Understanding Amortization: Why Most of Your Payment Is Interest

What Is Amortization?

Amortization is the process of paying off a loan through scheduled, fixed payments over time. Each payment covers both interest (the cost of borrowing) and principal (the actual loan balance). The key feature: the split between principal and interest changes with every single payment.

Why Do You Pay Mostly Interest at First?

Interest is calculated on your remaining balance. At the start of a loan, you owe the full amount — so interest is at its highest. As you pay down the balance, less interest accrues each month, leaving more of your fixed payment to reduce principal.

On a $300,000 mortgage at 6.5%:

• Month 1: Payment = $1,896. Interest = $1,625. Principal = $271. Remaining: $299,729
• Year 10 (Month 120): Payment = $1,896. Interest = $1,291. Principal = $605. Remaining: $238,000
• Year 25 (Month 300): Payment = $1,896. Interest = $604. Principal = $1,292. Remaining: $110,000
• Final months: Nearly all payment goes to principal

The Amortization Formula

The standard loan payment formula is: M = P × [r(1+r)^n] / [(1+r)^n − 1]

• M = monthly payment
• P = principal (loan amount)
• r = monthly interest rate (annual rate ÷ 12)
• n = total number of payments (years × 12)

This formula is designed to ensure that at the end of exactly n payments, the balance is zero — perfectly amortized.

Reading an Amortization Schedule

An amortization schedule is a table showing every payment broken down into principal and interest. It typically shows: payment number, payment amount, interest portion, principal portion, and remaining balance.

Use cases for reviewing your amortization schedule:

• Understand how much equity you've built at any point
• See the impact of extra payments before making them
• Determine the break-even point for refinancing
• Plan when to remove PMI (at 20% equity)
• Understand your tax-deductible interest for any given year

How Extra Payments Change Amortization

When you make an extra principal payment, you skip ahead on the amortization schedule. Every dollar of extra principal eliminates all future interest that would have been charged on that dollar. This is why early extra payments are especially powerful — they eliminate interest that would have compounded for years.

A $5,000 extra payment in month 12 of a 30-year mortgage at 6.5% eliminates approximately $15,000 in future interest — a 3x return on the extra payment.

Not All Loans Are Amortizing Loans

Interest-only loans, balloon loans, and some adjustable-rate mortgages (ARMs) don't follow standard amortization schedules. Be aware of what type of loan you have — and confirm with your lender how extra payments are applied (to principal vs. future payments).