How Do You Find Out Monthly Interest On A Mortgage?

If it's figured as "simple interest", take the interest rate and divide it by 365 (days in a year). Then multiply that figure by your current mortgage balance.  This figure will be your daily interest on this current mortgage balance.  Then multiply that figure by the number of days from the last payment paid to the date your next payment will be paid.  This will be the interest accrued.  If it's calculated on simple interest, as your balance goes down, so does your interest.