I have $x currently and want to save up to $y. What is the formula to tell
me how much I need to save each week for say 1 year to go from x to y given
a standard, fixed variable rate?
What's a "fixed variable rate"? Do you mean: given (a) a fixed rate,
and (b) a variable rate?
For a fixed annual nominal rate "r" (i.e. not the APY):
=-pmt(r/52, 52, -x, y, 1)
Caveat: This assumes that the savings account compounds at the same
frequency as the deposits, namely weekly. That is probably not the
case. But for such a short period, the difference should be small.
For a variable rate, you do not provide sufficient information.
However, it is unlikely that a simple formula can be devised. I would
set up a 52-week schedule that models the savings account, e.g. C2:
=(C1+$A$1)*(1+B2/52), where C1 is the initial balance (x), $A$1 is the
weekly payment and B2 is the annual nominal interest rate for the
week. Copy that formula down through C53, and fill in B3:B53
appropriately. Then vary $A$1 either manually or perhaps using Excel
Solver until C53 (the balance after 52 weeks) is the smallest amount
greater than "y".
