What is Amortization?
Amortization is the financial process of systematically repaying a debt over time through scheduled, equal payments. On a standard fixed-rate mortgage, your total monthly payment remains constant, but the internal split between interest paid to the lender and principal reducing the debt balance changes with every single payment.
In the initial years of a 30-year mortgage, the lender collects interest on the massive outstanding balance, causing up to 70--80% of each monthly installment to go purely toward interest charges. As the principal diminishes, the interest charge shrinks, accelerating equity buildup.
The Mathematical Formula
The standard fixed-rate amortization payment M is derived from the annuity present value equation:
Where:
Step-by-Step Calculation ($300,000 Loan @ 6.5% for 30 Years)
Let us compute the exact monthly payment for a $300,000 mortgage at 6.5% interest over 360 months:
The Principal vs Interest Curve
Here is how Month 1 breaks down versus Month 180 (Year 15) and Month 360 (Year 30):
Month 1:
Month 180 (Year 15):
Month 360 (Final Payment):
How Extra Payments Save Massive Interest
Because interest is billed strictly on the unpaid balance, every dollar of extra principal paid immediately stops accruing interest for the entire remaining life of the loan.
On a \300,000 mortgage at 6.5\%$:
