How Mortgage Amortization Works — A Complete Breakdown
Blog › Finance · 9 min read · Published 2026-03-03
Understand how mortgage payments are split between principal and interest, and how extra payments can save you thousands.
What Is Mortgage Amortization?
Amortization is the process of paying off a loan through regular payments that cover both principal and interest. In a fully amortized mortgage, each payment is the same size, but the split between principal and interest changes over time. Early payments are mostly interest; later payments are mostly principal. This front-loading of interest is what makes mortgages so profitable for lenders — and why understanding amortization can save borrowers tens of thousands of dollars.
The Amortization Formula
Monthly payment M = P[r(1+r)^n] / [(1+r)^n – 1], where P is the loan principal, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of payments. For a $300,000 loan at 6.5% for 30 years: r = 0.065/12 = 0.005417, n = 360. M = $300,000 × [0.005417(1.005417)^360] / [(1.005417)^360 – 1] = $1,896.20 per month.
How the Payment Split Changes
In the first month of a $300,000 loan at 6.5%, interest is $300,000 × 0.005417 = $1,625.00. Only $271.20 goes to principal. After 15 years (payment #180), the remaining balance is about $230,000 — you've paid $341,316 total but only reduced principal by $70,000. By payment #300 (year 25), the split reverses: roughly $1,200 goes to principal and $700 to interest.
Over the full 30 years, total payments are $682,632 — meaning $382,632 is pure interest, more than the original loan amount. This is the true cost of a 30-year mortgage.
The Power of Extra Payments
Adding $200/month to that $1,896 payment on a $300,000/6.5% loan saves $86,000 in interest and pays off the mortgage 5.5 years early. Even one extra payment per year (paying biweekly instead of monthly) cuts about 4 years off a 30-year mortgage. The key insight: extra payments go entirely to principal, reducing the balance that future interest is calculated on.
A lump sum of $10,000 applied in year 5 saves roughly $25,000 in interest over the remaining life of the loan. The earlier you make extra payments, the greater the impact due to the compounding nature of interest savings.
15-Year vs 30-Year Mortgages
A 15-year mortgage on $300,000 at 5.8% costs $2,520/month — $624 more than the 30-year. But total interest paid is only $153,600 versus $382,632. The 15-year borrower saves $229,032 and owns their home 15 years sooner. The tradeoff is higher monthly payments and less financial flexibility.
Amortization and Refinancing
Refinancing resets your amortization schedule. If you refinance a $250,000 balance into a new 30-year loan, you restart the interest-heavy early years. This is why the break-even period matters: you need to stay in the home long enough for rate savings to exceed closing costs and the amortization reset penalty.
Key Takeaways
Understanding amortization is crucial for making smart mortgage decisions. Front-loaded interest means you build equity slowly in early years. Extra payments, even small ones, can save tens of thousands in interest. Use an amortization calculator to visualize the payment schedule and model different extra payment strategies before committing to a mortgage structure.