Temperature Converter - Celsius, Fahrenheit, Kelvin

The three scales above don't just use different numbers. They were built around different reference points. Knowing which point anchors each scale explains most of the confusion people run into when converting.

Why water isn't a fixed reference any more

Celsius is usually taught as 0 for freezing water and 100 for boiling water. That only holds at standard atmospheric pressure of 101.325 kPa. Boiling point falls as pressure drops, by very roughly 1 °C for every 300 meters of elevation. That is why cooking instructions and candy-making recipes often carry separate high-altitude timings. Freezing point barely moves. So the 0 end of the scale is far more stable than the 100 end.

Since the SI redefinition took effect in 2019, the kelvin no longer depends on water at all. It's now fixed by the Boltzmann constant, at exactly 1.380649 × 10⁻²³ joules per kelvin. Any laboratory can reproduce that without needing a sample of water of a particular isotopic composition.

Kelvin takes no degree symbol

This one shows up constantly in student work and lab reports. Kelvin is written as 300 K, never 300°K. It's read as "three hundred kelvin" rather than "degrees kelvin". Celsius and Fahrenheit keep the degree symbol because they measure position on a scale with an arbitrary zero. Kelvin measures distance from absolute zero, so the degree convention doesn't apply.

The false precision of 98.6°F

Normal body temperature is widely quoted as 98.6°F, and that decimal place is an artifact of unit conversion rather than a measurement. The original nineteenth-century figure was 37°C, a rounded average, and converting 37 exactly gives 98.6. Later research has put the typical average somewhat lower, closer to 36.6°C, so treating 98.6 as a precise threshold reads more certainty into the number than it ever carried.

Rankine, and the point where two scales meet

There's a fourth scale you may meet in older American engineering material. Rankine is an absolute scale like kelvin, but it uses Fahrenheit-sized degrees, so absolute zero sits at 0°R and water freezes at 491.67°R. It still appears in some thermodynamics and HVAC calculations in the United States.

Finally, a useful sanity check when converting by hand: Celsius and Fahrenheit read the same value at exactly −40. If your conversion crosses that point and the two numbers don't swap order as expected, the multiplication and the offset have probably been applied in the wrong sequence.