Physicist Daniel Gabriel Fahrenheit established a temperature scale which was once widely used by English-speaking countries. Fahrenheit originally defined body temperature as 96°F and an ice/water/salt mixture as 0°F. He used these measurements as fixed reference points to measure other temperatures.

Later the scale was adjusted slightly. In the late 1960s most countries switched to the more convenient Celsius scale. People in the United States still use the Fahrenheit scale. It is important for them to be able to convert between Fahrenheit and Celsius.

On the Fahrenheit scale: Absolute zero is –459.69°F A degree is 1/180 of the temperature difference between water's boiling and freezing points. Normal body temperature is 98.6°F. Water (at sea level air pressure) freezes at 32°F. Water (at sea level air pressure) boils at 212°F.

The Rankine scale is an absolute temperature scale based on the Fahrenheit scale. William John Macquorn Rankine devised this absolute temperature scale in 1859. Zero Rankine is the lower limit of temperature. There are no negative Rankine temperatures. Rankine is hardly used by today's scientists.