I was messing around with my net worth projecting Excel file (actually a Google Sheet) and I wanted to add a field that would predict the day that we'd hit a certain target (e.g. $1.5M).

To do this, I determined our annual net worth growth rate (looking at the last 8 years, for which I have good data). This combines market increases (or decreases) with our additional investments -- about 17% per year.

I knew that the formula to then project that growth forward by Y years is: [current NW]*(1+[annual NW growth])^Y. I've been out of school for a long time, so I wasn't sure how to solve for Y (if I ever knew that). I asked ChatGPT -- I think this is the first time I've used it for anything. It gave me this:

Y = log base 1+[annual NW growth] (1500000/[current NW])

I asked ChatGPT to put that in Google Sheets terms, and it told me:

=LOG(1500000/[current NW], 1+[annual NW growth])

I then brought that into my Google Sheet, and added some date fields, giving me this:

=LOG(1500000/[current NW],1+[annual NW growth])*365.25+[today's date]

As of today, it's projecting that we'll hit $1.5 million (our FI target) on March 13, 2024.

Will this date end up changing? Almost certainly. At the same time, it was fun to figure this out.