How do I work out if the bank is correctly charging interest on my mortgage?

I have noticed that the interest charged to me by the bank on my mortgage fluctuates from month to month (fixed interest rate) an I was wondering if there is a formula used to work out what interest should be charged???How do I work out if the bank is correctly charging interest on my mortgage?
take the balance at the beginning of the month and multiply by your rate divided by twelve. Thatis how much interest you shoudl be charged for that month. The difference between that and your payment is the principal. Every month the interest will go down just a little and the principal up by the same amount.How do I work out if the bank is correctly charging interest on my mortgage?
The interest you are paying each month (or payment) should be slowly going down as your loan amount is lessening each time. Take the yearly interest rate and divide it by 12 (months/year), the multiply that number by the amount currently owing on your mortgage. Next month is going to be different, so do the same thing.
If you have a 30 year mortgage, you pay about 70% on the first 15 years. That's how most banks make their money. Call your bank and ask to send you an amortization that will give you a break down between interest/principal. If you can pay an extra amount even if its $50 to $100 each month, that should reduce the terms of the mortgage about 10 years.