This is generally where the “80-90% of your current expenditures” rule of thumb comes in. Note that it is expenditures, not income. In retirement you won’t be doing things like saving for retirement, so your expenditures will be much different than your income. Having a paid off mortgage, not having to commute to a job location etc will also impact this number.

Say your current expenditures are $10000/month, and you conservatively estimate that you’ll need 90% of that, or $9000, to retire. There’s your target.

Next, realize that despite all the fear mongering around social security, the program isn’t goin anywhere and you can/should bake it into your retirement assumption. Social security is designed to replace about 40% of your income in retirement. In this example we will say that 40% of your current income is $4000. So when you reach full retirement age social security will be covering a little under half of your expenses. You can also request a more accurate SS estimate straight from the SSA if you want a higher fidelity number of what to expect.

The remaining $5000 is up to you. If you have no other forms of income in retirement (pensions, rental properties, reverse mortgage, etc) then to figure out how much you need in your nest egg to comfortably produce $5000 in income just use the 4% rule, and times $5000*12*25 to determine the total nest egg you’ll need. In this case that would be $1,500,000.

If you have other forms of income then subtract them from the $5000 and do the math from there.

You can, and many do, overanalyze all of this to death but they do so because they enjoy doing it (whether they realize it or not), not because you have to.