A collection of tricks for converting all kinds of different units quickly in one’s head?

Question

I know that answerable questions are preferred and if this is closed that’s okay, but didn’t know a better place to ask.

I am sure that everyone has different ways of quickly thinking about unit conversions and if you shared some if your favourites perhaps this could turn into a comprehensive list of the best tips and tricks.

For a very basic example: divide pounds by 2, drop last number and subtract (approx -10%)

Answer 1

For converting between Celsius and Fahrenheit, what I find useful is to have memorized the table of multiples of $5^{\circ}{\rm C}$ in Fahrenheit.

Of course, everybody should know the freezing and boiling points $0^{\circ}{\rm C} = 32^{\circ}{\rm F}$ and $100^{\circ}{\rm C} = 212^{\circ}{\rm F}$. But besides that, I mostly use the $10 \rm{-} 30^{\circ}{\rm C}$ range. So:

• $10^{\circ}{\rm C} = 50^{\circ}{\rm F}$
• $15^{\circ}{\rm C} = 59^{\circ}{\rm F}$
• $20^{\circ}{\rm C} = 68^{\circ}{\rm F}$ (room temperature)
• $25^{\circ}{\rm C} = 77^{\circ}{\rm F}$
• $30^{\circ}{\rm C} = 86^{\circ}{\rm F}$

Then you need to know that an increment of $1^{\circ}{\rm C}$ correspons to an increment of $1.8^{\circ}{\rm F}$.