I'm trying to replicate the functionality of the Bankrate Simple Savings Calculator in Google Sheets. I've assumed that I need to use the FV function, but I can't get the numbers to match exactly. Here's what my Google Sheets spreadsheet looks like:

         [A]                 [B]
[1] Initial Deposit         $100,000                
[2] Monthly Contributions   $500                
[3] Over a period of        10        (years)       
[4] APY                     2.4%                
FV          $194,830.57      ...using =FV(B4/12,B3*12,-B2,-B1,0)            
Bankrate    $194,410.60      ...using https://www.bankrate.com/calculators/savings/simple-savings-calculator.aspx           
FV - Bankrate = $419.97 difference          ...why?

Using the FV function with a rate of 2.4/12, a number of periods of 10*12, a payment amount of -$500, and a present value of -$100,000 yields $194,830.57, but the same numbers in the Bankrate calculator come out to $194,410.60. This is a difference of $419.97.

I suspect that 2.4% APY in the Bankrate calculator is not the same as 2.4% in the FV function, but can someone explain why and how to fix the FV version so that it matches the Bankrate calculator? Thanks!


APR is an annualized rate, meaning the yield if the periodic interest is compounded for an entire year. To convert it to a monthly rate, use a slightly different formula;

r = (APY+1)^(1/12) - 1

That gives a monthly rate of 0.1978%, which if used in the FV formula gives exactly the $194,410.60

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