Amortization is the process by which a loan is repaid over time, one installment at a time. On a fixed-rate mortgage using the French system — the standard in the US, UK, Spain and Latin America — you pay the same amount every month, but that payment is split between interest (the cost of borrowed money) and principal (the part that actually reduces your balance). What surprises most homeowners is how dramatically that split changes over time.
The formula behind the payment
The monthly payment on a fully amortizing fixed-rate loan is: M = P · r / (1 − (1 + r)⁻ⁿ), where P is principal, r is the monthly interest rate (annual rate divided by 12) and n is the total number of payments (years × 12). The payment only changes if one of those three variables changes, for example after a rate reset on an ARM or after a partial prepayment that resets the schedule.
How each month works
Every month, the current interest rate is applied to the outstanding balance at the start of the period. That gives the interest portion. Whatever is left of your payment reduces principal. Next month the same process repeats, but on a slightly smaller balance, so interest drops a little and principal rises a little. Multiply that by 360 months and you get a very lopsided distribution.
Worked example: $250,000 at 6.5% for 30 years
Fixed monthly payment: about $1,580. In month one, interest is 250,000 × 0.065/12 ≈ $1,354; only $226 reduces principal. After twelve payments totaling about $18,960, you have paid down only about $2,800 of the loan. By month 240 (year 20), interest has fallen to roughly $530 and principal repayment is about $1,050. In the final months, almost the whole payment reduces principal. Total paid over 30 years: about $569,000 on a $250,000 loan — around $319,000 in interest.
The amortization schedule
An amortization schedule is a table showing, month by month, how much of your payment is interest, how much is principal, and how much you still owe. Every lender is required to provide it before closing. Read it carefully: it is the clearest way to see the total interest you are committing to and how the actual (not theoretical) debt evolves.
Where extra principal payments bite
Because interest is heavily front-loaded, every extra dollar you send early eliminates interest that would otherwise compound for decades. Prepaying $5,000 in year two might save $10,000–$14,000 in interest. Prepaying the same $5,000 in year 25 saves only a few hundred dollars.
When you prepay you usually have two options: reduce the payment (same term, lower monthly cost) or reduce the term (same payment, fewer years). Reducing the term always saves more interest; reducing the payment protects cash flow. If you carry expensive credit-card debt or have thin liquidity, prepaying the mortgage may not be the right call at all.
Prepay vs refinance
Refinancing makes sense when the new rate is at least 0.5–1 percentage point below the current one and you plan to stay in the home long enough to recover the closing costs (typically 2–4% of the new loan). If you might sell in 2–3 years, refinancing rarely pays off, even with a lower rate.
Common mistakes reading a schedule
- Thinking “I’m ten years in, so the loan is almost gone.” Often you still owe 75% of the original balance after ten years.
- Comparing two mortgages by payment alone. A longer term lowers the payment but explodes total interest.
- Confusing the nominal rate with the APR. Insurance, points and fees change the true cost.
- Not checking the new schedule after a prepayment: the servicer may default to reducing the payment when you wanted to reduce the term.
How to use the calculator to decide
Run three scenarios in the mortgage calculator: your current offer, the same loan with a term five years shorter, and the same loan with $100 extra per month. Compare the monthly payment, total interest and total paid. You will almost always see that shortening the term, or adding roughly 10% to the payment, saves tens of thousands in interest without changing your interest rate.