Temperature Conversion

Throughout the world, there are many different units of measuring temperature. The three most important ones are Fahrenheit (F), Kelvin (K), and Celsius (C). 

Key Facts & Summary

  • Celsius and Fahrenheit are degree scales. The degree symbol is not used to report temperature using the Kelvin scale, instead, they are noted as Kelvins.
  • Water boils at 100 degrees Celsius or 212 degrees Fahrenheit, or 373.15 Kelvins.
  • Water freezes at 0 degrees Celsius or 32 Degrees Fahrenheit, or 273.15 Kelvins.
  • Absolute zero is 0 Kelvins. This is the lowest temperature any substance can drop down to.
  • Celsius and Fahrenheit are the same at – 40 degrees since the scales converge.
  • Celsius and Kelvins become equal at high temperatures as the difference of 273.15 between them gets lost in the noise.
  • 0 degrees Celsius is equal to 32 degrees Fahrenheit. The basic formula is (°C × 9/5) + 32 = °F.
  • 0 degrees Celsius is equal to 273.15 Kelvins. The basic formula is °C + 273.15 = K.
  • The basic formula for converting Fahrenheit into Celsius is (°F − 32) × 5/9 = °C.
  • To convert Fahrenheit degrees into Kelvins, (°F − 32) × 5/9 + 273.15 = K.
  • To convert Kelvins into Celsius degrees, the formula is K − 273.15 = °C, while the formula of converting Kelvins into Fahrenheit degrees is (K − 273.15) × 9/5 + 32 = °F.
  • Temperature can be simply defined as a measure of the hotness or coldness of an object.
  • Temperature is measured using a thermometer – we observe the effect of temperature on the substance inside it.

Temperature Conversion

Whether we use Celsius, Fahrenheit, or Kelvin in our efforts to measure temperatures, is not that important. However, if we find ourselves in certain regions where the temperature measurement is different, knowing the differences between the three most used systems may come in handy.

All three are correct ways of noting temperatures and they all can be derived from one another through formulas since they are related to each other.

But are these units of measurement any different from each other except for their different values? Yes and no, and here is a bit about every one of them.


Also called centigrade, is a scale based on 0 degrees for the freezing point of water and 100 degrees for the boiling point of water. This system was invented in 1742 by the Swedish astronomer Anders Celsius.

Some still call it the centigrade scale because of the 100-degree interval between the defined points. The Celsius scale is part of the metric system and is used to measure the temperature in many countries.

It is the easiest scale to use and though it is used throughout the world, there is one exception, the United States. In the US, the Fahrenheit scale prevails but the Celsius scale is also not accepted in certain places where Kelvin is used.

The normal human body temperature is 32°C, and the absolute zero value is set at -273.15°C, on this scale. To convert Celsius to Fahrenheit is pretty simple:

°C to °F: Multiply by 9, then divide by 5 and then add 32.

Example: How to convert 28°C to Fahrenheit (°F).
First step: 28°C × 9/5 = 252/5 = 50.4.
Second step: 50.4 + 32 = 82°F.

The Celsius scale and the Fahrenheit scale coincides at -40°, which is the same for both. When it comes to converting Celsius degrees into Kelvins, the formula is even simpler.

0 degrees Celsius is equal to 273.15 Kelvins. The basic formula is °C + 273.15 = K. Kelvin to Celsius: Add 273.

Example: How to convert 28°C to Kelvin (K).

First step: 28°C + 273.15 = 301.15 K


This temperature scale is based on 32 degrees for the freezing point of water, and 212 degrees for the boiling point. The interval between the two points is divided into 180 equal parts.

The German physicist Daniel Gabriel Fahrenheit devised this scale in 1724. The average human body temperature is 98.6°F, while the absolute zero is -459.67°F on this scale.

The Fahrenheit scale is accepted and is used in the United States and some of its territories. To convert Fahrenheit degrees into Celsius, here is the formula:

°F to °C: Subtract 32, then multiply by 5, then divide by 9. The basic formula is  (°F – 32) × 5/9 = °C or precisely (F – 32)/1.8.

Example: How to convert 98.6° Fahrenheit into Celsius (°C)
First step: 98.6°F – 32 = 66.6.
Second step: 66.6 × 5/9 = 333/9 = 37°C.

When it comes to converting Fahrenheit degrees into Kelvins, the formula is even simpler.

Subtract 32, multiply by 5, divide by 9, then add 273.15.

(°F − 32) × 5/9 + 273.15 = K.

Example: How to convert 98.6° Fahrenheit into Kelvin (K)

(98.6°F − 32) × 5/9 + 273.15 = 310.15K


This temperature scale is named after the British mathematician and physicist William Thomson Kelvin, who proposed it in 1848. It is an absolute temperature scale, having an absolute zero below which temperatures do not exist.

