In This Article
Key Takeaways
- Mortgage interest is calculated monthly on the remaining loan balance, not the original amount
- In a 30-year mortgage, you pay more interest than principal in the first ~18 years
- Making extra payments toward principal in the early years has the biggest impact on total interest
- Fixed-rate mortgages lock your interest rate, while ARMs can adjust after an initial period
What Is Mortgage Interest?
Mortgage interest is the cost you pay to borrow money for a home purchase. It's expressed as an annual percentage rate (APR) but calculated and applied monthly. When you make a mortgage payment, part goes toward the interest owed that month and the rest goes toward reducing your loan balance (principal).
For example, on a $280,000 loan at 6.5%, your first month's interest charge is approximately $1,517 ($280,000 x 6.5% / 12). If your monthly payment is $1,770, only $253 goes toward principal in the first month. Use our mortgage calculator to see exactly how this breaks down for your situation.
How Mortgage Interest Is Calculated
Mortgage interest is calculated using a method called amortization. Each month, interest is computed on the current remaining balance:
Because the balance decreases with each payment, the interest portion also decreases over time. This means your fixed payment gradually shifts from mostly interest to mostly principal. You can see this progression with our amortization calculator.
Why Interest Is Front-Loaded
One of the most important concepts in mortgage finance is that interest is front-loaded. In a standard 30-year mortgage at 6.5%:
- Year 1: About 86% of each payment goes to interest
- Year 10: About 74% goes to interest
- Year 20: About 51% goes to interest
- Year 25: About 31% goes to interest
This front-loading is why refinancing early in a mortgage term can be beneficial, while refinancing in the last 10 years often isn't worth it. It's also why extra payments in the early years have an outsized impact on total interest savings.
Fixed-Rate vs Adjustable-Rate Interest
Fixed-rate mortgages lock your interest rate for the entire loan term (typically 15 or 30 years). Your payment never changes, making budgeting predictable. This is the most popular option, especially when rates are historically low.
Adjustable-rate mortgages (ARMs) offer a lower initial rate (often 0.5-1% below fixed rates) for a set period, then adjust based on a market index. A "5/1 ARM" means the rate is fixed for 5 years, then adjusts annually. ARMs carry risk if rates rise but can save money if you plan to sell or refinance before the adjustment period.
Strategies to Reduce Total Interest Paid
The total interest on a mortgage can exceed the original loan amount over 30 years. Here are proven strategies to reduce it:
- Make biweekly payments: Instead of 12 monthly payments, make 26 half-payments per year (equivalent to 13 full payments). This can shave 4-5 years off a 30-year mortgage.
- Round up payments: If your payment is $1,770, pay $1,800 or $2,000. The extra goes directly to principal.
- Make one extra payment per year: Apply your tax refund or bonus directly to mortgage principal.
- Refinance when rates drop: If rates drop 0.75-1% below your current rate, refinancing typically makes sense if you plan to stay in the home 3+ more years. Use our refinance calculator to compare.
- Choose a shorter term: A 15-year mortgage has higher payments but typically a lower rate and dramatically less total interest.
APR vs Interest Rate: What's the Difference?
The interest rate is the annual cost of borrowing, expressed as a percentage. The APR (Annual Percentage Rate) includes the interest rate plus other loan costs like origination fees, discount points, and mortgage insurance, spread over the loan term.
APR is always higher than the interest rate and gives a more complete picture of the true cost of borrowing. When comparing lenders, APR is the better metric because it accounts for fees that vary between lenders.