Fahrenheit To Celsius - How To Convert

Article Category: Units  |   

A temperature gauge showing fahrenheit and celsius

In this article, we will show you the easiest and most accurate ways to convert temperatures from Fahrenheit to Celsius and from Celsius to Fahrenheit.

At first glance, the two scales do not seem to relate to each other at all and this is because of the different methodologies of the scientists after which they are named.

As a note, before you start reading. You will find, on the second page of this article, some handy reference charts and a converter, to allow you to quickly convert between the units. Quick links to those are included below.

The Fahrenheit scale

Fahrenheit icon

The German physicist Daniel Gabriel Fahrenheit (1686– 1736) came first. In 1714, he invented the mercury-in-glass thermometer that 300 years on is still one of the most accurate and most widely used means of measuring temperature.

Ten years later, Fahrenheit first proposed his temperature scale that set freezing point at 32 degrees (32°F) and water boiling point at 212°F.

Why these points? For a start, Fahrenheit realised that the temperature at which water starts to freeze is a fair bit warmer than the coldest temperature measurable, which he calibrated by placing a thermometer into a mixture of iced water and ammonium chloride salt. This formed his zero point.

He then observed that the temperature at which ice starts to form on the surface of water was 32 degrees warmer than zero. His third reference point was human body temperature (measured by placing the thermometer in the mouth or under the arm), which he calibrated at 96 degrees (later refined to 98.6°F)

Fahrenheit’s theoretical ultimate temperature was the boiling point of mercury, calculated at 600 degrees. It was work by others that showed that on Fahrenheit’s scale, the boiling point of water is about 179 degrees above its freezing point and this was standardised to 180 degrees, which is a nicely rounded (composite) number that is divisible by 2, 3, 4, 5, 6, 8, 9, and 10.

The Celsius scale

Celsius icon

The year Fahrenheit published his original scale, young Anders Celsius (1701 – 1744) was a budding physicist, astronomer and mathematician. He went on to found Uppsala Astronomical Observatory in his native Sweden and a year later, in 1742, proposed his own temperature scale.

By this point, Celsius had done a lot of research on temperature observing, for instance, that the boiling point of water varies with barometric pressure.

Since he was aiming for a scale that could be used worldwide, Celsius set water boiling and freezing points exactly 100 degrees apart, calling his scale Centigrade from the Latin for ‘100 steps’. Eccentrically, Celsius set freezing point at 100°C and water boiling point at 0°C. After the Great Man’s death, fellow Swede Carl Linnaeus suggested to the Swedish Academy that this be reversed and a more intuitive scale was adopted in 1745.

Kelvin scale

We should also note that most scientists and engineers would not use either of these scales but would prefer a third one, the kelvin system developed by British physicist William Thomson, Baron Kelvin. This is based on recognition that heat is actually caused by movement of molecules inside a substance, with a theoretical Absolute Zero point at which there is no molecular activity at all. Just how cold this is can be understood when you note that on the Kelvin scale, water freezing (actually called the ‘triple point’ at which water, ice and water vapour can all co-exist) occurs at 273.16K, Boiling point is set exactly 100 points higher at 373.16K. Thus the kelvin represents the same magnitude of heat as the degree Celsius.

Switching from Fahrenheit to Celsius

From the mid-18th to mid-20th centuries, Fahrenheit was the system most widely used in English-speaking countries, while continental Europeans preferred ‘The Swedish Scale’. With the drive to standardisation and metrication from the 1950s, Celsius became the adopted temperature scale for the EEC (later EU), with the UK and Ireland converting (somewhat reluctantly) after joining the European community.

Lady sitting under parasol on a sun-kissed beach
Is it 88°F or 31.1°C? You decide...

This reluctance was partly due to human preference. For while the Celsius scale was developed for convenience, Fahrenheit’s scale was based on observation and measurement. Also, whereas Fahrenheit had developed a ratio scale, Celsius had produced an interval scale. These are very different things, as we shall explain in a future article.

This may help explain why many people prefer Fahrenheit readings as rather closer to actual experience. In Celsius, a chilly day at 11°C does not seem greatly different from a really hot one at 35°C. On the Fahrenheit scale, these respectively measure at about 52°F and 95°F, with the latter traditionally taken as license for red top tabloids to start running 48 point headlines containing words like ‘Phew’, ‘Flaming’ and ‘Scorcher’.

Incidentally, while the UK’s Met Office started publishing temperatures in both Celsius and Fahrenheit from 1962 and dropping use of Fahrenheit altogether from official reports in 1970, British media persisted in using Fahrenheit in weather reports well into the 21st century. Indeed it became the fashion to use Celsius when describing extreme winter conditions and Fahrenheit for heat waves - a nice example of double standards in action.

Thermometer readings

On the next page we look at the formulae for converting between Fahrenheit and Celsius, the most effective methods for quickly estimating conversions and we feature reference charts for comparison and a Fahrenheit and Celsius converter to help you check your conversions.

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Last update: 18 July 2016

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