Thermometers measure temperature according to well-defined scales of measurement. The three most common temperature scales are Fahrenheit, Celsius, and Kelvin. Temperature scales are created by identifying at least two reproducible temperatures. The freezing and boiling temperatures of water at standard atmospheric pressure are commonly used as reference points to these scales.

The table below shows the most common temperature reference points at all three scales:

Table with four columns titled reference point, Kelvin, Celcius, and Fahrenheit. The first reference point for boiling water is 373.15 Kelvin, 100 Celcius and 212 Fahrenheit. Ice water is 273.15 Kelvin, 0 Celcius, and 32 Fahrenheit. Boiling nitrogen is 77 Kelvin, negative 195.79 Celcius and negative 320 Fahrenheit.

And the equations below show how to convert from one temperature scale to another:

T (in oC) + 273.15 = T (in K)

T (in oF) = T(in oC) × 9/5 + 32