convert the temperatures into a form that you understand.

The most common scales between which you may need to convert temperatures are Fahrenheit and Celsius. The names Fahrenheit and Celsius are used to refer both to the relevant scales of measurement and to the individual units that are used on this scale. Thus, it is possible to both to the Fahrenheit scale and to a measurement, such as 1 degree Fahrenheit.

**Fahrenheit**

The Fahrenheit scale of temperature was created in 1724 by the physicist after whom it was named. This scale has been largely replaced by the Celsius scale in most of the world, but it is still used in some countries, such as Belize and the United States, and it is sometimes used alongside the Celsius scale, as in the United Kingdom.

The Fahrenheit scale was based upon experimentation. Fahrenheit determined the zero point for his scale by placing a thermometer in a mixture of brine. He determined the 100 degrees point using the level that the liquid in the thermometer reached when it was placed inside the mouth (i.e. the temperature of the human body). The scale was later refined by other scientists, who had noticed that the points at which water froze and boiled on this scale were almost 180 degrees apart. They adjusted to scale to make this difference exactly 180 degrees. This shifted the body temperature of a human from 100 degrees on the original scale to 98.6 degrees on the refined scale.

The Fahrenheit scale became the standard form for measurements of temperature in many countries, particularly in the English-speaking world, until the 1960s. It is still used in some countries such as the US, Liberia and Jamaica as the most common scale for temperature. However, it is less commonly used by scientists, even in those countries that continue to rely upon Fahrenheit for everyday use.

On the Fahrenheit scale, water freezes at 32 degrees and boils at 212 degrees. This means that the freezing and boiling points are separated by exactly 180 degrees.

**Celsius**

The Celsius scale is named after a Swedish astronomer who created a very similar scale of temperature in 1742. On Celsius’ original scale, the boiling point of water was set as 0 degrees, with the freezing point being 100 degrees, and the scale working in reverse to the modern one. A higher value indicated a colder temperature.

The scale was reversed by Linnaeus in 1744, although other scientists are also believed to have independently decided to reverse the scale. Dealing with temperature values that increase as the temperature rises comes more naturally to the human mind. On these newer versions of the Celsius scale, 0 degrees is the freezing point of water, while 100 degrees is the temperature at which water boils.

The name centigrade was used for this temperature scale for more than two centuries. However, in some languages, particularly in French and Spanish, the word centigrade was already used in the measurement of angles, so its use as a measurement of temperature could become confusing. The term centigrade is still sometimes used in reference to temperature, but since 1948 the scientific community has adopted the use of Celsius in order to prevent confusion.

The Celsius scale of temperature is now the most commonly used scale in the world, even though some countries have been reluctant to adopt it.

The Celsius scale was originally defined by the freezing and boiling points of water, at 0 and 100 degrees Celsius, but the scientific community now uses another means of defining the scale, based upon the points of absolute zero and what is known as the triple point of a specially prepared type of water called VSMOW. It is this newer definition of the scale that is used to relate it precisely to the scale of temperature that is commonly used in science, the Kelvin. Absolute zero is -273.15 degrees Celsius, while the triple point for water is at 0.01 degrees Celsius.

On the Celsius scale, water freezes at 0 degrees and boils at 100 degrees. This means that the freezing and boiling points are separated by exactly 100 degrees.

**Kelvin**

The Kelvin scale is not commonly used in everyday life, but it has become the standard international scale for measuring temperature in science. The Kelvin scale is related very closely to the Celsius scale. A single degree on either scale has the same size, so a change in temperature of 1 degree is the same whether it is being measured on the Celsius or Kelvin scales.

The Kelvin scale, like the modern Celsius scale, is based around the temperatures of absolute zero and the triple point for water. Absolute zero is 0K or Kelvin (-273.15 degrees Celsius). The triple point for water is 273.16K (0.01 degrees Celsius). To convert a temperature measurement between the Celsius and Kelvin scales in very easy. To convert from Celsius to Kelvin, you add 273.15. To convert from Kelvin to Celsius, you subtract 373.15.

Both the Celsius and the Kelvin scales of temperature are used by scientists. Since conversion between the two forms is easy and the size of a degree is the same on both scales, this does not lead to confusion, as long as the scale that is being used is made clear.

