I'm sure this has already been asked, but I've had difficulty finding the answer by searching. Most investment formulas I've found explained don't include a starting balance.
Given payment size, interest rate per period, inflation rate per period, existing investment balance, and a target size of investment, how can I calculate the number of payment periods still required to reach that size of investment?
EDIT: I'm looking for the mathematical formula, not an online calculator, that I can refer to while building a spreadsheet.
EDIT 2: If you'd like to give me a spreadsheet-specific answer, I'm using Google Sheets.
EDIT 3: Here's what I've been trying with NPER.
Google provides this documentation on their NPER function:
NPER(rate, payment_amount, present_value, [future_value, end_or_beginning])
rate- The interest rate.
payment_amount- The amount of each payment made.
present_value- The current value of the annuity.
future_value- [ OPTIONAL ] - The future value remaining after the final payment has been made.
end_or_beginning- [ OPTIONAL -
0by default ] - Whether payments are due at the end (0) or beginning (1) of each period.
Intuitively, it seems like I should be calculating each parameter as follows:
rate- rate of return, either straight average investment return or maybe average investment return minus inflation.
payment_amount- amount I plan to pay into the investment per period.
future value- This is where it gets confusing. Should I put the current balance in present value or future value? In which should I put my target value? Either way I get the error "Scenario in function NPER is not possible."