Understanding Mortgage Interest

How Interest is Calculated

Mortgage interest is calculated monthly based on your outstanding principal balance. Each month, your lender takes your annual interest rate, divides it by 12 to get a monthly rate, and multiplies it by whatever you still owe.

• Formula: Annual rate ÷ 12 = Monthly rate
• Monthly charge: Outstanding balance × Monthly rate = Interest for that month

On a $300,000 loan at 6.5%: $300,000 × (6.5% ÷ 12) = $1,625 interest for the first month. The next month, you owe slightly less ($299,729 after your first payment), so the interest is slightly less ($1,623), and slightly more goes to principal ($273 instead of $271). This snowball effect accelerates over time.

Front-Loaded Interest

In the early years of a mortgage, most of your payment goes to interest, not principal. This is the nature of amortization — your balance is highest at the start, so interest charges are highest. On a 30-year loan:

• Year 1: About 70% goes to interest, 30% to principal
• Year 10: About 60% goes to interest, 40% to principal
• Year 15: About 50% goes to interest — the crossover point where principal and interest are roughly equal
• Year 25: About 30% goes to interest, 70% to principal
• Year 30: Nearly 100% goes to principal

On a $300,000 loan at 6.5%, you'll pay approximately $382,000 in total interest over 30 years — more than the original loan amount. This is why the interest rate matters so much: a 0.5% difference can mean tens of thousands of dollars over the life of the loan.

Reducing Total Interest

There are several effective strategies to reduce the total interest you pay:

• Make extra payments: Even $100/month extra in the first 5 years can save $20,000+ in interest and shave years off your loan. Use our extra payment calculator to see the impact on your specific loan.
• Choose a shorter term: 15-year loans typically have rates 0.5-0.75% lower than 30-year loans, and you pay interest for half the time. On $300,000 at 6.0%, a 15-year saves about $190,000 in interest vs a 30-year.
• Refinance when rates drop: If rates drop 0.5-1% below your current rate, refinancing can reduce your monthly payment and total interest. Use our refinance calculator to check your break-even point.
• Make biweekly payments: Paying half your mortgage every two weeks results in 13 full payments per year instead of 12, effectively making one extra payment annually. This alone can cut 4-6 years off a 30-year loan.
• Put more money down: A larger down payment means a smaller loan balance, which means less interest charged every month for the life of the loan.

Fixed vs Variable Interest

The type of interest rate you have affects how your payments change over time:

• Fixed rate: Your interest rate stays the same for the entire loan term. Monthly payments are predictable and never change. Best for long-term homeowners who want stability.
• Adjustable rate (ARM): Your rate is fixed for an initial period (5, 7, or 10 years), then adjusts based on market conditions. The initial rate is typically lower, but payments can increase after the fixed period. See our ARM calculator to understand potential adjustments.

For a detailed comparison, read our ARM vs Fixed-Rate Mortgage guide.