The Celsius scale, created in the mid-18th century, is part of the metric system, and is today the most common form of temperature measurement. Because of the near-universal adoption of the metric scale, Celsius is the official form of temperature used in the vast majority of countries worldwide.

The scale now known as the Celsius scale was first proposed in the 18th century. In 1742, Swedish scientist Anders Celsius created a temperature scale, using the boiling point of water as the zero degree measurement, and its freezing point as the 100 degree measurement. One year later, a similar scale, called ​centigrade,​ was invented by French scientist Jean Pierre Cristin. Cristin placed the freezing point at zero degrees and the boiling point at 100 degrees instead. Cristin's placements of the freezing and boiling points became those used on the scale today. The scale was known interchangeably as Celsius and centigrade until 1948, when an international meeting on measurements officially designated the scale as Celsius.

Celsius temperature is part of the ​metric system​ of measurement, first developed in 18th century France. Like Celsius, other metric units – such as kilometers, grams and liters – are based on multiples of 10. The metric system was established as an international standard of measurement in 1875, and became the official form of standardized measurement for most European countries and their colonies by the end of the 19th century. Since the Celsius scale was the major temperature scale of the metric system, it became the official temperature scale for most of the world.

The only exceptions to the quick adoption of metric scales, and thus Celsius, were English-speaking countries that used the ​imperial system​, such as the United Kingdom, India and South Africa. These countries used Fahrenheit, an imperial unit of temperature. However, by the mid-20th century, even these English-speaking countries began to adopt the metric scale, and thus Celsius. India switched in 1954, the U.K. in 1965, and Australia and New Zealand in 1969. Today, only three countries do not use the metric system: the United States, Liberia and Burma.

The relationship between Celsius, C, and Fahrenheit, F, temperatures is given by the following formula:

So, freezing point – zero degrees Celsius – is 32 degrees Fahrenheit, and the boiling point at 100 degrees Celsius is 212 degrees Fahrenheit.

When the temperature is -40 degrees, it is the same in both Celsius and Fahrenheit.

Because of the widespread adoption of the metric system, most countries worldwide – including non-metric Liberia and Burma – use Celsius as their official temperature scale. Only a few countries use Fahrenheit as their official scale: the United States, Belize, Palau, the Bahamas and the Cayman Islands. Fahrenheit is still sometimes used in Canada, although Celsius is more common and is the official Canadian temperature scale.