How to properly store and handle different units of measure for ingredients in a foodservice CMS?

I have a backend system for restaurants and food. At the front end, people use a huge number of units for recipes and such (kilogram, cup, gallon, pint, small spoon, large spoon, pinch of salt, dash of pepper, etc.). But I need to transform all of this into something small and indivisible in order to do calculations, reports and analytics and backend. However, in order to show some information and reports to the user, I have to convert all the data back to "convenient" units.

So it can lead to numerical errors (rounding errors, etc.).

What is the best practice for solving this problem? Thank!