I need some help figuring out a formula to project how many months a non-profit will retain donors.
What I have so far is a calculator that collects the number of donors, calculates the current retention rate, and projects that performance over several years. Currently, each year is recalculated base on the number of donors from the previous year.
What should be possible (but what I cannot figure out) is the formula for the number of months it takes to reach ZERO donors.
For example, if we started with 1,000 donors and lose 60% of them every year, how many months (or years) before they're all gone?
Any idea how would that be written?
It seems that this should be simple but I'm not getting it.
BTW: I am familar with NPER in Excel, but unfortunately that is not an option. I need the formula.
Thanks for any help!
