Your requirement isn't really clear. A numerator and denominator of what?
What is your numerator? What is your denominator?
If you'd like specific guidance you will need to upload a Tableau Packaged Workbook including sample data as well.
I can't really provide you with specific advice without a workbook.
A numerator and a denominator are both just components in a fraction. They could be the count of something or the sum of something, you haven't specified that at all.
If I want to know the total of profit divided by sales, then my Numerator is SUM(Profit) and my denominator is SUM(Sales). I would implement that in a calculated fieldin tableau as SUM(Profit) / SUM(Sales)