Temperature conversion calculator

liquid thermometer. temperature conversion calculator
Liquid thermometer showing both Celsius and Fahrenheit / Marco Verch via  Flickr

Did you know that scientists have developed seven different scales for measuring temperature? Many people have trouble enough with the two most familiar ones. Here is a helpful temperature conversion calculator.

Most people are familiar with two ways of measuring temperature: the Fahrenheit scale and the Celsius scale. Sometimes it helps to be able convert one to the other. It’s not hard, really. Going up or down five degrees on the Celsius scale exactly equals going up or down nine degrees on the Fahrenheit scale.

A few correspondences are easy to remember. Water freezes at 0ºC and 32º F. And so,

  • 0º C = 32º F
  • 5º C = 41º F
  • 10º C = 50º F
  • 15º C = 59º F
  • 20º C = 68º F
  • 25º C = 77º F
  • 30º C = 86º F

Most of the world uses Celsius. Speaking of weather, temperatures in the 20s Celsius mean it’s comfortable out. Temperatures in the 30s begin to be uncomfortably hot. Americans using the Fahrenheit scale need a nice jacket when going out in the 30s and a heavy coat in the 20s.

Below freezing or thinking of temperatures for anything but weather, the math gets harder to memorize. You don’t have to do any math, though. Enter a temperature in front of any scale on this temperature conversion calculator and watch the equivalent in seven other temperature scales appear on the screen.

But what are those other six scales? And for that matter, what is temperature? 

Temperature Conversion

What is temperature?

Let’s begin with the last question first.

All matter comprises atoms and molecules, and the atoms and molecules are always vibrating. Temperature means the speed or momentum of that vibration. That is, the faster the molecules vibrate, the higher the temperature. 

If you want to know more, you can read this encyclopedia article or this fuller explanation.

Basically, temperature has to do with energy and thermodynamics. Everything in physics influences everything else, so things such as atmospheric pressure or volume all affect temperature.  

We measure temperature with a thermometer. A thermometer shows the degrees of temperature according to a fixed scale, such as Celsius or Fahrenheit.

A brief history of the thermometer

At the outset of the Scientific Revolution of the 17th century, the thermoscope became a common way of demonstrating differences in temperature. It involved suspending a glass tube in a liquid and watching the level of the liquid rise and fall within the tube. It lacked any kind of scale for measuring the differences. 

Galileo Galilei often receives credit for inventing it. Ferdinand II, the Grand Duke of Tuscany, created a sealed design, in which the liquid resided in a bulb at the thermoscope’s base.

We can think of early thermometers as thermoscopes with some kind of scale that can actually measure temperature. They used all manner of liquids, especially alcohol and mercury. Nowadays, liquid thermometers often display both Fahrenheit and Celsius temperature scales.

Each liquid has its own limitations. For example, alcohol thermometers cannot display temperatures above 172º F (78º C) because alcohol boils at that temperature. Mercury thermometers cannot display temperatures below -37.89º F (-38.83º C) because mercury solidifies at that temperature. 

Nowadays, other thermometer technologies exist. You have probably used a battery-operated thermometer for taking your body temperature. Measuring in Kelvin requires electronic thermometers. 

Temperature scales

Fahrenheit and Celsius

digital thermometer. temperature conversion calculator

Digital thermometer for taking body temperature / Santeri Viinamäki via Wikimedia Commons

Our most common temperature scales, Fahrenheit and Celsius, were invented by two scientists at the beginning of the 18th century. Both scales have been modified since their deaths.

Daniel Fahrenheit, who was born in Poland and spent much of his life in Amsterdam, introduced his scale in 1724. It uses three fixed points. A slurry of ice, salt, and water provided the lowest temperature he could achieve in his laboratory, so it became 0º. Using ice water with no salt, he established his second fixed point as the 32nd degree. He tripled that figure and established human body temperature as the 96th degree. 

After Fahrenheit’s death, other scientists altered his scale. They added 180 degrees to Fahrenheit’s 32 and defined the boiling point of water as the third fixed point, 212º.

