What Is Amortization
Amortization is the process of paying off a loan through fixed monthly payments that cover both principal and interest. Each payment is the same dollar amount, but the split between principal and interest shifts over time. Early payments are mostly interest. Late payments are mostly principal. A 30-year mortgage at 7% doesn't hit a 50/50 split until about year 18.
Your lender calculates each month's interest on the remaining balance. As you pay down principal, less interest accrues, so more of your fixed payment goes toward the loan itself. This is why extra payments early in a loan have outsized effects—every extra dollar goes straight to principal and reduces the base that generates interest for the rest of the term.
How Extra Payments Save You Money
Extra payments reduce your outstanding balance immediately, which means less interest accrues on every future payment. The savings compound over the remaining life of the loan. Even modest extra payments create significant results because of this compounding effect.
| Extra/Month | Interest Saved | Years Saved | Total Paid |
|---|---|---|---|
| $0 | — | — | $718,527 |
| $100 | $66,366 | 4.2 | $652,161 |
| $200 | $114,916 | 7.3 | $603,611 |
| $500 | $213,581 | 13.1 | $504,946 |
| $1,000 | $291,452 | 18.0 | $427,075 |
*Based on a $300,000 loan at 7% for 30 years. Interest saved and years saved compared to making minimum payments only.
Principal vs. Interest Over Time
In year one of a $300,000 mortgage at 7%, you'll pay about $20,900 in interest and only $3,050 in principal. By year 15, the split is roughly even. By year 25, you're paying $16,600 toward principal and $3,750 in interest. This front-loaded interest structure is why most of your equity builds in the second half of the loan.
This also explains why refinancing after 15 years into a new 30-year loan can be costly—you restart the amortization clock right when principal payments were about to accelerate. If you refinance, keep the same payoff date by choosing a shorter term or making extra payments to offset the reset.
When Refinancing Makes Sense
Refinancing is worth running the numbers on when rates drop at least 0.75–1% below your current rate and you plan to stay in the home long enough to recoup closing costs. A common rule: divide your closing costs by your monthly savings to find the break-even month. If you'll stay past that point, refinancing pays off.
Watch for the term trap. Refinancing from a 30-year loan 10 years in to a new 30-year loan extends your total repayment to 40 years. If the rate improvement is small, you may end up paying more total interest despite the lower rate. Refinance into a 15 or 20-year term to lock in both rate savings and a faster payoff.
For a full mortgage breakdown including taxes, insurance, and PMI, use the mortgage calculator. To compare different loan scenarios with varying amounts and terms, try the loan payment calculator.