The question every borrower should ask
When a lender quotes a monthly mortgage payment, most people accept the number and move on. But that single figure hides the most important story of the loan: how much of each payment chips away at what you borrowed versus how much simply pays the bank for the privilege. That split — and the way it shifts over time — is amortization, and understanding it changes how you think about overpaying, refinancing, and the true cost of a long loan.
Amortization is the process of paying off a debt with a series of equal, scheduled payments. Each payment is identical, but its composition is not: early on, you pay mostly interest; later, mostly principal. The schedule that maps this out, month by month, is called an amortization table.
How the monthly payment is calculated
A fixed-rate mortgage uses one formula to find the constant payment that will clear the balance exactly at the end of the term:
M = P × r × (1 + r)ⁿ ÷ ((1 + r)ⁿ − 1)Here P is the amount borrowed, r is the monthly interest rate (the annual rate divided by 12), and n is the number of monthly payments. Borrow $300,000 at 6% over 30 years and you get a monthly rate of 0.06 ÷ 12 = 0.005 across 360 payments, producing a payment of about $1,799 a month.
Why early payments are almost all interest
Interest is always charged on the balance that remains. In month one of that $300,000 loan, interest is 300,000 × 0.005 = $1,500. Since the payment is $1,799, only $299 actually reduces the principal. The balance barely moves, so month two's interest is almost as large. This is why, in the first years, it can feel like you are paying a fortune and owing nearly as much as you started with.
| Payment # | Interest | Principal | Balance after |
|---|---|---|---|
| 1 | $1,500 | $299 | $299,701 |
| 60 (year 5) | $1,388 | $411 | $277,200 |
| 180 (year 15) | $1,022 | $777 | $203,600 |
| 360 (final) | $9 | $1,790 | $0 |
The crossover — where principal finally exceeds interest in each payment — arrives surprisingly late on a 30-year loan, often around year 18 to 21 at typical rates. Everything before that point is front-loaded interest.
The lever most borrowers overlook: extra principal
Because interest is charged on the remaining balance, any extra money you put toward principal removes not just that dollar of debt but all the future interest it would have generated. The effect is largest earliest, when the balance is highest.
- One extra payment a year on a 30-year loan typically shortens it by four to six years and saves tens of thousands in interest.
- Rounding up — paying $1,900 instead of $1,799 — quietly accelerates payoff with no painful change to your budget.
- A lump sum early beats the same lump sum late, because it stops more compounding interest.
The catch is that extra payments are illiquid: once paid, that money is in the house, not your bank account. Weigh prepayment against keeping an emergency fund and against investments that might earn more than your mortgage rate.
What the term length really costs
A longer term lowers the monthly payment but raises the total interest dramatically, because you owe money for longer. On that $300,000 loan at 6%:
- 15-year term: about $2,532 a month, roughly $156,000 total interest.
- 30-year term: about $1,799 a month, roughly $347,000 total interest.
The 30-year option is $733 cheaper each month but costs more than $190,000 extra over the life of the loan. Neither is “correct” — it is a trade between monthly affordability and lifetime cost. Run both through our mortgage calculator (and the loan calculator for non-property debt) to see the full schedule before you commit.