Temperature Converter

Features

• Four scales updating simultaneously
• Warns when you go below absolute zero
• Plain-English context for the temperature entered
• Reference table of familiar temperatures
• Handles negatives and decimals

How to use it

1. Type a temperature into any of the four boxes.
2. The other three update immediately.
3. Read the context line for a sense of scale.
4. Copy all four values at once if you need them.

Where these scales come from

Celsius fixes 0 at the freezing point of water and 100 at its boiling point at standard atmospheric pressure, which makes it convenient for everyday use and for science. Fahrenheit's reference points are historical rather than natural: Daniel Fahrenheit set 0 at the freezing point of a brine solution and originally calibrated 96 to approximate human body temperature. The scales cross at −40, which is the same temperature in both.

Kelvin starts at absolute zero, the point where classical thermal motion stops, and uses degrees the same size as Celsius, so K = °C + 273.15. It has no negative values by definition, which is why entering one here produces a warning. Rankine does the same for Fahrenheit-sized degrees, starting at absolute zero with °R = °F + 459.67. It survives mainly in some US engineering contexts.

One notational detail: Kelvin takes no degree symbol and is written as 300 K rather than 300°K, because it measures absolute thermodynamic temperature rather than a position on a scale between two arbitrary reference points. Since 2019 the kelvin has been defined by fixing the Boltzmann constant, making it independent of any physical substance.