Loans and mortgages
How Mortgage Amortization Works
Reviewed by the Smart Finance Calculators Editorial TeamLast reviewed:
Amortization is the schedule by which a loan is paid off through fixed, regular payments. With a standard fixed-rate mortgage, you pay the same amount every month, but the way that payment is split between interest and principal changes over the life of the loan.
Understanding this split explains why paying a mortgage feels slow at first, why extra payments early are so effective, and how lenders arrive at your monthly number.
Key takeaways
- Each payment covers that month’s interest first; the remainder reduces the principal balance.
- Early payments are mostly interest because the balance is largest at the start.
- The monthly payment stays fixed, but the principal portion grows every month.
- Total interest paid over a 30-year loan can approach or exceed the amount borrowed.
What the monthly payment is made of
On a fixed-rate mortgage, each monthly payment has two parts: interest for that month and principal repayment. Interest is calculated on the outstanding balance, so at the beginning — when you owe the most — interest eats up most of the payment.
As the balance falls, the monthly interest charge falls with it. Because the total payment is fixed, the leftover amount going to principal grows month after month. This is why the balance drops slowly early on and much faster later.
Worked example: $300,000 at 6.5% for 30 years
A $300,000 loan at a 6.5% annual rate over 30 years (360 payments) has a monthly principal-and-interest payment of about $1,896.
In month one, interest is $300,000 × (0.065 / 12) = $1,625, so only about $271 goes to principal. By year 15, the interest portion has fallen to roughly $1,100 and over $780 goes to principal each month.
Across all 360 payments you pay about $682,600 in total — roughly $382,600 of it interest, more than the original amount borrowed. That total is exactly why the payment split matters.
Payment split at different points in a $300,000, 6.5%, 30-year loan
| Payment # | Interest portion | Principal portion | Remaining balance |
|---|---|---|---|
| 1 | $1,625 | $271 | $299,729 |
| 60 (year 5) | $1,502 | $394 | $276,900 |
| 180 (year 15) | $1,113 | $783 | $204,700 |
| 360 (year 30) | $10 | $1,886 | $0 |
Approximate figures for a fixed-rate loan; taxes and insurance are not included.
The amortization formula
The fixed monthly payment M is calculated as M = P × [ r(1 + r)^n ] / [ (1 + r)^n − 1 ], where P is the loan principal, r is the monthly interest rate (annual rate divided by 12), and n is the total number of monthly payments.
You rarely need to compute this by hand, but it explains why lowering the rate or shortening the term changes the payment so much — both r and n appear as exponents.
How to use the Mortgage Calculator
Enter your loan amount, rate, and term to see your monthly payment and a full amortization schedule showing the interest-vs-principal split for every payment.
Open the Mortgage CalculatorCommon mistakes to avoid
- Judging affordability by the monthly payment alone and ignoring total interest over the term.
- Forgetting that property taxes, homeowners insurance, and PMI add to the real monthly cost.
- Assuming a slightly lower rate is trivial — on a large balance it can save tens of thousands.
- Overlooking that extra principal payments are most powerful in the early years.
Practical takeaways
- Compare loans on total interest, not just the monthly payment.
- Even small extra principal payments early shorten the loan and cut interest sharply.
- A shorter term raises the payment but dramatically lowers lifetime interest.
- Always confirm whether your quoted payment includes taxes and insurance.
Frequently asked questions
Interest is charged on the outstanding balance, which is at its highest when the loan begins. Because your fixed payment must first cover that large interest charge, only a small slice is left to reduce principal. As the balance shrinks, the interest charge shrinks and more of each payment attacks the principal.
Yes. On a 30-year loan, adding roughly one extra monthly payment each year can shave several years off the term and save a substantial amount of interest, because every extra dollar of principal permanently removes future interest on that dollar. Confirm your lender applies extra amounts to principal.
The interest rate determines your monthly interest charge. The APR (annual percentage rate) folds in certain lender fees and points, so it is usually slightly higher and gives a fuller picture of the loan’s cost. Use APR to compare offers with different fee structures.
PITI stands for principal, interest, taxes, and insurance — the four common components of a full mortgage payment. Amortization calculators typically show only principal and interest, so remember to budget for the taxes and insurance separately.
The principal-and-interest portion stays fixed. However, if taxes or insurance are paid through an escrow account, your total monthly payment can rise or fall when those amounts are reassessed each year.
A 15-year loan has higher monthly payments but far less total interest and builds equity faster. A 30-year loan has lower, more flexible payments but costs much more over time. The right choice depends on your budget stability and other financial goals.
Related calculators
Related guides
Sources and methodology
This article is based on general financial principles and information published by the authoritative primary sources below. Figures are illustrative and rounded to explain the concepts.
About this article
Written and reviewed by the Smart Finance Calculators Editorial Team.
Published · Last reviewed
Financial disclaimer: Results are estimates for educational purposes only and are not professional financial, tax, legal or investment advice. Figures may not reflect your exact situation. Consult a qualified professional before making financial decisions.