I have a conversion app for stuff like this.
Over 4 million downloads! The FASTEST way possible to convert units. Just type in a unit without first selecting the category. As you type, a list of units appear. Tap a unit to select it. For most units, 1 or 2 letters is enough to find the unit you want. Automatically saves units into a...
apps.apple.com
It will convert millimeters to inches, US dollars to Mexican pesos, or nearly anything else.
I also have a calculator that works with fractions.
Fraction Calculator Plus - the best and easiest solution for fractions in everyday life. Calculate fractions: add, subtract, divide and multiply. Convert fractions to decimals easily. - Would you like time to check your children's maths exercises? Now that's no longer a problem because you...
apps.apple.com
For stuff I deal with routinely, I don’t need either. For stuff that’s outside my usual usage or to confirm what I “think” is right or for fractional calculations that get irritatingly cumbersome I like them very much. I don’t use them a ton so I can put up with the ads in exchange for free. If I used them more, I’d pay for them.