Question 1

What are the exact formulas the Fahrenheit to Celsius converter uses?

Accepted Answer

°C = (°F − 32) × 5/9 and °F = °C × 9/5 + 32. These are exact algebraic conversions between the two scales as defined against the International Temperature Scale of 1990 (ITS-90) published by the Bureau International des Poids et Mesures. Examples: 68°F = (68 − 32) × 5/9 = 20°C; 100°C = 100 × 9/5 + 32 = 212°F. The two scales cross at −40° (−40°F = −40°C), which is the standard cold-weather sanity check.

Question 2

How do Fahrenheit and Celsius relate to Kelvin, Rankine, and absolute zero?

Accepted Answer

K = °C + 273.15 and °R = °F + 459.67. Absolute zero is 0 K = −273.15°C = −459.67°F = 0°R and is defined by ITS-90 as the temperature at which the enthalpy of an ideal gas reaches its minimum. The 2019 SI redefinition set the Boltzmann constant to exactly 1.380 649 × 10⁻²³ J/K, so kelvin is now defined thermodynamically rather than via the triple point of water. Water's triple point is 273.16 K = 0.01°C exactly by pre-2019 definition and is still the primary defining fixed point on ITS-90.

Question 3

What are the reference points for water on each scale?

Accepted Answer

At standard atmospheric pressure (101.325 kPa, defined by BIPM), pure water freezes at 32°F = 0°C = 273.15 K and boils at 212°F = 100°C = 373.15 K. Both boiling and freezing points shift with pressure (about −0.28°C per kPa for the boiling point near sea level), which is why cooking instructions specify altitude adjustments above 3,000 feet. NIST Special Publication 811 (2008 revision, still current) documents these reference values and the SI unit conventions used across U.S. federal science and engineering practice.

Question 4

Is normal human body temperature really 98.6°F?

Accepted Answer

The 98.6°F (37.0°C) figure comes from Carl Wunderlich's 1868 monograph averaging one million axillary temperatures. Modern re-measurement with electronic thermometers and larger datasets found a true 21st-century mean closer to 97.5–97.9°F (36.4–36.6°C). Protsiv, Ley, Lankester, Hastie, and Parsonnet's analysis of 197 years of U.S. body temperature records in eLife (2020) documented a monotonic decline of about 0.03°C per decade since the 1860s, attributed to reduced background inflammation, warmer indoor environments, and lower resting metabolic rate. Clinical fever thresholds have not been formally revised, so 100.4°F (38.0°C) remains the operational cutoff for infection screening.

Question 5

Which countries use Fahrenheit and why did the U.S. never fully switch to Celsius?

Accepted Answer

Only the United States, Liberia, the Cayman Islands, and a handful of Caribbean territories continue to use Fahrenheit as the primary everyday temperature scale; the rest of the world uses Celsius, and scientific work worldwide uses kelvin. The U.S. adopted the Metric Conversion Act of 1975 (15 U.S.C. §§ 205a–205l) declaring the metric system “the preferred system of weights and measures for U.S. trade and commerce,” and Executive Order 12770 (1991) required federal agencies to use metric internally. Both are voluntary for consumer weather and commerce; NIST continues to publish the SI usage guide (SP 811) as the reference for federal and scientific practice.

Fahrenheit to Celsius Converter