According to a letter Fahrenheit wrote to his friend Herman Boerhaave, his scale was built on the work of Ole Rømer, whom he had met earlier. In Rømer's scale, brine freezes at 0 degrees, ice melts at 7.5 degrees, body temperature is 22.5, and water boils at 60 degrees. Fahrenheit multiplied each value by four in order to eliminate fractions and increase the granularity of the scale. He then re-calibrated his scale using the melting point of ice and normal human body temperature (which were at 30 and 90 degrees); he adjusted the scale so that the melting point of ice would be 32 degrees and body temperature 96 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).
Fahrenheit observed, somewhat incorrectly, that water boils at about 212 degrees using this scale. Later, other scientists decided to redefine the degree slightly to make the freezing point exactly 32oF, and the boiling point exactly 212oF or 180 degrees higher. It is for this reason that normal human body temperature is approximately 98o (oral temperature) on the revised scale (whereas it was 90o on Fahrenheit's multiplication of Rømer, and 96o on his original scale).