I have a conversion app for stuff like this.
Download Convert Any Unit Lite by Cider Software LLC on the App Store. See screenshots, ratings and reviews, user tips, and more games like Convert Any Unit…
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.
Download Fraction Calculator Plus #1 by DigitAlchemy LLC on the App Store. See screenshots, ratings and reviews, user tips, and more games like Fraction…
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.