How to convert between fahrenheit and celsius

How to Convert Between Fahrenheit and Celsius

In every science class, you must know how to use and interpret the Celsius temperature scale. But this can be difficult for students who are used to using Fahrenheit instead. How do you convert Celsius to Fahrenheit? What about Fahrenheit to Celsius?

We answer these questions below, giving you the mathematical formulas for converting between these two temperature units, a handy conversion chart, and a quick conversion trick you can use without having to grab a calculator.

Celsius vs Fahrenheit: Key Differences

Before we explain how to convert Celsius to Fahrenheit (and Fahrenheit to Celsius), let’s review the main differences between the two temperature scales.

Celsius (written as °C and also called Centigrade) is the most common temperature scale in the world, used by all but five countries. It’s part of the International System of Units (SI), or what you might know as the metric system, which is typically used in science classes (think centimeters, meters, kilograms, milliliters, etc.) and in science as a whole.

By contrast, Fahrenheit (written as °F) is only used officially by five countries in the world:

  • United States
  • Belize
  • Cayman Islands
  • Palau
  • Bahamas

How to Convert Celsius to Fahrenheit Quickly

How to Convert Between Fahrenheit and Celsius

Measuring temperature on a day-to-day basis is generally pretty simple: you’re either warm or cold, and the specific numbers really don’t matter all that much. In science class, however, temperature becomes all about the numbers.

Suddenly, you’re expected not only to quantify temperature, but to attach one of two units: Fahrenheit or Celsius. To make matters worse, you’ll have to learn how to convert between these two measurements.

If you’re sick of reaching for a calculator every time you have to deal with these units, you’re in the right place. Read on to find out how to quickly convert Celsius to Fahrenheit.

What is Celsius?

Let’s start with the basics: what is celsius? Celsius—sometimes called the centigrade scale—is the most common method of quantifying temperature. It is employed by the International System of Units, or the metric system, which is used in all countries except:

  • United States
  • Bahamas
  • Belize
  • Cayman Islands
  • Liberia

Named after Anders Celsius, this system has been around since the early 1700s. In today’s form, you will see it depicted as a degree symbol followed by a capital ‘c’ ( °C).

What is Fahrenheit?

If you live in one of the five countries listed above, you’re probably already familiar with Fahrenheit. The Fahrenheit system’s zero degrees (written as 0°F) came about from the temperature of brine, a combination made from equal parts ice, water, and salt.

Fahrenheit to Celsius conversion

How to Convert Between Fahrenheit and Celsius

22 C = 71 F on a gauge with both Celsius and Fahrenheit readings

© Public Domain/Wikimedia Commons

How hot is it today? Well, that depends where you are — in more ways than one. If you are in a hot country, the temperature is higher, and in a cold country the temperature is lower. Okay, so what else is new? Well there is also the issue of how we measure temperature.

In this step we have a look at the important conversion between the Celsius and Fahrenheit systems, illustrating the idea of a linear relation which is not a direct proportionality.

Celsius versus Fahrenheit

In most countries around the world, temperature is measured in the Celsius system, where water freezes at (
ormalsize{0}) and boils at (
ormalsize{100}).

Such countries include those in Europe, most of Asia and Africa, and Australia.

However there are some countries, including notably the USA, where the Fahrenheit system is used, with water freezing at (
ormalsize{32}) and boiling at (
ormalsize{212}). Canada uses a mix of Celsius and Fahrenheit.

So a natural question is: how do we convert from one temperature to another? One way is with a chart:

Celsius Fahrenheit
32
10 50
20 68
30 86
40 104
50 122
60 140
70 158
80 176
90 194
100 212

A more visual way is to plot corresponding points. Let’s denote the two variables by (
ormalsize{F}) and (
ormalsize{C}), standing (of course) for Fahrenheit and Celsius respectively.

A linear relationship

The value (
ormalsize{F=32}) goes with (
ormalsize{C=0}), while the value (
ormalsize{F=212}) goes with the value (
ormalsize{C=100}). So let’s make a graph.

Suppose we want primarily to convert from Celsius values to Fahrenheit. Then we would treat (
ormalsize{C}) as the horizontal variable, and (
ormalsize{F}) as the vertical variable. Here is then our (
ormalsize{C-F}) plane, with the two points that we know:

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For the line (
ormalsize{F=mC+b}), the (
ormalsize{F})-intercept is (
ormalsize{b}), which we can see is (
ormalsize{32}). How about the slope of the line? It is

[Large{m=frac{Delta F}{Delta C} = frac{212-32}{100-0}= frac{9}{5}.}]

So

[Large{F=frac{9}{5}C+32}.]

Converting temperatures

Let’s get some practice converting from one system to the other. You can use the formula, or possibly eyeball things from the graph of the line.

  • Q1 (E): (room temperature) So using our formula, find (
    ormalsize{F}) when (
    ormalsize{C=22}).
  • Q2 (E): (hot day) Find (
    ormalsize{F}) when (
    ormalsize{C=30}).
  • Q3 (E): (cold day) Find (
    ormalsize{F}) when (
    ormalsize{C=-10}).

