The Celsius scale is used for measuring temperature. Measurements in the Celsius scale are indicated by the suffix °C.

The scale is designed such that the freezing point of water is 0°C, and the boiling point of water is 100°C at a pressure of one atmosphere.

The Celsius thermometer (and hence the corresponding scale) was invented in 1742 by a Swedish astronomer, Anders Celsius, and published in a paper to the Annals of the Royal Swedish Academy of Science. It was already widely suggested that the freezing point of water was a good temperature calibration point, and Fahrenheit had determined that a relationship existed between barometric pressure and boiling point. Celsius' scientific contributions were:

Celsius' most important contribution was the following procedure for calibrating a mercury thermometer to this scale, allowing the production of standardized thermometers:

1. Put the cylinder AB of the thermometer (i.e., the bulb) in thawing snow and mark the freezing point of water C, which should be at such a height over the cylinder at A that the distance AC is half the distance between C and the water boiling point mark D.
2. Mark the boiling point of water D at a pressure of "25 tum 3 linier" (approximately 755 Hgmm).
3. Divide the distance in 100 equal parts or degrees; so that 0 degree corresponds to the boiling point of water D, and 100 to the freezing point of water C. When the same degrees have been continued below C all the way down to A the thermometer is ready.
(Translation from the original Swedish from www.santesson.com.)

You will notice from this description that the Celsius had the boiling and freezing points reversed from what we use today. In fact his original order never lasted much past his experimental efforts, as the instrument maker responsible for the manufacture of the thermometers reversed the order before many were made.

Originally the scale was known as the centigrade scale, owing to the one hundred graduations between freezing and boiling. In 1948 the name was officially changed by the 9th General Conference on Weights and Measures. This was partly in honor of its inventor, but mostly to remove what was by then over a hundred and fifty years of confusion caused by the metric use of the centi prefix for 1/100th and many European languages' existing words for "degree" being similar to "grade". (Note: centigrade predates metric by about 50 years.)

To convert from °F to °C and vice-versa use these formulas (F = temp in °F, C = temp in °C).

C = (5/9) * (F - 32)

F = ((9/5) * C) + 32