A mortgage is likely the largest debt most people ever take on. Understanding exactly how interest is calculated — and how it changes over the life of the loan — can save you tens of thousands of dollars through better decisions about extra payments, refinancing, and loan term selection.
How Monthly Interest Is Calculated
Monthly interest = (Annual interest rate ÷ 12) × Outstanding balance. On a $400,000 mortgage at 6%, the first month's interest is (0.06 ÷ 12) × $400,000 = $2,000. Your monthly payment might be $2,398 — so only $398 goes to principal in month one. As you pay down the balance, more of each payment goes to principal.
Amortization: Why You Pay More Interest Early
Mortgage payments are structured so the monthly payment stays constant throughout the loan term. This is called amortization. Early in the loan, most of your payment goes to interest (because the balance is high). As the balance decreases over years, the interest portion shrinks and the principal portion grows. In month 1 of a 30-year loan, you might pay 84% interest; by year 25, you might pay only 30% interest.
Total Interest Over the Loan Life
- $400,000 at 6% for 30 years: total interest paid ≈ $463,000
- $400,000 at 6% for 15 years: total interest paid ≈ $207,000
- The 15-year loan saves $256,000 in interest but has higher monthly payments ($3,375 vs $2,398)
- One extra payment per year on a 30-year loan can cut the loan term by 5–6 years
Strategies to Reduce Total Interest
- Make extra principal payments when possible — apply them directly to principal
- Refinance to a lower rate when rates drop significantly (typically 1%+ lower)
- Choose a 15-year over a 30-year term if you can afford the higher payment
- Make bi-weekly payments instead of monthly — you'll make one extra full payment per year
- Avoid extending the loan term when refinancing — a new 30-year on a 20-year-old loan adds cost
Points: Paying Interest Upfront
Mortgage points are upfront fees paid to lower your interest rate. One point = 1% of the loan amount. Paying one point ($4,000 on a $400,000 loan) might lower your rate from 6.5% to 6.25%. Break-even analysis: if the rate reduction saves $50/month, you break even in 80 months (~6.7 years). If you'll stay longer, points make sense.
Use an amortization calculator to see exactly how extra payments reduce your total interest. Even $100/month extra on a 30-year mortgage can save $30,000+ in interest and cut years off your loan.