Temperature Converter

What this converter does

This converter turns a temperature from Celsius, Fahrenheit, or Kelvin into either of the other two, instantly and exactly. Enter a number, pick what you are converting from and what you want it in, and you get a clean result. It is the quick answer for a recipe written in the wrong scale, a US weather report when you think in Celsius, or a chemistry problem that wants Kelvin.

How to use it

It is barely a step more than reading the answer.

• Type your temperature into the value box.
• Set From to the scale you have, and To to the scale you want. It opens on Celsius to Fahrenheit, the most common swap.
• Press Calculate.

The result reads back in plain words, like "100 °C is equal to 212 °F." The Swap button flips the two scales if you want to go the other way, and Reset clears it.

The conversion formulas

Most unit conversions are a single multiply. A kilometre is always 1,000 metres, so you scale and you are done. Temperature is the odd one out, because the three scales do not share a starting point. Zero Celsius, zero Fahrenheit, and zero Kelvin are three completely different temperatures. So converting temperature means doing two things at once: shifting the scale to line the zeros up, and stretching it to match the size of a degree.

Here are the formulas the converter uses:

• Celsius to Fahrenheit: F = C × 9/5 + 32
• Fahrenheit to Celsius: C = (F - 32) × 5/9
• Celsius to Kelvin: K = C + 273.15
• Kelvin to Celsius: C = K - 273.15
• Fahrenheit to Kelvin: K = (F - 32) × 5/9 + 273.15
• Kelvin to Fahrenheit: F = (K - 273.15) × 9/5 + 32

The 9/5, or 1.8, is the stretch: one Celsius degree is 1.8 Fahrenheit degrees, because Celsius splits the gap between freezing and boiling water into 100 steps while Fahrenheit uses 180. The +32 and the 273.15 are the shifts that slide each zero into place. Behind the scenes the tool routes every conversion through Kelvin as a shared base, which is what keeps all three scales lined up exactly.

A quick example

Say you are boiling water and the recipe is in Celsius: 100 °C. To get Fahrenheit, multiply by 9/5 and add 32:

100 × 9/5 + 32 = 180 + 32 = 212 °F

That is the boiling point of water, the number on the back of every kettle in the US. Going the other way, normal body temperature of 98.6 °F becomes (98.6 - 32) × 5/9 = 37 °C, the figure most of the world uses. And if you have no calculator handy, a rough trick for Celsius to Fahrenheit is to double it and add 30: 20 °C gives about 70, close to the real 68.

Common temperatures at a glance

A few landmarks worth knowing, side by side:

Absolute zero is the coldest anything can get, the point where molecular motion stops, and it is exactly where the Kelvin scale begins at zero.

The three scales, and where each is used

Three scales, three different jobs.

Celsius is what most of the world uses day to day. Anders Celsius, a Swedish astronomer, built it around water, with 0 for freezing and 100 for boiling. Clean and intuitive, which is why it runs weather forecasts and kitchens across most of the planet.

Fahrenheit is the one the United States kept. Daniel Gabriel Fahrenheit set it up in the early 1700s using a colder zero, so everyday weather mostly lands between 0 and 100, which some people find handy for describing how a day feels. If you are converting a US oven setting or a weather app, this is usually the scale you are wrestling with.

Kelvin is the scientist's scale. Lord Kelvin anchored it to absolute zero, the true bottom of the temperature range, so it never goes negative. It uses the same step size as Celsius, just shifted, and you will meet it in physics, chemistry, and anywhere temperatures run to extremes. One quirk: it drops the degree symbol, so it is written "300 K", not "300 °K".

Questions people ask

References

1. National Institute of Standards and Technology (NIST). The kelvin (SI base unit of temperature). https://physics.nist.gov/cuu/Units/kelvin.html