© “Sahara Desert” by wonker/Flickr CC BY 2.0

Q4 (E): (hottest day on record) Find (
ormalsize{F}) when (
ormalsize{C=56.7}) (recorded on 10 July 1913 at Greenland Ranch, Death Valley, California, USA).

Q5 (M): (coldest day on record) When (
ormalsize{F=−128.6}) find (
ormalsize{C}) (recorded on 21 July 1983 at the Soviet Vostok Station in Antarctica).

Fahrenheit to Celsius

Q6 (M): Rearrange the equation ({
ormalsize F=frac{9}{5}C+32}) to find (
ormalsize{C}) in terms of (
ormalsize{F}).

Answers

A1. (room temperature) (
ormalsize{F=71.6})

  1. A2. (hot day) (
    ormalsize{F=86})
  2. A3. (cold day) (
    ormalsize{F=14})
  3. A4. (hottest day) (
    ormalsize{F=134})

A5. (coldest day) (
ormalsize{C=-89.2})

A6. You can (if you already know how) construct the line using the two points on the graph. A faster method is to rearrange the equation ({
ormalsize F = frac{9}{5}C + 32}) into

[{Large C=frac{5}{9}F-frac{160}{9}.}]

We could also rewrite this as

[{Large C=frac{5}{9}(F-32).}]

How to Convert Celsius to Fahrenheit Without a Calculator

You know that moment when it's time to cook, but the recipe you want to make provides temperature information that you're unfamiliar with? Don't worry, when you don't have a calculator on hand, there are other ways how to convert Celsius to Fahrenheit. 

What's the difference between Celsius and Fahrenheit? 

David Zambuto

Celsius and Fahrenheit are two measurements of temperature. Key differences between the two are that the boiling point of water is 212 degrees Fahrenheit and 100 degrees Celsius and that the freezing point of water is at 32 degrees Fahrenheit and 0 degrees Celsius. 

Why do some recipes use Celsius and others Fahrenheit? 

Camilla Caffo Recipes either appear in Celsius and/or Fahrenheit due to their country of origin's use of the metric system (as Europe does) or imperial units (as the United States does). All countries in the world except for the United States, Myanmar and Liberia use the metric system.

What are some ways to convert Celsius to Fahrenheit without a calculator? 

David Zambuto

Without a calculator, there are many means to convert Celsius to Fahrenheit.

1. By Hand

David Zambuto

Multiply the Celsius temperature by 1.8 and add 32 to get the Fahrenheit conversion With this method you get the exact temperature conversion degree. 

  • Example: What is the temperature in Fahrenheit from 10 degrees celsius? 
  • (10*C x 9/5) + 32= *F
  • (10*C x 1.8) + 32= *F
  • (18) + 32 = *F
  • 50= F
  • You find that 10 degrees Celsius is equal to 50 degrees Fahrenheit. 
  • 2. Visual Conversion

Brooke Buchan

Use an infographic chart to visually convert Celsius to Fahrenheit. With this method, there are no mathematic calculations necessary. 

3. Unconventional Mathematics Formula 

David Zambuto

Multiply the Celsius degree by 2, subtract by the first digit of the Celsius temperature then add 32. It's a unconventional method but simpler in comparison to using the value 1.8 by hand. Please note that using this method may require subtracting a degree or two for a complete accurate conversion. 

  1. Example: What is the temperature of Fahrenheit from 10 degrees Celsius? 
  2. ((10*C x 2) – 1 ) + 32 = ?F
  3. ((20)-1)) + 32 = ?F
  4. 19 + 32 =?F
  5. 51=?F
  6. You find that 10 degrees Celsius is roughly equal to 51 degrees Fahrenheit. 

4. Reference Recipes with Similar Conversions

Missy Miller

Consult other recipes that contain similar temperature units. Recipes often account for differences in temperature units, so they list the degrees in Fahrenheit and Celsius, which provides a method that's free of any mathematics. 

How does one convert with a calculator?

How to Convert Fahrenheit to Celsius

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.

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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.

First, you need the formula for converting Fahrenheit (F) to Celsius (C):

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.

  1. Subtract 32 from the Fahrenheit temperature.
  2. Multiply this number by five.
  3. 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:

  1. 80 F – 32 = 48
  2. 5 x 48 = 240
  3. 240 / 9 = 26.7 C

So the temperature in Celsius is 26.7 C.

If you want to convert a normal human body temperature (98.6 F) to Celsius, plug the Fahrenheit temperature into the formula:

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

Fahrenheit To Celsius Formula, Charts and Conversion

By Nick Valentine|
Last update: 17 June 2017

In -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

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

Fahrenheit to Celsius

Format DecimalFractions

Accuracy Select resolution 1 significant figure 2 significant figures 3 significant figures 4 significant figures 5 significant figures 6 significant figures 7 significant figures 8 significant figures

Note: Fractional results are rounded to the nearest 1/64. For a more accurate answer please select 'decimal' from the options above the result.

Note: You can increase or decrease the accuracy of this answer by selecting the number of significant figures required from the options above the result.

