Interest is the price you pay to borrow money. When it comes to a mortgage, because the loan amounts are so large and the terms are so long, interest is usually the most expensive part of buying a home.
Annual vs. Monthly Rates
When a lender quotes you a "6.5% interest rate," they are quoting the Annual Percentage Rate. However, mortgages do not calculate interest once a year. They compound interest monthly.
To find the actual rate applied to your loan each month, you divide the annual rate by 12.
- Annual Rate: 6.5% (or 0.065 as a decimal)
- Monthly Rate: 0.065 / 12 = 0.005416
The Monthly Calculation
Every single month, the bank looks at your remaining principal balance and multiplies it by the monthly interest rate. That is the interest fee for that specific month.
Example: If your remaining balance is $240,000, and your monthly rate is 0.005416, your interest charge for that month is:
$240,000 * 0.005416 = $1,300
If your total fixed monthly payment is $1,517, then $1,300 goes to the bank as profit, and only $217 goes toward actually paying down the $240,000 debt.
Long-Term Cost
Because mortgage interest is calculated on the remaining balance every single month for 30 years (360 months), the total amount of interest you pay over the life of the loan is staggering.
On a $240,000 loan at 6.5% for 30 years, you will pay approximately $306,315 in total interest. That means you pay more in interest than the original cost of the house itself!
Explore the Formula
Curious how the bank calculates the exact monthly payment needed to bring the balance to zero in exactly 360 months? Check out our Mortgage Formula Explorer.
How to Reduce Interest Costs
Because interest is a function of the balance, the rate, and the time, you can reduce total interest paid by:
- Getting a lower rate: Refinancing when rates drop.
- Shortening the time: Choosing a 15-year mortgage instead of a 30-year.
- Lowering the balance faster: Making extra principal payments.