Conversion of Temperature
Quick Celsius (°C) / Fahrenheit (°F) Conversion:Just type a value in either box:
Or use the Interactive Thermometer,
Or this method:

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 this is how it works out:
To convert from Celsius to Fahrenheit, first multiply by 180/100, then add 32 
To convert from Fahrenheit to Celsius, first subtract 32, then multiply by 100/180 
Note: 180/100 can be simplified to 9/5, and likewise 100/180=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 that 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 you could 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 you 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 404 = 36
 36+32 = 68° F
Add 40, Multiply, Subtract 40
Since both scales cross at 40° (40° C equals 40° F) you 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
 9040 = 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)