S = 25 * N
Where S is the amount of savings you need, denominated in anything you want (present dollars, bars of gold, whatever) and N is the amount you want to spend per year.
The derivation of this formula is decidedly less simple. It comes down to "The overwhelming majority of portfolios with a standard 60:40 stock/bond allocation which model the historical returns of the US stock market will not be depleted within a human life span if you restrict the withdraw rate to 4% per year. Go above that and things start to become dicey, quickly. This is backtestable but, obviously, not forwardtestable."
Thus, if your desired standard of living requires (present day dollars) $40k a year, you would want a million present day dollars saved to retire today.