Google Sheets and how BOI calculate interest

[LonseaLM]

I want to have my own Google Sheet to keep track of my mortgage repayments, work out the various outcomes if I overpay etc. I have it all working however, the issue is how BOI calculate interest, it seems to be every 3 months. So my numbers are off, but every three months or so are close enough.

Does anyone know if there is a formula I can use to mimic how BOI calculate?

 

[Savvy]

I don't know how BOI calculated but they could be calculating daily but posting it to the account quarterly

 

[elcato]

They calculate daily for the amount of days between the 3 monthly payments. This varies depending on the amount of days in the months.

 

[dwyershane]

it sounds similar to how AIB calculate it, though they accrue daily and then add the interest back onto the account every quarter on the 16th. I assume you pay capital and interest payment monthly, and then every 3 months the interest is added to your account or you mean BOI only calculate the interest on the balance at the end of the 3 month period?
if its latter, it should just be the closing balance on this date* rate* number of days in that period. if its the former, have a line item for each month showing opening balance/interest/closing balance, then on the month you have the interest you need to split in 2, first part of the month on the old balance, second part on the new opening balance.