I'm trying to find a very simple FI calculator to show my friend who is not mathematically or spreadsheet inclined.

She wants to know how much she needs to save in order to become financially independent and sustain her current cost of living ($20k/year).

Does anyone have recommendations?

For the really really short version:

According to the 4% rule, if 20,000 is the gross income that you need:

N is your number.

(.04)xN = 20,000

N = (20,000)/(.04)

=(20,000)x(25) = 500,000.

In your first year of retirement, you can safely withdraw 4% of your portfolio. If you have $500,000, that would be $20,000. After that, increase your annual withdrawal amount by last year's inflation rate.

Of course, the hard part is convincing someone how or why the 4% rule is applicable. And of course, not everyone believes in the 4% rule, but it's a good place to start.