In case anyone else cares about the math behind coasting age:

Deconstructing this, its simply calculating the number of years it will take to get from your current portfolio balance to your target portfolio balance assuming a constant rate of return with no additional cash flows.

If current portfolio balance is P, and I return i each year, the P*(1+i)^n = T can be solved for N, such that (1+i)^n = T/P. This Solving for n requires the ln. ln{(1+i)^n} = ln{T/P} becomes n = ln{T/P}/ln{1+i}. In this case, Current Age (A) + n = Coasting Age (C).

The math done to derive the formula in the prior post starts the same, P(1+i)^(C-A)=T

=P*(1+i)^C/(1+i)^A = T

=P*(1+i)^C = T*(1+i)^A

= (1+i)^C = T/P*(1+i)^A

=ln((1+i)^C) = ln(T/P*(1+i)^A)

=C*ln(1+i) = ln(T/P)*(1+i)^A)

So then the prior formula: Coasting Age = ln((Target Portfolio/Current Portfolio)*(1+Portfolio Return)^Current Age)/ln(1+Portfolio Return)

The alternative is to do the following in Excel:

PV = C

FV = T

i = Portfolio Return

Use the NPER() function:

Coasting Age = NPER(i,0,-C,T) + Current Age.

This also allows you to add an amount to your portfolio each year to accelerate your time to Coast. Simply replace 0 with the annual amount. So if I'm going to add 10k per year at the end of the year:

Coasting Age = NPER(i,-10,000,-C,T,0) If you add 10k at the begining of the year: NPER(i,-10,000,-C,T,1)

The formula assumes a loan, so the amount of PV is assumed to be negative, and payments are assumed to be negative. Since this isn't a loan, but a positive value, adding positive values, you flip the sign to make it work.

Hope that simplifies this for folks.

EDIT: Wouldn't you know this was covered already with financial calculators! Reiterating the prior point, if you want to account for inflation, just set i in the formula equal to your target portfolio return minus the expected inflation percentage. So if you think your portfolio will return 6%, and inflation will be 2.5%, use 3.5% as i in the formula and it will work out. Sorry for the duplication!