When I first came over the United States, I was faced with new units for almost every measurement. One of the more common ones was temperature, which could be frustrating when I was talking to relatives and trying to tell them how hot (or cold) it was. I knew there was a conversion formula that involved 9/5 (or was it 5/9?), but I never managed to remember which way was which.
Passing the Northern Trust bank at Adams and Wacker most days, I get to see their time/temperature display which, like most of them, shows both Fahrenheit and Celsius. That inspired me to make the effort to learn how to make the conversion in my head.
The temperature in Celsius is defined to be 0° at the melting point of water and 100° at the boiling point. The equivalent range in Fahrenheit is from 32° to 212°. That means that a 100 degree interval in Celsius is equivalent to 180 [212 - 32] degrees Fahrenheit so 1.8 degrees Fahrenheit equals one degree Celsius. (1.8 is the same number as 9/5 but I find the decimal version easier to remember.)
If you're converting temperatures you need to normalize for the 32 degree offset. When converting to Fahrenheit, you need to add 32; when going the other way, you subtract the 32.
- Fahrenheit to Celsius
- c = (f - 32) / 1.8
- Celsius to Fahrenheit
- f = c * 1.8 + 32
To convert quickly in my head I just round the numbers, so
- Fahrenheit to Celsius
- subtract thirty, then halve
- Celsius to Fahrenheit
- double, then add thirty
The approximations are good enough to get an intuitive idea of how hot or cold it is. For normal temperatures that we humans can tolerate, they're usually within a few degrees. For more extreme temperatures, they're really not very accurate.
| Fahrenheit | Approximate | Exact |
|---|---|---|
| 30°F | 0°C | -1.1°C |
| 50°F | 10°C | 10°C |
| 70°F | 20°C | 21.1°C |
| 90°F | 30°C | 32.2°C |
| 110°F | 40°C | 43.3°C |
I have a friend in Canada originally from the Czech Republic. She does the same approximate thing you do in her head --subtract thirty and halve the result.
Very logical and easy to remember. Hats off to you for discovering this eary to caluculate method .
In the past, I struggled to remember the conversion formula, but using the method you outlined here, I can now convert temperatures in my head. Thanks!
thanks for the approximation formula, its really great
The "double and add 30" method is very good, but if you can remember three steps you can be more consistently right:
1. double
2. discount (take ten percent off)
3. add 32.
example: 10 degrees celsius
1. double: 10 x 2 = 20
2. discount: 10% of 20 = 20 so 20 - 2 = 18
3. add 32 (18 + 32 = 50)