Temperature Converter
- 20
- Celsius (°C)
- 68
- Fahrenheit (°F)
- 293.15
- Kelvin (K)
- 527.67
- Rankine (°R)
The temperature converter shows all four scales at once (Celsius, Fahrenheit, Kelvin and Rankine) and flags any value below absolute zero. Temperature is the one common conversion that needs an offset as well as a scale factor, which is why the familiar formula has a +32 in it.
How it works
F = C x 9/5 + 32 C = (F - 32) x 5/9
- 9/5
- a Fahrenheit degree is 5/9 the size of a Celsius degree, so the numbers grow faster
- 32
- the offset: water freezes at 0 °C but 32 °F
Every conversion here routes through Kelvin, the only one of the four scales whose zero is absolute zero. Celsius and Fahrenheit both have arbitrary zero points, so converting between them directly requires handling the offset in both directions.
- Kelvin uses the same degree size as Celsius, offset by 273.15.
- Rankine uses the Fahrenheit degree size with an absolute zero point, so it relates to Fahrenheit as Kelvin does to Celsius.
- Absolute zero is 0 K, −273.15 °C, −459.67 °F and 0 °R. The tool flags anything below it.
Examples
Body temperature
Value
37
Scale
Celsius
Result
98.6 °F · 310.15 K · 558.27 °R
37 × 9/5 = 66.6, plus 32 = 98.6. The famous 98.6 is a conversion artefact. The original measurement was 37 °C, a two-significant-figure value that became three in Fahrenheit.
Where the scales cross
Value
-40
Scale
Celsius
Result
−40 °F · 233.15 K · 419.67 °R
Minus forty is the single point where Celsius and Fahrenheit agree, because the offset and the scale factor cancel exactly.
Below absolute zero
Value
-300
Scale
Celsius
Result
Flagged as impossible. Absolute zero is −273.15 °C
The arithmetic still works and produces a negative Kelvin figure, which is why the tool checks rather than silently converting a physically meaningless value.
Frequently asked questions
Why is normal body temperature 98.6 °F when it is 37 °C?
Because 98.6 is a false-precision artefact. Carl Wunderlich measured 37 °C in the 1860s, two significant figures, and the exact conversion produced 98.6 °F, which was then treated as a precise figure. Modern studies put the average nearer 36.6 °C, and it varies by person, time of day and measurement site.
At what temperature do Celsius and Fahrenheit read the same?
Minus forty. It is the only point where they agree, because that is where the 32-degree offset and the 9/5 scale factor cancel. It also makes a useful check that a converter is handling the offset correctly.
What is Rankine and who uses it?
An absolute scale using Fahrenheit-sized degrees, so 0 °R is absolute zero and water freezes at 491.67 °R. It appears in US engineering thermodynamics, where absolute temperature is needed but the working units are Fahrenheit.
Why does the tool flag values below absolute zero?
Because the arithmetic happily produces them and nothing else would stop you. A negative Kelvin value is not a cold temperature. It is a sign the input was wrong, usually a missing minus or a wrong scale selection.