<prompt> Human: Im looking for something that can tell me my net gain over investing money over 20 years vs the principal I would receive the interest I would avoid paying and any other relevant factors.  <response>: I ended up rolling my own in Excel, which is essentially an amortization table with extra columns for a growing balance.But you can get a pretty good ballpark by just using simple time value of money formulas in Excel. Like if we dont take taxes into account, then the value is the future value formula.Example, 7 returns, 4 mortgage, 500month invested results in 164k more net worth after 20 years. FV7-412, 2012, -500 164,151Now lets apply capital gains taxes. Say 15, so that means instead of 7, its 7-157 FV7-157-412, 2012, -500 146,619If you deduct your mortgage interest, and say youre at 24 marginal, we can approximate this. Note we ignore the fact you dont really benefit from the full deduction, as you forfeit the standard deduction. With the std deduction so large now, Id actually just approximate it by saying instead of saving 24, youre only saving, say, 12: FV7-157-4-41212, 2012, -500 139,405Now these arent perfect, the 7 return is not the annualized return, but rather 7 with the monthly compounding, so the annualized return is really 6.8 or so 16.81212 7. And when comparing interest rates, I believe its more accurate to do 1-1a1b, so 4 and 7 is really a difference of 2.8, not 3. But growth estimates are going to be approximations, anyway.If anyone has any feedback on this, please share. There may be easier andor more accurate ways of doing this. 