Understanding loan amortization
What is amortization?
Amortization is the process of paying off a loan in equal, scheduled installments. Each payment covers the interest that accrued on your outstanding balance, and whatever remains goes toward reducing the principal. Because your balance shrinks a little with every payment, the interest charge shrinks too — so with each successive payment, a larger slice goes to principal and a smaller slice to interest. An amortization calculator does this month-by-month arithmetic for you and lays out the entire life of the loan in a single schedule.
Fixed-rate mortgages, auto loans, personal loans, and many student loans are all amortizing loans. By the time you make your final payment, the balance reaches exactly zero — provided you make every payment on schedule.
Principal vs. interest: how each payment is split
Every payment you make is split two ways. The principal is the part that pays down what you actually borrowed. The interest is the lender's charge for borrowing, calculated as your annual rate divided by 12, applied to the balance that month. Early in the loan the balance is at its largest, so interest takes the biggest share of your payment. On a $300,000, 30-year loan at 6% APR, the first payment of $1,798.65 includes about $1,500 in interest and only about $299 in principal.
This is why the early years of a mortgage can feel like treading water: the balance barely moves even though you are paying thousands of dollars. The amortization schedule above shows exactly where this crossover point happens for your loan — the month when principal finally overtakes interest.
How extra payments save money
Interest is charged on your current balance, so every extra dollar you put toward principal eliminates all of the interest that dollar would have accrued in future months. That is what makes extra payments so powerful: a single $100 extra payment in year one can remove several times that amount in interest over the life of the loan. Enter any amount in the extra payment field and the calculator will show your interest saved, months saved, and new payoff date.
The savings are largest early in the loan and on high-interest loans, because that is when future interest charges per dollar of balance are greatest. One popular strategy is adding a fixed amount to every monthly payment; another is making one extra payment per year. Note that the schedule above already accounts for extra payments reducing your balance — if your lender charges prepayment penalties or applies extra payments differently, check your loan terms. And if you are comparing a home equity line of credit — which typically has an interest-only draw period rather than level payments — use our HELOC Payment Calculator instead.
How to use this amortization schedule calculator
Enter your loan amount, annual interest rate, and term in years. The monthly payment, total interest, total cost, and payoff date update instantly. Add an optional extra monthly payment to see how much faster you would pay off the loan and how much interest you would avoid. The schedule defaults to a compact yearly summary; click "Show monthly view" for the full payment-by-payment breakdown with principal, interest, extra, and balance columns.
The calculator also handles special cases correctly: a 0% APR loan simply divides the balance evenly across the term, very short terms work down to a single year, and an extra payment at or above your regular payment will pay the loan off early, with the final payment capped at what you actually owe.
Reading your amortization schedule
Each row of the schedule shows one period of your loan. Payment is your fixed required payment, principal and interest show how it was split, extra is any additional principal you chose to pay, and balance is what you still owe after that period. Scan the balance column to watch your debt decline, and compare the interest column across years to see how the lender's share of your money falls over time.