There are two main temperature scales:

°C, the Celsius Scale (part of the Metric System, used in most countries)

°F, the Fahrenheit Scale (used in the US)

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

measure/images/thermometer.js?mode=boxes

Type a value in either box

Or use the slider

Or the Interactive Thermometer

Or this method:

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

()

(only bold are exactly the same)

    °C °F Description
    220   430   Hot Oven  
    180   360   Moderate Oven  
    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!)  

For these three conversions we can "flip the digits" (accurate within 1°):

°C   °F
28 ⇄ 82
16 ⇄ 61
04 ⇄ 40

Oven Temperatures in the range 150°C to 200°C: double °C to get °F (accurate within 8°F):

°C   °F
Estimate   °F
Actual  
200   400   392  
180   360   356  
160   320   320  
150   300   302  

Going the other way: for the range 300°F to 400°F we can halve °F to get °C (accurate within 4°C).

Celsius and Fahrenheit 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 180100, then add 32

from Fahrenheit to Celsius: first subtract 32, then multiply by 100180

180100 can be simplified to 95, and 100180 can be simplified to 59, so we get this:

°C to °F: Divide by 5, then multiply by 9, then add 32

°F to °C: Subtract 32, then multiply by 5, then divide by 9

Example: Convert 25° Celsius (a nice warm day) to Fahrenheit

First: 25° / 5 = 5
Then: 5 × 9 = 45
Then: 45 + 32 = 77°F

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

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

We can swap the order of divide and multiply if we want, but don't change the add or subtract. So this is also OK:

Example: Convert 98.6° Fahrenheit to Celsius (again)

First: 98.6° − 32 = 66.6
Then: 66.6 / 9 = 7.4
Then: 7.4 × 5 = 37° C

(Same answer as before, was it easier or harder this way?)

Celsius to Fahrenheit:   (°C × 95) + 32 = °F
Fahrenheit to Celsius:   (°F − 32) × 59 = °C

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

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

Like this:

Celsius to Fahrenheit:   Add 40, multiply by 9/5, then subtract 40
Fahrenheit to Celsius:   Add 40, multiply by 5/9, then subtract 40

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

10+40 = 50

50×9/5 = 90

90−40 = 50° F

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 object's particles are moving.

1041, 1042, 1043, 1044, 3724, 3725, 3726, 3727, 3728, 3729

Copyright © 2025 Rod Pierce