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MortgagesEver wonder why your $1,900 mortgage payment barely dents the balance in year one, yet in year 25 the same payment is mostly principal? That slow shift is amortization — the engine behind every fixed-rate mortgage.
Amortization simply means paying off a loan in equal installments over a set period. Each payment is split into two parts: interest (the lender's cut, computed on your current balance) and principal (the part that actually reduces what you owe). Because the interest is recalculated on a shrinking balance every month, the mix tilts over time — from mostly interest to mostly principal — while your payment stays identical.
Try it yourself: Plug in your own loan and watch the interest/principal split month by month. Mortgage Calculator →
The math behind your payment
Lenders compute your fixed monthly payment with the amortization formula:
Payment = P × r(1+r)n ÷ ((1+r)n − 1)
P = loan amount · r = monthly rate · n = number of payments
P = loan amount · r = monthly rate · n = number of payments
You don't need to memorize it — but it explains one key fact: the payment is engineered so that after exactly n payments, the balance hits zero. Interest is always charged first on what's owed; whatever is left over from the fixed payment chips away at the principal.
A real example: $300,000 at 6.5% for 30 years
The monthly payment comes to about $1,896. Watch how the split changes:
| Payment | Interest | Principal | Balance after |
|---|---|---|---|
| #1 | $1,625 | $271 | $299,729 |
| #60 (year 5) | $1,550 | $346 | $285,750 |
| #120 (year 10) | $1,448 | $448 | $266,900 |
| #240 (year 20) | $1,075 | $821 | $197,700 |
| #360 (year 30) | $10 | $1,886 | $0 |
The painful truth: after 10 years and $227,000 in payments, the balance has fallen by only about $33,000. The bulk of early payments is interest — which is exactly why extra principal payments early on are so powerful.
Why extra payments beat the schedule
An extra principal payment doesn't just shorten the loan by one month — it reduces the balance on which all future interest is computed. Adding just $100/month to the loan above cuts the total interest from roughly $382,000 to about $320,000 and pays the loan off nearly 4 years early. Early extra payments have the biggest effect because they prevent interest from compounding on a larger balance.
Two things to confirm with your lender first: that extra payments are applied to principal (not just advanced future payments), and that the loan has no prepayment penalty.
What an amortization schedule shows you
- Payment number and date — every payment from #1 to the last.
- Interest vs principal — the exact split each month.
- Remaining balance — what you still owe after each payment.
- Cumulative interest — how much the loan will really cost you.
Before signing any mortgage, ask the lender for the full schedule. It converts a vague "30 years of payments" into a concrete number — the true price of the house.
Frequently asked questions
Amortization is the gradual payoff of a loan through regular fixed payments. Each payment is split into interest (a percentage of the current balance) and principal (what reduces what you owe). Because the balance shrinks each month, the interest portion falls and the principal portion rises, even though the payment stays the same.
Interest is charged on the outstanding balance, which is largest at the start. On a $300,000 loan at 6.5%, the first month's interest alone is about $1,625 of the $1,896 payment, leaving only $271 to reduce principal. As the balance falls, the split shifts — by year 20, roughly two-thirds of each payment is principal.
Yes, if they are applied to principal. An extra principal payment lowers the balance immediately, so every future month's interest is computed on a smaller amount. Even $100 extra a month on a 30-year loan can shave off several years and tens of thousands in interest. Always confirm with your lender that extra payments go to principal, not just to prepay future installments.
The principle is the same, but when an adjustable rate resets, the lender recalculates the payment needed to amortize the remaining balance over the remaining term at the new rate. Your payment can rise or fall, and the amortization schedule restarts from the new balance and rate.
Disclaimer: Calculator content is for education and planning only — not financial advice. Loan terms, rates, fees and tax rules vary by lender and country; confirm important figures with your lender or a licensed financial adviser before deciding.