: How can I figure out my Financial independence targets and years to FI? Trying to figure out my target for financial independence and is about to start building an investment portfolio for 3 lifetimes. (3 lifetimes as in

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First, notice that .04 = 1/25, so your first two calculations are really doing the exact same thing. Multiplying by 25 is just given as an easier way to do the calculation, but it will always give you the same number as dividing by 0.04.

A withdrawal timeframe of 240 years requires practically the same target as an infinite timeframe, but it is simpler to calculate for the infinite case.

Note that if your ROI is only 4%, then 4% cannot also be your SWR. You need your ROI to be at least than your SWR + inflation to be able to withdraw 4% indefinitely (and you need it to be almost equal to SWR + inflation to be able to withdraw for 240 years).

Other considerations:

Do you have or expect to have just one kid who will also have exactly one kid? If you expect to have more than one then you might want to plan to have an even bigger gap between your ROI and your SWR to allow the investment to double or triple in size before your kids inherit it
The 240 year target is still quite strange. For you to have a grandkid alive in 240 years, assuming you have a kid around now and your kid has a kid around 30, your grandkid would be 210 years old before 240 years from now. Maybe that is possible, 240 years is a long time for healthcare innovations in the modern era; but it doesn't correspond well with your 80x3 calculation.
If you plan to have years of overlap, where the withdrawals are doubled or tripled, because you and your descendants are withdrawing at the same time, then you need to plan for your 4% SWR to include double your annual expenses. That means doubling your target. You cannot just add up the years of overlap and count them as extra years, because you would be withdrawing more than your SWR during those years and you can't do that without your investments being depleted.
You may want to consider just saving for your own indefinite retirement and teaching your kid to do the same (instead of saving for your kid's). That way you don't have to worry about things like living an unexpectedly long life. You could live to 110 and your kid could be 80 before you stop needing the investment income. It would sure be nice if your kid had also become financially independent themselves well before that. I believe that one of the best gifts you can give to your children is the peace of mind that you can stick it out in this world for as long as you like, without ever becoming a financial burden. Then they can focus on their own goals without having to worry about yours.

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There's a great (fake) anecdote about your long term thinking:

A Texas oil baron is asked about his wealth, compared with his ancestors and his future descendants. He says "My grandparents took the bus, my parents drove a twenty year old truck, I drive a Mercedes, my children will drive Bentleys, and my grandchildren will take the bus."

Among the implications you can glean from the above, is that the oil baron has a limited ability to control his children's lifestyles, and by the time the grandchildren are adults, the family money may well be gone. Planning ahead for your grandchildren is admirable, but futile. Don't worry about what will happen 160 years after your death; there is enough stress in one lifetime already.

Paulpro's answer has some great resources on both your specific questions, and also general thoughts about how your math will play out differently than you expect (ie: if you have 3 kids and each of your 3 kids has 3 kids, there will be 9 grandchildren withdrawing form the inheritance).