I always take the middle class as the threshold where you have sufficient passive income to afford a dignified lifestyle without needing to work anymore but may choose to.

Examples would be landlords with a decent portfolio, business owners where all the work is done for them, and people with substantial savings and investments.

If you have to work to pay the bills, no matter how much you’re on, you’re working class.

If you’re so rich that you no longer need to care about the value of money then you are upper class.