Originally shared on the public FactorioPrints / Factorio School archive (factorioprints.com · factorio.school). Authorship preserved from the public catalog.

Simple Percentage Calculator

Instructions:

Put what your target value is in the Constant Combinator to the right of the lights. This will be your 100%.
In the multiply Arithmetic Combinator, put what you want to track in the first slot. In the screenshot's case I wanted to track Uranium Ore.
Connect your network to the same Arithmetic Combinator.
[OPTIONAL]: Hover your mouse over the division Arithmetic Combinator. The output signal is the percentage.
[OPTIONAL]: Change the color using the Constant Combinator to the left.
[OPTIONAL]: Decider Combinator to the right can be used as a binary output.