I've seen this formula from MDM (who said it came from Heart of Tin):

Time in years to FI = Ln((S + i*E/WR) / (S + i*A)) / Ln(1 + I)

A = amount currently invested in funds you will draw upon in retirement

E = total expenses in retirement (including taxes, in today's dollars)

I = Real return on invested retirement funds

S = Annual amount invested in funds you will draw upon in retirement

WR = withdrawal Rate planned for retirement (e.g. 4%)

I have a few questions regarding the definitions:

is E per year? Or total for all years I plan to be retired?

For S, is it the amount that I will invest each year until retirement?

Does it assume that you want the principal to remain untouched?

I've also been using forum search and google site: search to try to find the post from someone whose money only needs to last x years until their pension / defined benefit plan kicks in. Anyone remember that?

Thanks,

TexasAnnie