How to convert a temperature
- Enter the temperature figure.
- Choose which scale it is currently in.
- All four scales update at once.
- Check the breakdown for how far above absolute zero it sits.
- Values below absolute zero are rejected, because they are not physically possible.
What this converter shows
- Celsius, Fahrenheit, Kelvin and Rankine simultaneously.
- A check against absolute zero, with a clear error rather than a nonsense figure.
- Recognition of landmark temperatures such as freezing and boiling point.
- Conversion from any of the four scales as the starting point.
- The distance above absolute zero, which is what Kelvin actually measures.
- Live results with nothing transmitted.
Why temperature conversion needs offsets
Converting metres to feet is a multiplication because both scales start at zero meaning "no length". Temperature scales do not share a zero. Celsius puts zero at the freezing point of water; Fahrenheit puts it at the freezing point of a brine mixture; Kelvin puts it at absolute zero.
So converting requires shifting as well as scaling: multiply by 9/5 and add 32 to go from Celsius to Fahrenheit. Getting the order wrong, or forgetting the offset, produces answers that are plausible at some temperatures and badly wrong at others.
The one temperature where they meet
At −40 degrees, Celsius and Fahrenheit read the same. It is the single point where the two scales cross, and it is a useful sanity check, if a conversion of −40 does not give −40, the arithmetic is wrong.
The everyday approximation worth knowing is doubling and adding 30: 20 °C becomes roughly 70 °F, which is close to the correct 68. It drifts at extremes but is good enough for deciding what to wear.
Kelvin and absolute zero
Kelvin starts at absolute zero, the point where particles have minimum possible motion. That is −273.15 °C, and nothing can be colder. Kelvin uses the same step size as Celsius, so converting is just adding or subtracting 273.15.
Because it starts at a genuine zero, Kelvin is the only one of the four where ratios are meaningful. 200 K really is twice as hot as 100 K in a physical sense. Saying 20 °C is twice as hot as 10 °C is meaningless, because the zero point is arbitrary.
Rankine, and where it survives
- Rankine starts at absolute zero, like Kelvin.
- It uses Fahrenheit-sized degrees rather than Celsius-sized ones.
- Absolute zero is 0 °R, water freezes at 491.67 °R.
It exists so that engineers working in Fahrenheit can do thermodynamic calculations that require an absolute scale. It appears in some American engineering contexts and essentially nowhere else, which is why most people have never encountered it.
Reference points worth remembering
Water freezes at 0 °C / 32 °F and boils at 100 °C / 212 °F at sea level. The boiling point drops with altitude, which is why high-altitude cooking takes longer. Normal human body temperature is about 37 °C / 98.6 °F, although that figure comes from a nineteenth-century study and modern averages run slightly lower.
A comfortable room is 20 to 22 °C, a fever starts around 38 °C, and a domestic oven runs between 160 and 220 °C. For other unit conversions the unit converter and length converter cover distance and volume. Everything is calculated in your browser.
Why weather forecasts feel different in each scale
Fahrenheit degrees are smaller than Celsius ones. There are 180 of them between freezing and boiling against 100, which means Fahrenheit gives finer resolution without decimals. That is part of why it persists for weather in the United States: the range people actually experience, roughly 0 to 100 °F, maps neatly onto "very cold" and "very hot".
Celsius anchors on water instead, which is more useful scientifically and less intuitive for describing a day. Both work; they simply optimise for different things. The conversion matters most when reading a recipe or a technical specification from another country, where being 20 degrees out is the difference between a warm oven and a burnt dinner.