The common calculation is that you need some multiplier of your yearly salary to retire. Say 15x. While that number would likely end up being a solid retirement number, it's a clunky way to look at it from my perspective.
A more accurate way is to project your yearly expenses when in retirement, then multiply my 25x. Based on historical market performance and using a Safe Withdrawal rate of 4%, you should theoretically never run out of money.
Keep in mind, we are talking about purely expenses. I personally track current expenses in two categories - mandatory (property taxes, electricity, groceries, gas, etc) and discretionary (travel, entertainment, shopping, etc).
While this seems like a lot, there are some passive impacts. Once in retirement, you will no longer need to save towards retirement and your income taxes are likely to be much lower than working years.
Now if you are like me, that's a little to simple and I can think of several variables that would decrease my confidence level in that calculation i.e. Illness, major market changes, etc. That is where passive investments like real estate, businesses, etc can help smooth out the risk.
While younger myself, my plan is to work and save like hell (and enjoy life) to get to a ballpark number that I feel comfortable all expenses will be covered then supplement with several rental properties for cash flow purposes. Once I reach that number, I'll decide if I want to keep working or maybe move to part time or take a flyer on a high risk/high reward job (100% commission sales, etc). It won't matter too much because I know all expenses are covered. Anything else is fun money.
Apologies for any typos, I wrote this from my phone. I'll share some articles when back on a computer.
