Why the United States Still Uses Fahrenheit: A Scale Set by One Man's Body Temperature in 1724
In the winter of 1714, a glassblower in Danzig named Daniel Gabriel Fahrenheit made a mercury thermometer that worked better than anything before it. Mercury moved smoothly, did not freeze in mild winters, and expanded predictably in a linear relationship with heat. What remained was calibration: what should the numbers mean? Fahrenheit needed fixed reference points, and he chose three. The lowest was the freezing point of a salt-water brine mix, which he called zero. The middle was the freezing point of plain water, which landed at 32. The top was his estimate of human body temperature, which he set at 96. From those three anchors, everything else followed.
His 1724 paper in the Philosophical Transactions of the Royal Society described the scale in careful detail. Fahrenheit was already recognized as the leading thermometer maker in Europe, and his instruments were used by researchers including Giovanni Maria Lancisi in Italy and Herman Boerhaave in the Netherlands. What Fahrenheit probably did not consider was that his choice of reference points would become one of the most persistently argued measurement decisions in history.
Temperature was one of the last physical quantities to be quantified precisely. Length, weight, and time had relatively straightforward material references: a length of barleycorns, a weight of grain, the arc of the sun. Temperature resisted easy measurement because heat is not a substance that can be weighed or held. The thermometer itself preceded calibration by more than a century: Galileo's thermoscope of the 1590s showed that temperature could be measured by the expansion of air, but without fixed reference points, comparisons between different instruments were meaningless.
The Scale's Odd Numbers and Their Origin
If you have ever wondered why water freezes at 32 degrees Fahrenheit instead of zero, or boils at 212 instead of 100, the answer is that the scale was not designed with water as the primary reference. Fahrenheit was calibrating against brine freezing and estimated body temperature first. Water fell where it fell: 32 degrees at freezing, 212 degrees at boiling. The 180-degree span between those points is not an accident; it is divisible by many small integers, which made the arithmetic of interpolation easier with pen and paper.
Fahrenheit's original body temperature anchor of 96 degrees was slightly off from what we now consider physiologically normal. That divergence led to the figure 98.6, which came from a German physician named Carl Wunderlich. Between 1851 and 1868, Wunderlich recorded axillary temperatures from roughly 25,000 patients in Leipzig, published in his 1868 book "On the Temperature in Diseases." His calculated average was 37.0 degrees Celsius, which converts to 98.6 Fahrenheit. That number entered textbooks and popular awareness as "normal body temperature."
A 1992 study in JAMA by Philip Mackowiak, Stephen Wasserman, and Myron Levine re-examined the question with modern instrumentation and found the true average oral temperature in healthy adults is approximately 98.2 degrees Fahrenheit, not 98.6. More recent research published in eLife in 2020 found average human body temperature has been decreasing since the 19th century, now averaging closer to 97.9 degrees Fahrenheit, potentially reflecting reduced systemic inflammation in populations with better access to antibiotics and reduced chronic infections. The 98.6 figure persists in common knowledge despite being incorrect by Wunderlich's own methodology and certainly by modern measurement.
The Celsius Alternative and Its Adoption
Twenty-eight years after Fahrenheit published his scale, the Swedish astronomer Anders Celsius proposed a competing system in a 1742 paper to the Swedish Royal Society of Sciences. Celsius's original version was inverted from what we use today: he placed 0 at the boiling point of water and 100 at the freezing point. After Celsius's death in 1744, his colleagues reversed the scale. The botanist Carl Linnaeus at Uppsala University is often credited with the reversal, though the French geologist Jean-Pierre Christin independently proposed the same corrected orientation the same year.
The revised Celsius scale had an elegant property Fahrenheit's lacked: the two most important phase transitions of water sat at exactly 0 and 100, making mental estimation straightforward. The scale also aligned naturally with the decimal metric system the French were developing during the same period.
In 1948, the General Conference on Weights and Measures formally renamed the centigrade scale to Celsius in honor of Anders Celsius, standardizing the terminology internationally. The International Temperature Scale of 1990 (ITS-90), developed by the Consultative Committee for Thermometry, provides the current technical definition of the Celsius scale as an offset from Kelvin: 0 degrees Celsius equals 273.15 Kelvin.