Note: For a pure decimal result please select 'decimal' from the options above the result.

There's a simple rule to convert Fahrenheit to Celsius that should be good enough for general use. Simply take 30 off the Fahrenheit value, and then half that number.

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Note that this value isn’t perfect, but it might save you having to reach for a calculator (or our site!)

Absolute Zero

Parity

Zero

Freezing point

Body Temperature

Boiling point

-459.67°F -273.15°C
-40°F -40°C
0°F -17.78°C
32°F 0°C
98.6°F 37°C
212°F 100°C

Definition of Fahrenheit and Celsius

In the Fahrenheit scale, water freezes at 32 degrees, and boils at 212 degrees. Boiling and freezing point are therefore 180 degrees apart. Normal body temperature is considered to be 98.6 °F (in real-life it fluctuates around this value). Absolute zero is defined as -459.67°F.

The Celsius scale is nowadays set in such a way that Zero degrees C is the temperature at which ice melts (note : not the temperature at which it freezes, which is different!) . At the other end of the scale, 100 degrees Celsius is the boiling point of water.

The scientific definition of Celsius is now defined against degrees Kelvin. Zero degrees Celsius is 273.15K. One degree Celsius is equal to one Kelvin, so we can say that the boiling point of water is equal to 273.15 + 100 = 373.15 Kelvin.

Fahrenheit to Celsius formula

  • °C =
  • °F – 32
  • ______
  • 1.8000

Most things we measure – length, width, time etc. have one thing in common – their values all start from zero.

We all know exactly how long zero centimeters or inches is, and can convert zero of any of those units into another type of unit very easily. Zero centimeters = zero meters = zero inches. Taking inches and centimeters as an example, to go from zero inches to 1 inch we need to add one inch. So far, so obvious.

Similarly, to go from zero centimeters to 1 centimeter, we need only add 1 centimeter. The only difference between adding one inch or one centimetre is the amount of distance we're adding. The relationship between an inch and a centimetre is that 1 inch is 2.54 centimetres.

So we can say that adding 1 inch is the same as adding 2.54 centimetres. Because they both start at zero, the formula to convert between the two very easy (in = cm * 0.39370)

Temperature units aren't built in the same simple way, because they don't all start in the same place at zero. If we pegged absolute zero to be 0°F, 0°C and 0K, converting between them would be much easier, but Fahrenheit and Celsius were defined before we could tell where absolute zero was, and as a result Fahrenheit, Celsius and Kelvin all start from different values.

Because these temperature units don’t share a common zero point, we need to add or subtract an offset before we do our division or multiplication. It's not a difficult extra step, but it seems to be something that can cause confusion. As a rough rule of thumb:
To go from Fahrenheit to Celsius, take 30 off the Fahrenheit value, and then half that number.

For a 100% accurate answer, subtract 32 and divide by 1.8 (or use the calculator above!)

Start

Increments Increment: 1000 Increment: 100 Increment: 20 Increment: 10 Increment: 5 Increment: 2 Increment: 1 Increment: 0.1 Increment: 0.01 Increment: 0.001 Fractional: 1/64 Fractional: 1/32 Fractional: 1/16 Fractional: 1/8 Fractional: 1/4 Fractional: 1/2

Accuracy Select resolution 1 significant figure 2 significant figures 3 significant figures 4 significant figures 5 significant figures 6 significant figures 7 significant figures 8 significant figures

Format DecimalFractions

Print table < Smaller Values Larger Values >

Fahrenheit Celsius
0°F -17.78°C
1°F -17.22°C
2°F -16.67°C
3°F -16.11°C
4°F -15.56°C
5°F -15.00°C
6°F -14.44°C
7°F -13.89°C
8°F -13.33°C
9°F -12.78°C
10°F -12.22°C
11°F -11.67°C
12°F -11.11°C
13°F -10.56°C
14°F -10.00°C
15°F -9.44°C
16°F -8.89°C
17°F -8.33°C
18°F -7.78°C
19°F -7.22°C
Fahrenheit Celsius
20°F -6.67°C
21°F -6.11°C
22°F -5.56°C
23°F -5.00°C
24°F -4.44°C
25°F -3.89°C
26°F -3.33°C
27°F -2.78°C
28°F -2.22°C
29°F -1.67°C
30°F -1.11°C
31°F -0.56°C
32°F 0.00°C
33°F 0.56°C
34°F 1.11°C
35°F 1.67°C
36°F 2.22°C
37°F 2.78°C
38°F 3.33°C
39°F 3.89°C
Fahrenheit Celsius
40°F 4.44°C
41°F 5.00°C
42°F 5.56°C
43°F 6.11°C
44°F 6.67°C
45°F 7.22°C
46°F 7.78°C
47°F 8.33°C
48°F 8.89°C
49°F 9.44°C
50°F 10.00°C
51°F 10.56°C
52°F 11.11°C
53°F 11.67°C
54°F 12.22°C
55°F 12.78°C
56°F 13.33°C
57°F 13.89°C
58°F 14.44°C
59°F 15.00°C

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