Temperature conversion involves a simple but easily forgotten equation involving two temperature scales: Fahrenheit and Celsius. Less common but often just as important to science are the Kelvin scale and the related Rankine scale, both of which are based upon absolute zero. This article discusses conversion among these scales; for they history behind the Fahrenheit, Celsius and Kelvin Scales, click here. The following temperature scales are too rare to be considered for the following article, but may prove interesting to the reader: Romer, Newton, Delisle and Reaumur.

The Fahrenheit scale is used widely in the United States and occasionally more globally in non-scientific applications. The Celsius scale on the other hand has global acceptance as the preferred scientific unit. To convert from a Fahrenheit temperature to a Celsius temperature, say to use an American thermometer for a science experiment, use the following equation:

C = (5/9) * (F – 32)

The best way to remember this equation is by memorizing the freezing point and boiling point in each scale. The freezing point in Fahrenheit is 32, while the Celsius freezing point is at 0 degrees. From this you can remember you must subtract 32 from the Fahrenheit value to find the Celsius value. To remember the 5/9 coefficient, look at the two boiling points: 212 in Fahrenheit is 100 in Celsius. First subtract 32 from 212 to find 180, then divide into 100. This gives you the ratio of 100/180 = 5/9, which makes sense because 5/9 is about half and 100 is about half of 180.

To go in the opposite direction, from Celsius to Fahrenheit, you must do the same operations as the previous equation, only in reverse. This would be done, for example, when you want to think about scientific data recorded in Celsius in terms of Fahrenheit numbers you may be used to in other contexts. The equation useful in this case is:

F = (9/5) * C + 32

If you think about the first equation, the second equation comes from solving for F. To do this, you would first multiply both sides by 9/5 to cancel out 5/9. Then, because (F-32) remains, add 32 to isolate F on one side. Just like above, this equation is easy to remember if you think about the freezing and boiling points. At freezing, 0 degrees Celsius is simply added to 32 to get the Fahrenheit temperature. At boiling, 100 * (9/5) = 180, 32 degrees short of 212 Fahrenheit.

Now, absolute zero is an important concept in chemistry and physics and this is reflected in two other scales: Kelvin and Rankine. The Kelvin scale is directly based on the Celsius scale: it’s units are equal but the scale starts at absolute zero rather than freezing. Because in Celsius freezing is 0 and absolute zero is 273.15, we get these two equations for conversion between Kelvin and Celsius:

K = C + 273.15

C = K – 273.15

Note: In some contexts, 273.15 is rounded to 273.

Less often, you may need to convert between Kelvin and Fahrenheit temperatures. To do this, either use a two step process combining the two conversions discussed above with an intermediate Celsius value, or use the following equations:

K = (5/9) * (F + 459.67)

F = (9/5) * K – 459.67

These equations can be seen as the combination of the two preceding sets. The – 459.67 arises from the Fahrenheit value for absolute zero, equivalent to 273.15 degrees Celsius.

The Rankine scale uses the same units as the Fahrenheit scale but, like the Kelvin scale, starts at absolute zero. To convert between Rankine and Kelvin scales, use the following equations:

K = (5/9) * R

R = (9/5) * K

Note: the Rankine scale is far less common than the three other scales considered above. Also, the Rankine scale can be found directly from the Fahrenheit scale using the Fahrenheit absolute zero of 459.67 mentioned above, but this is rarely needed.