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Fahrenheit and Celsius are the scales most often used for reporting room, weather, and water temperatures. The Fahrenheit scale is used in the United States, while the Celsius scale is used worldwide.
Indeed, most countries around the world measure their weather and temperatures using the relatively simple Celsius scale. But the United States is one of just a few remaining countries that use Fahrenheit, so it's important for Americans to know how to convert one to the other, especially when traveling or doing scientific research.
How to Convert Temperatures
First, you need the formula for converting Fahrenheit (F) to Celsius (C):
- C = 5/9 x (F-32)
The notation C represents the temperature in Celsius, and F is the temperature in Fahrenheit. After you know the formula, it is easy to convert Fahrenheit to Celsius with these three steps.
- Subtract 32 from the Fahrenheit temperature.
- Multiply this number by five.
- Divide the result by nine.
For example, suppose the temperature is 80 degrees Fahrenheit and you want to know what the figure would be in Celsius. Use the above three steps:
- 80 F - 32 = 48
- 5 x 48 = 240
- 240 / 9 = 26.7 C
So the temperature in Celsius is 26.7 C.
Fahrenheit to Celsius Example
If you want to convert a normal human body temperature (98.6 F) to Celsius, plug the Fahrenheit temperature into the formula:
- C = 5/9 x (F - 32)
As noted, your starting temperature is 98.6 F. So you would have:
- C = 5/9 x (F - 32)
- C = 5/9 x (98.6 - 32)
- C = 5/9 x (66.6)
- C = 37 C
Check your answer to ensure it makes sense. At ordinary temperatures, a Celsius value is always lower than the corresponding Fahrenheit value. Also, it's helpful to keep in mind that the Celsius scale is based on the freezing and boiling points of water, where 0 C is the freezing point and 100 C is the boiling point. On the Fahrenheit scale, water freezes at 32 F and boils at 212 F.
You often don't need an exact conversion. If you're traveling to Europe, for example, and you know the temperature is 74 F, you might want to know the approximate temperature in Celsius. The website Lifehacker offers this tip on making an approximate conversion:
Fahrenheit to Celsius: Subtract 30 from the Fahrenheit temperature and then divide by two. So, using the approximation formula:
- 74 F - 30 = 44
- 44 / 2 = 22 C
(If you go through the previous formula's calculations for the exact temperature, you arrive at 23.3.)
Celsius to Fahrenheit: To reverse the approximation and convert from 22 C to Fahrenheit, multiply by two and add 30. So:
- 22 C x 2 = 44
- 44 + 30 = 74 C
Quick Conversion Table
You can save even more time by using predetermined conversions. The Old Farmer's Almanac offers this table for making quick conversions from Fahrenheit to Celsius.
|-40 F||-40 C|
|-30 F||-34 C|
|-20 F||-29 C|
|-10 F||-23 C|
|0 F||-18 C|
|10 F||-12 C|
|20 F||-7 C|
|32 F||0 C|
|40 F||4 C|
|50 F||10 C|
|60 F||16 C|
|70 F||21 C|
|80 F||27 C|
|90 F||32 C|
|100 F||38 C|
Note how the Fahrenheit and Celsius scales read the same temperature at -40.
Invention of Fahrenheit
While you're mastering these conversions, it might be interesting to learn how the Fahrenheit temperature scale came into existence. The first mercury thermometer was invented by German scientist Daniel Fahrenheit in 1714. His scale divides the freezing and boiling points of water into 180 degrees, with 32 degrees as water's freezing point, and 212 as its boiling point.
On Fahrenheit's scale, zero degrees was determined as the temperature of a temperature-stable brine solution of ice, water, and ammonium chloride. He based the scale on the average temperature of the human body, which he originally calculated at 100 degrees. (As noted, it's since been adjusted to 98.6 degrees Fahrenheit.)
Fahrenheit was the standard unit of measure in most countries until the 1960s and 1970s when it was replaced with the Celsius scale in a widespread conversion to the more useful metric system. In addition to the United States and its territories, Fahrenheit is still used in the Bahamas, Belize, and the Cayman Islands for most temperature measurements.