Each unit on this scale is called a Kelvin rather than a degree. For this reason, just the K, not the degree symbol °, is used when reporting temperatures in Kelvin. There are no negative numbers on the Kelvin scale, as the lowest number is 0 K.

Absolute zero cannot technically be achieved. It is the temperature at which molecules would stop moving – it is thus “infinitely cold.”

The freezing point of water in Kelvin is 273.15K and 373.15K the boiling point. The idea for the Kelvin scale was sparked by a discovery in the 1800s of a relationship between the volume and the temperature of a gas.

When it comes to the other scales, Kelvins and Celsius become equal at high temperatures as the difference of 273.15 between them gets lost in the noise.

To convert Kelvin into Celsius, the formula is quite simple. 

K to °C: Add 273

K − 273.15 = °C

Example: How to convert 35°C into Kelvin (K)

35°C + 273.15 = 308.15K

To convert Kelvin into Fahrenheit, the formula is:

(K − 273.15) × 9/5 + 32 = °F. 

Subtract 273.15, multiply by 1.8, then add 32.

Example: How to convert 75°F into Kelvin (K)

(75°F − 32) × 5/9 + 273.15 = 297.039K

Fun Facts

  1. The hottest temperature recorded on Earth is 57.8°C / 136°F, recorded in Al’Aziziyah, Lybia on September 13, 1922.
  2. The coldest temperature recorded on Earth is  -89.2°C / -128.6°F, recorded at Vostok Station, Antarctica on July 21, 1983.
  3. In the US, one of the highest temperatures ever recorded was 56.7°C / 134°F, recorded in Death Valley, California on July 10, 1913.
  4. In New Zealand, the highest temperature recorded is 42.4°C / 108.3°F, recorded in both Rangiora and Marlborough on February 7, 1973.
  5. In Africa, the lowest temperature ever recorded is -24°C / -11°F, recorded in Ifrane, Morocco on February 11, 1935.
  6. The inventor of the Kelvin scale also wrote the second law of thermodynamics, which states that heat will not flow from a colder body to a hotter body.

Conversion Formulas

  • Kelvin to Fahrenheit: Subtract 273.15, multiply by 1.8, then add 32
  • Fahrenheit to Kelvin: Subtract 32, multiply by 5, divide by 9, then add 273.15
  • Kelvin to Celsius: Add 273
  • Celsius to Kelvin: Subtract 273
  • Fahrenheit to Celsius: Subtract 32, multiply by 5, then divide by 9
  • Celsius to Fahrenheit: Multiply by 9, divide by 5, then add 32


Mercury thermometers

Not too long ago, thermometers had mercury inside the glass. If the temperature got hotter, the mercury expanded and moved up through the narrow tube.

We could see what the temperature was by reading it off the scale of numbers on the tube. However, mercury is very toxic and it was soon replaced with different liquids.

The concept remains the same regardless, as the temperature goes up, the liquid expands and rises, and when the temperature goes down, the liquid shrinks and drops down the tube.

Did you know?

  1. The Kelvin scale isn’t the only absolute temperature scale. There is also the Rankine temperature scale. It is used mostly by engineers.
  2. It is unknown if Daniel Gabriel Fahrenheit was a freemason, however, many believe that he was because there are 32 degrees of enlightenment,  and he chose to use 32 as the melting temperature of the water in his scale.
  3. Anders Celsius invented his temperature scale in 1742 using the mercury thermometer.


Image sources:

  1. https://static.bhphotovideo.com/explora/sites/default/files/styles/top_shot/public/Color-Temperature.jpg?itok=yHYqoXAf
  2. https://www.worldatlas.com/r/w1200-h701-c1200x701/upload/0c/ce/3f/shutterstock-618369506.jpg
  3. https://res.cloudinary.com/dk-find-out/image/upload/q_80,w_1920,f_auto/A-Carolina-Pontes-Daniel-Fahrenheit_zljtwo.jpg
  4. https://nationalmaglab.org/media/k2/items/cache/72d71c3e1a75301c5a2942cd172af179_L.jpg
  5. https://www.researchgate.net/profile/Augusto_Belendez2/publication/237266333/figure/fig21/AS:647921112449027@1531487839191/Figura-31-William-Thomson-Lord-Kelvin-desde-1892-18241907.png
  6. https://upload.wikimedia.org/wikipedia/commons/thumb/b/b1/Anders-Celsius-Head.jpg/1200px-Anders-Celsius-Head.jpg
  7. https://images.squarespace-cdn.com/content/5a9762f0b98a78c859db8f5e/1552325547898-ERZ1ED821S61UJ0HTZ8Q/Screen+Shot+2019-03-11+at+12.32.23+PM.png?content-type=image%2Fpng