How Mortgage Amortization Works
The short answer
Every mortgage payment you make is split into two parts: principal (the amount you borrowed) and interest (the cost of borrowing it). Early in your loan, the vast majority of each payment goes to interest — not to paying down what you owe. By the end of the loan, that flips: almost all of each payment reduces your balance. This gradual shift is called amortization.
It can feel counterintuitive — you're making the same payment every month, but the effect of each payment changes dramatically over time.
Why it works that way
Interest is charged on your remaining balance, not on your original loan amount. In month one, your balance is at its highest — so the interest charge is at its highest. Your fixed monthly payment covers that interest charge first, and whatever's left chips away at principal.
As your balance slowly decreases, the monthly interest charge decreases with it. That frees up more of your fixed payment to reduce principal — which reduces the balance faster — which reduces interest further. It's a compounding effect, but it takes years to gain momentum.
Here's what that looks like on a $400,000 loan at 6.5% over 30 years (monthly payment: $2,528.27):
| Payment | To interest | To principal | Remaining balance |
|---|---|---|---|
| Month 1 | $2,166.67 | $361.60 | $399,638.40 |
| Month 180 (year 15) | $1,577.27 | $951.01 | $290,236.56 |
| Month 360 (final) | $13.62 | $2,514.65 | $0 |
Notice that at the halfway point — 15 years in — you've made half your payments but still owe about 73% of the original balance. That's amortization in action.
Go deeper — the formula
M = P × [r(1+r)^n] / [(1+r)^n − 1]
M— monthly paymentP— loan principal (amount borrowed)r— monthly interest rate (annual rate ÷ 12). For 6.5%: 0.065 ÷ 12 = 0.005417n— total number of payments (30 years × 12 = 360)
Plugging in the $400,000 example: M = 400,000 × [0.005417 × (1.005417)^360] / [(1.005417)^360 − 1]. Since (1.005417)^360 ≈ 6.993, that gives M = 400,000 × 0.03788 / 5.993 ≈ $2,528.27. Over 30 years you'll make $910,177 in total payments — meaning $510,177 goes to interest.
What this means for you
Refinancing resets the clock. When you refinance, you start a new amortization schedule. If you're 10 years into a 30-year loan and you refinance into another 30-year loan, you go back to paying mostly interest again. The lower rate may still save you money overall — but run the numbers before assuming it will.
Extra payments have the biggest impact early. Any extra amount you pay goes directly toward principal. Early in your loan, reducing principal by $1,000 saves you years of compounding interest. The same extra payment made in year 25 saves comparatively little.
Shorter loan terms shift the curve. A 15-year mortgage has a steeper amortization curve — you build equity much faster and pay far less total interest, but your monthly payment is higher. We'll cover that tradeoff in a separate guide.
If you want to see how your own loan amortizes — including how much of each payment goes to interest vs principal at any point — the calculator can show you that.
Key takeaways
- Amortization is the process of paying off a loan through fixed monthly payments over time.
- Early payments are mostly interest; late payments are mostly principal — the split shifts gradually throughout the loan.
- Interest is charged on your remaining balance, so as you pay down principal, your interest charge shrinks each month.
- On a $400k / 30-year / 6.5% loan, you'll pay $510,177 in interest over the life of the loan.
- Extra payments applied early in a loan reduce principal — and therefore future interest — far more than the same payments made later.
- Refinancing starts a new amortization schedule, which may increase total interest paid even if the rate is lower.
Ready to run the numbers?
See how your own loan amortizes over time.