**Conversion**

The difference between the Fahrenheit and Celsius scales can be understood by comparing the intervals between the freezing (or melting) and boiling points of water. The difference is 180 degrees on the Fahrenheit scale and 100 degrees on the Celsius scale. This means that a single degree on the Fahrenheit scale is 1/180th of the interval between the freezing and boiling points of water. On the Celsius scale, a single degree is 1/100th of the interval between the freezing and boiling points.

A single degree of Fahrenheit is, therefore, equivalent to 5/9 of a degree on the Celsius scale. A degree on the Celsius scale is larger than one on the Fahrenheit scale.

The difference in the size of a degree on either scale is not the only difference between the two scales. They also start at different temperatures, with zero degrees being colder on the Celsius scale than on the Fahrenheit. The difference between the starting points of the two scales is 32 degrees. 0 degrees Celsius, the freezing point of water, is 32 degrees Fahrenheit.

The Fahrenheit and Celsius scales actually converge at one point. A temperature of -40 degrees Fahrenheit is the same as a temperature of -40 degrees Celsius.

In order to convert between these two temperature scales, it is necessary to account both for the difference in the size of the degree in each scale and for the different starting points around which the scales are based. The Celsius scale uses a larger degree interval than the Fahrenheit scale, with one degree Fahrenheit being equivalent to 5/9 of a degree Celsius. The Fahrenheit scale is also based upon a colder zero point and a hotter 100 degree point. In the Fahrenheit scale, 0 degrees is the temperature of a brine solution. In the Celsius scale, 0 degrees is the point at which water freezes. In the Fahrenheit sale, 100 degrees is slightly higher than the temperature of the human body. In the Celsius scale, 100 degrees is the point at which water boils.

Taking these two differences into account, it is possible to convert between the two scales of temperature. You can perform the conversion in your head, on paper or using a calculator, depending on your mathematical ability and preferences. You need to multiply by 5/9 or 9/5 to deal with the difference in the sizes of the degrees in either scale and to add or subtract 32 to deal with the different starting points of the two scales.

Converting Fahrenheit to Celsius

To convert a temperature from Fahrenheit to Celsius, it is necessary to multiply by 5/9 and subtract 32. If you prefer to use decimals rather than fractions, then 5/9 is equivalent to about 0.55.

**Examples**

To convert 350 degrees Fahrenheit into Celsius

350 x 5/9 -32

or

350 x 0.55 -32

= 162.4

350 degrees Fahrenheit is 162.4 degrees Celsius.

You can also divide by 9/5 or by 1.8 rather than multiplying, if you prefer.

For example:

To convert 30 degrees Fahrenheit into Celsius

30 / 9/5 -32

or

30 / 1.8 – 32

= -15.3

30 degrees Fahrenheit is -15.3 degrees Celsius.

**Converting Celsius to Fahrenheit**

To convert a temperature from Celsius to Fahrenheit, it is necessary to multiply by 9/5 and add 32. If you prefer to use decimals rather than fractions, then 9/5 is equivalent to 1.8.

For example:

To convert 30 degrees Celsius to Fahrenheit

30 x 9/5 + 32

or

30 x1.8 + 32

= 86

30 degrees Celsius is 86 degrees Fahrenheit

You can also divide by 5/9 or by 0.55 rather than multiplying, if you find it easier.

For example:

To convert 150 degrees Celsius to Fahrenheit

150 / 5/9 + 32

or

150 / 0.55 + 32

= 302

150 degrees Celsius is 302 degrees Fahrenheit

**Other methods**

Rather than performing the conversion yourself, you can use one of the many tools that are available online in order to convert one type of temperature measurement into another. All you will need to do is to enter the temperature you want to convert and the computer will convert it for you.

Another option is to estimate the conversion rather than to calculate it exactly. Although this is not a good idea when precision is important, for example when you are dealing with temperatures in a scientific or medical setting, it can be a quick and easy way of converting between Fahrenheit and Celsius when you are cooking, or when you do not need to be accurate.

If you are not worried about accuracy, then you can simply convert a temperature from Fahrenheit to Celsius by halving it. To convert from Celsius to Fahrenheit, you can just double the temperature.

For example, if a recipe instructs you to turn your oven on at 400 degrees Fahrenheit, then you can estimate that your oven needs to be set at 200 degrees Celsius.

If you were to perform and accurate conversion, then you would have taken 400 degrees Fahrenheit, deducted 32 to make it 368 and multiplied it by 5/9 to end up with 204.44 degrees. The difference between the estimated and calculated results will not be significant when you are cooking, although it could make a big difference to a scientific experiment.