Learn English – How to say 0.01% in a formal way

If you just want to know how to say the number, the convention that scientists and engineers use is to call out each digit to the right of the decimal point. For your example it would be:

zero point zero zero zero one seven eight two percent

This generally not done for numbers to the left of the decimal point. For example, if you wanted to say 1,234.567, it would be:

one thousand two hundred and thirty four point five six seven

Alternatively, you can just call out all the digits:

one two three four point five six seven