Most of the world adopted Celsius during the metric movements of the 18th and 19th centuries. Britain officially converted weather reporting to Celsius in 1962, though informal Fahrenheit use persisted in popular communication for decades. Canada converted to Celsius in weather and public communications in 1975 as part of its Metric Conversion Act program. Australia completed its switch in 1972.
America's Almost-Switch and Its Failure
The United States came very close to following. Congress passed the Metric Conversion Act in 1975, which created the United States Metric Board and established a national policy of voluntary conversion to the metric system, including temperature. The National Bureau of Standards (now NIST) began publishing metric-first documents. Highway signs in kilometers were proposed and piloted on some interstate segments. Weather forecasts in some US markets showed Celsius alongside Fahrenheit.
The Metric Board was dissolved in 1982 after President Reagan's administration eliminated it during budget cuts justified by its voluntary and therefore ineffective mandate. The voluntary conversion policy, as it turned out, produced no conversion. The Omnibus Trade and Competitiveness Act of 1988 declared the metric system the "preferred system of weights and measures for United States trade and commerce," but preference without mandate produced the same result as the earlier voluntary approach.
Today the United States, alongside Liberia and Myanmar, continues to use Fahrenheit for everyday temperature. The reasons are less about the scale's merits and more about switching costs: millions of appliances, weather broadcasts, recipes, building codes, medical charts, and decades of accumulated human intuition encoded in Fahrenheit. A person who has spent forty years interpreting 75 degrees as a pleasant afternoon and 20 degrees as cold weather has calibrated their perception to a scale that a number change cannot easily overwrite.
Kelvin and Its Scientific Applications
Lord Kelvin, the British physicist William Thomson, established the absolute temperature scale in 1848, building on work by Nicolas Sadi Carnot and James Prescott Joule on the thermodynamics of heat engines. Thomson recognized that there was a theoretical lower limit to temperature, a point at which all thermal motion would cease, and he proposed a scale anchored at that absolute zero rather than at the freezing point of any particular substance. Absolute zero is -273.15 degrees Celsius, or -459.67 degrees Fahrenheit.
The Kelvin scale is the SI unit of temperature and is used in physics, astronomy, and thermodynamics. The temperature of the cosmic microwave background radiation, the remnant heat of the Big Bang, is approximately 2.725 Kelvin. The surface temperature of the Sun is approximately 5,778 Kelvin. Industrial processes including semiconductor fabrication use Kelvin because many physical relationships, such as the Stefan-Boltzmann law relating radiated power to temperature, require absolute temperature in their formulas.
Conclusion
Temperature conversion between Fahrenheit, Celsius, and Kelvin is a routine need for travelers, cooks, scientists, and anyone working across national contexts where different standards apply. The mathematical relationships are fixed: to convert Celsius to Fahrenheit, multiply by 9/5 and add 32. To convert Fahrenheit to Celsius, subtract 32 and multiply by 5/9. To convert Celsius to Kelvin, add 273.15. ToolHQ's temperature converter handles any direction instantly, which matters precisely because three centuries after Fahrenheit chose his brine reference points, his scale and the Celsius alternative still coexist as daily realities in different parts of the world.
Frequently Asked Questions
Why does the Fahrenheit scale have such odd numbers like 32 and 212?
Because Fahrenheit did not use water as his primary reference. He calibrated against brine freezing and body temperature first, and water's phase transitions landed where they did on the resulting scale.
Which countries still use Fahrenheit?
The United States, Liberia, and Myanmar use Fahrenheit for everyday temperatures. Most other countries use Celsius, and scientific work worldwide uses Kelvin or Celsius.
What is the formula to convert Celsius to Fahrenheit?
Multiply by 9/5, then add 32. So 20 Celsius becomes (20 x 1.8) + 32 = 68 Fahrenheit.
What is Kelvin and when is it used?
Kelvin is an absolute temperature scale starting at absolute zero (-273.15 Celsius). It is used in physics, astronomy, and thermodynamics where negative temperatures would complicate calculations.