[GNC] Calculating interest rates

[john]

> On Sep 13, 2021, at 7:46 AM, Steve Hill <steve at nexusuk.org> wrote:
> 
> 
> Despite much Googling I haven't been able to find anything explaining how to calculate this...
> 
> I know:
> - The balance of a savings account at the start of the year;
> - The amounts and dates of payments into the account; and
> - The amount of interest paid at the end of the year.
> 
> I want to calculate the percent of interest received, assuming that the account compounds daily.
> 
> If there were no payments into the account it would be easy (interest_amount/start_balance=interest_percent), but I can't figure out how to take account of irregular payments into the account throughout the year.

You're looking for internal rate of return. Most spreadsheets have an XIRR function that takes an array of dates and cash flows along with a starting guess and calculates the actual daily rate. The documentation for LibreOffice's version is at https://wiki.documentfoundation.org/Documentation/Calc_Functions/XIRR <https://wiki.documentfoundation.org/Documentation/Calc_Functions/XIRR>

Regards,
John Ralls