from wikipedia:
There are several competing versions of the story of how Fahrenheit came to devise his temperature scale. One states that Fahrenheit established the zero (0 °F) and 96 °F points on his scale by recording the lowest outdoor temperatures he could measure, and his own body temperature. He took as his zero point the lowest temperature which he measured in the harsh winter of 1708 through 1709 in his hometown of Danzig (now GdaÅsk, Poland) (â17.8 °C). (He was later able to reach this temperature under laboratory conditions using a mixture of ice, ammonium chloride and water.)
Fahrenheit wanted to avoid the negative temperatures that the Rømer scale had produced in everyday use. He fixed his own body temperature as 96 °F (normal body temperature is closer to 98.6 °F, suggesting that his thermometer was inaccurate, or lastly it is believed that he may have used a cow’s temperature instead of his own), and divided his original scale into twelve divisions; later dividing each of these into 8 equal subdivisions producing a scale of 96 degrees. Fahrenheit noted that his scale placed the freezing point of water at 32 °F and the boiling point at 212 °F, a neat 180 degrees apart.
Another story holds that Fahrenheit established the zero of his scale (0 °F) as the temperature at which an equal mixture of ice and salt melts (some say he took that fixed mixture of ice and salt that produced the lowest temperature); and ninety-six degrees as the temperature of blood (he initially used horse blood to calibrate his scale). Initially, his scale only contained 12 equal subdivisions, but later he subdivided each division into 8 equal degrees ending up with 96.
A fourth well-known version of the story, as described in the popular physics television series The Mechanical Universe, holds that Fahrenheit simply adopted Rømer’s scale, at which water freezes at 7.5 degrees, and multiplied each value by 4 in order to eliminate the fractions and increase the granularity of the scale (giving 30 and 240 degrees). He then re-calibrated his scale between the melting point of water and normal human body temperature (which he took to be 96 degrees); the melting point of ice was adjusted to 32 degrees so that 64 intervals would separate the two, allowing him to mark degree lines on his instruments by simply bisecting the interval six times (since 64 is 2 to the sixth power).
A fifth version maintains that Fahrenheit based 0 degrees on an estimate of the temperature someone would freeze to death, and 100 degrees on the temperature someone would die of heat exhaustion from, therefore making 0 to 100 the livable range for human beings (this, however, is not feasible with current knowledge because the human body has been proven to survive at temperatures well above and below these thresholds due to its thermoregulatory capabilities).
A sixth version maintains that Fahrenheit marked the melting point of ice, normal human body temperature, and the boiling point of water. He then divided the span from melting to boiling into 180 degrees. Setting the normal human body temperature as 96 resulted in the freezing point and boiling point being 32 and 212, respectively.
His measurements were not entirely accurate; by his original scale, the actual melting and boiling points would have been noticeably different from 32 °F and 212 °F. Some time after his death, it was decided to recalibrate the scale with 32 °F and 212 °F as the exact freezing and boiling points of plain water. That change was made to easily convert from Celsius to Fahrenheit and vice versa, with a simple formula. This change also explains why the body temperature once taken as 96 °F by Fahrenheit is today taken by many as 98.6 °F (it is a direct conversion of 37 °C), although giving the value as 98 °F would be more accurate.