What are some of the most useful unit conversions?

I'm building a converter App(Yes I know it a little over done but I want to start off with something basic) and I want some people's opinion about what type of conversions that I should put in there. I know some of the most basic would be meter to kilometer, degrees to celsius, and cups to liters. But I want to offer a bit more variety to people.What are some of the most useful unit conversions?
Absolute temperatures. An item that is cooled to absolute 0 has no molecular movement. 0 Rankine = -459.99 degrees F and 0 Kelvin is - 273.16 degrees C



Pressures:

Converting between PSI and MPa

converting mm of Mercury and PSI



Converting money: US dollars, Euro, Yen, Pound Sterling, and so on. However, keep in mind these fluctuate.



The ounce used for the value of gold is not the same as the ounce used for other things such as hamburger, soap, etc. see link below. Troy ounce is for precious metals.What are some of the most useful unit conversions?
The easiest are celsius to fahrenheit, and fahrenheit to celsius, because these are constants.

Students probably need millimeters to centimeters and then to decimeters and so on.

The third is the group for liquid measure.

dermalogica