Temperature Conversion: Celsius, Fahrenheit, Kelvin Explained

Temperature Scales: A Brief History

The three major temperature scales were created at different times for different purposes.

Fahrenheit (degrees F) was developed by German physicist Daniel Gabriel Fahrenheit in 1724. He set 0 degrees F as the coldest temperature he could produce with an ice-salt mixture and 96 degrees F as human body temperature (later adjusted to 98.6 degrees F). Fahrenheit remains the primary scale in the United States for everyday use.

Celsius (degrees C), originally called centigrade, was proposed by Swedish astronomer Anders Celsius in 1742. He defined 0 degrees C as the freezing point of water and 100 degrees C as the boiling point at sea level, creating a convenient 100-degree scale for scientific use. Now the standard in most of the world.

Kelvin (K) was defined by Lord Kelvin (William Thomson) in 1848. It starts at absolute zero - the theoretical temperature at which all thermal motion ceases. There are no negative Kelvin values. Kelvin is the SI base unit of temperature, essential for physics and chemistry.

Conversion Formulas

Celsius to Fahrenheit

°F = (°C * 9/5) + 32
Example: 100°C -> (100 * 1.8) + 32 = 212°F

Fahrenheit to Celsius

°C = (°F - 32) * 5/9
Example: 98.6°F -> (98.6 - 32) * 5/9 = 37°C

Celsius to Kelvin

K = °C + 273.15
Example: 0°C -> 273.15 K

Kelvin to Celsius

°C = K - 273.15
Example: 373.15 K -> 100°C

Quick Reference Table

Note that -40 degrees is the same value in both Celsius and Fahrenheit - the one point where the scales intersect.

Temperature in Science and Engineering

The ideal gas law uses Kelvin because gas properties are proportional to absolute temperature:

PV = nRT   (P=pressure, V=volume, n=moles, R=gas constant, T=Kelvin)

You cannot substitute Celsius or Fahrenheit into this formula - only Kelvin works correctly because it starts from true zero.

Rankine: The Fourth Scale

Rankine (degrees R) is the Fahrenheit equivalent of Kelvin - an absolute scale where zero is absolute zero, but degrees are the same size as Fahrenheit degrees:

°R = °F + 459.67
°R = K * 9/5

Rankine is used in some American engineering applications.

Everyday Temperature References

• -40 degrees: The one temperature that is identical in both Celsius and Fahrenheit
• 0°C (32°F): Water freezes at standard pressure
• 20°C (68°F): Standard comfortable room temperature
• 37°C (98.6°F): Normal human body temperature
• 100°C (212°F): Water boils at sea level (lower at high altitude)
• 160°C (320°F): Safe minimum internal temperature for poultry

Climate Change Context

Global temperature anomaly is measured in Celsius relative to pre-industrial baseline. The Paris Agreement targets limiting warming to 1.5-2°C above pre-industrial levels, which translates to 2.7-3.6°F.

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