The most common alternative temperature scale used to be called centigrade until scientists less than a century ago decided the world would be a better place if they renamed it after its inventor, the Swedish scientist Anders Celsius. At least it wasn’t necessary to change the standard abbreviation.

Celsius introduced his 100-degree scale in 1742. Sweden accepted it immediately. The French adopted it during the Revolution. The number 100 had special appeal to philosophers of the French Enlightenment. International acceptance soon followed. Celsius’ proposal seems backwards, though. He defined 0º as the boiling point of water and 100º as its freezing point. It didn’t take the world long to turn the scale around. 


Both the Fahrenheit and Celsius scales adequately serve every-day purposes, but they share important flaws that renders them too imprecise for science. For example, they both arbitrarily chose water as a standard simply because it was easy given the technology of the time. Water does not always boil at the same temperature, which varies according to altitude and atmospheric pressure. Thus, one of the fixed points isn’t fixed, afterall.

In the late 17th century, French physicist Guillaume Amontons studied the relationship between pressure and temperature in gases. He established that pressure rises and falls with temperature. He speculated that a temperature existed where air would have no pressure at all. From that condition, it would be impossible to achieve a colder temperature. 

This insight led to the concept of absolute zero. Later scientists attempted to define this temperature. In 1848, William Lord Kelvin established it as -273º C. 

Kelvin also proposed a new temperature scale defining 0º as absolute zero. It rises from there with the same size increments as the Celsius scale. Scientists who use the scale, however, do not refer to degrees. Water freezes at 273 Kelvins and boils at 373 Kelvins.

Other historically important temperature scales

Fahrenheit, Celsius, and Kelvin are the most important temperature scales but not the only ones. Others are historically important but no longer of much practical value. Still, it might be fun to know the temperature conversions to them. The temperature conversion calculator instantly shows them all.


Isaac Newton worked for the Royal Mint and needed to understand the melting point of metals. He developed a scale with 0º defined as the melting point of water. He also measured the melting points of iron and tin. His linseed oil thermometer could measure no higher temperature. On his scale, water boils at 33º.


Danish astronomer Ole Christensen Rømer established the first practical temperature scale in 1701. Before Rømer, each thermometer had its own gradations. They were apparently based on the hottest and coldest days of the year at the place where they were manufactured. 

Rømer proposed using two fixed points: the freezing point of a saltwater mixture and the boiling point of unsalted water. Any lab could duplicate them His scale had 60 degrees between these two temperatures. 

Fahrenheit visited Rømer, learned of his scale, and thought that essentially tripling the number of degrees between these points would enable more precise measurement of temperature. 


French entomologist René de Réaumur proposed a scale from 0º as the melting point of water and 80º as its boiling point in 1730. It was widely adopted and continued in use into the 20th century. The French Revolution, however, saw it replaced by Celsius, which used what seemed like the more logical 100. Most of Europe followed suit in the following century. Some dairies and candy makers still use Réaumur.


Russian Tsar Peter the Great invited French astronomer Joseph-Nicolas Delisle to St. Petersburg. There Delisle build a mercury thermometer. He chose 0º as the boiling point of water and measured the contraction of the mercury as it cooled. 

Originally, the Delisle scale had 2,400 or more gradations. Josias Weitbrecht recalibrated it to define the freezing point of water as 150º D in 1738. In this form, Russia continued to use the Delisle scale for about a century. Weitbrecht sent his version of the Delisle scale to Celsius. Celsius’ use of it explains why his original scale also started with 0º as the boiling point of water. 


Scottish physicist Macquorn Rankine proposed a temperature scale in 1859 that, like Kelvin, starts at absolute zero. Instead of degrees equal to Celsius, however, the Rankine scale uses Fahrenheit degrees, where absolute zero is -459.67º. 

Neither the Kelvin scale nor the Rankine scale use the word “degree,” however. Scientists speak of Kelvins or Rankines. And since the names of three inventors of temperature scales begin with R, the Rankine is often abbreviated Ra, although not on this temperature conversion calculator. With Rankine, water freezes at 491.67 R (or Ra) and boils at 671.64 R (or Ra).

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