Conversion of Temperature

Quick Celsius (°C) / Fahrenheit (°F) Conversion:

Conversion Tool

Just type a value in either box:

°C:
°F:

 

Or use the Interactive Thermometer,

 

Or this method:

°F to °C Deduct 32, then multiply by 5, then divide by 9
°C to °F Multiply by 9, then divide by 5, then add 32

(Explanation Below ...)

Typical Temperatures

°C °F Description
100 212 Water boils
40 104 Hot Bath
37 98.6 Body temperature
30 86 Beach weather
21 70 Room temperature
10 50 Cool Day
0 32 Freezing point of water
−18 0 Very Cold Day
−40 −40 Extremely Cold Day
(and the same number!)
(bold are exact)

16 is about 61
28 is about 82

Explanation

There are two main temperature scales:

  • °F, the Fahrenheit Scale (used in the US), and
  • °C, the Celsius Scale (part of the Metric System, used in most other countries)

They both measure the same thing (temperature!), but use different numbers:

  • Boiling water (at normal pressure) measures 100° in Celsius, but 212° in Fahrenheit
  • And as water freezes it measures 0° in Celsius, but 32° in Fahrenheit

Like this:

Looking at the diagram, notice:

  • The scales start at a different number (0 vs 32), so we will need to add or subtract 32
  • The scales rise at a different rate (100 vs 180), so we will also need to multiply

And so, to convert:

from Celsius to Fahrenheit: first multiply by 180/100, then add 32
from Fahrenheit to Celsius: first subtract 32, then multiply by 100/180

 

But 180/100 can be simplified to 9/5,
and 100/180 can be simplified to 5/9, so this is the easiest way:

°C to °F   Multiply by 9, then divide by 5, then add 32
°F to °C   Deduct 32, then multiply by 5, then divide by 9

We can write each as a formula like this:

Celsius to Fahrenheit:   (°C × 9/5) + 32 = °F
Fahrenheit to Celsius:   (°F − 32) x 5/9 = °C

Example: Convert 26° Celsius (a nice warm day) to Fahrenheit

First: 26° × 9/5 = 234/5 = 46.8
Then: 46.8 + 32 = 78.8° F

Example: Convert 98.6° Fahrenheit (normal body temperature) to Celsius

First: 98.6° − 32 = 66.6
Then: 66.6 × 5/9 = 333/9 = 37° C

 

Other Methods That Work

Use 1.8 instead of 9/5

9/5 is equal to 1.8, so we can also use this method:

Celsius to Fahrenheit:   °C × 1.8 + 32 = °F
Fahrenheit to Celsius:   (°F − 32) / 1.8 = °C

To make "×1.8" easier we can multiply by 2 and subtract 10%, but it only works for °C to °F:

Celsius to Fahrenheit:   (°C × 2) less 10% + 32 = °F

Example: Convert 20° Celsius (A nice day) to Fahrenheit

  • 20x2 = 40
  • less 10% is 40−4 = 36
  • 36+32 = 68° F

 

Add 40, Multiply, Subtract 40

Since both scales cross at −40° (−40° C equals −40° F) we can:

  • add 40,
  • multiply by 5/9 (for °F to °C), or 9/5 (for °C to °F)
  • subtract 40

Example: Convert 10° Celsius (A cool day) to Fahrenheit

  • 10+40 = 50
  • 50×9/5 = 90
  • 90−40 = 50° F

To remember 9/5 for °C to °F think "F is greater than C, so there are more °F than °C"

 

Quick, but Not Accurate

Celsius to Fahrenheit:   Double, then add 30
Fahrenheit to Celsius:   Subtract 30, then halve

Examples °C → °F:

  • 0° C → 0+30 → 30° F (low by 2°)
  • 10° C → 20+30 → 50° F (exact!)
  • 30° C → 60+30 → 90° F (high by 4°)
  • 180° C → 360+30 → 390° F (high by 34°, not good)

Examples °F → °C:

  • 40° F → 10/2 → 5° C (almost right)
  • 80° F → 50/2 → 25° C (low by about 2°)
  • 120° F → 90/2 → 45° C (low by about 4°)
  • 450° F → 420/2 → 210° C (low by about 22°, not good)

 

Footnote: Temperature is a measure of how fast an objects particles are moving.