What an amortization schedule shows
An amortization schedule is the period by period record of how a fixed payment loan is repaid. Each row shows how much of the payment went to interest, how much reduced the principal, and what the balance became. The payment stays constant while its composition shifts steadily from interest toward principal.
The mechanics are the same every period. Interest for the period equals the current balance times the periodic rate. Principal equals the payment minus that interest. The new balance equals the old balance minus the principal portion. Because the balance falls, the interest portion falls, and since the payment is fixed, the principal portion rises by exactly the same amount.
This calculator computes the payment from the standard formula, payment = P * i * (1 + i)^n / ((1 + i)^n - 1), then iterates month by month and aggregates the results into annual rows. Each row of the table shows the principal paid, interest paid, and ending balance for that year, which is compact enough to read while still showing the shape of the loan.
Worked example: 250,000 at 6.5 percent over 30 years
Using the calculator defaults of 250,000 borrowed, a 6.5 percent annual rate, and a 30 year term, the monthly rate is 0.065 / 12 = 0.00541667 and n is 360. The payment is 1,580.17. Across all 360 payments that is 568,861.22, of which 318,861.22 is interest.
The first year is heavily weighted toward interest. Twelve payments of 1,580.17 total 18,962.04, and of that, 16,167.73 is interest while only 2,794.31 reduces the principal. The balance at the end of year one is 247,205.69, barely one percent below where it started.
The final year is the mirror image. In year 30, 18,310.96 of the same annual total goes to principal and only 651.08 to interest. The crossover point, where a single payment sends more to principal than to interest, arrives in month 233 of 360, which is more than nineteen years in. That asymmetry is the central fact of long term amortization.
Why early payments barely move the balance
The reason is that interest is charged on the outstanding balance, and at the start the outstanding balance is the entire loan. On the example above, the first month accrues 250,000 * 0.00541667 = 1,354.17 in interest, leaving only 226.00 of the 1,580.17 payment to reduce principal. Everything else is the price of having borrowed the money for that month.
As a consequence, the effect of a prepayment depends entirely on when it is made. A 5,000 lump sum applied in year one removes 5,000 of balance that would otherwise have accrued interest for 29 more years. The same 5,000 applied in year 25 saves interest for only five years. Prepayments are worth the most at the beginning, which is also when they are usually hardest to make.
It also explains why selling early feels like you have built no equity. After five years on the example loan, roughly 16,000 of principal has been repaid out of nearly 95,000 in payments. Add selling costs and a buyer in a flat market can walk away with less than the down payment they put in.
What this calculator does not model
It covers principal and interest only. Property tax, homeowners insurance, mortgage insurance, and HOA dues are not included, so the payment shown is smaller than what a mortgage servicer will collect. The mortgage calculator on this site adds those components for a home loan.
Extra payments are not supported here. The schedule assumes the exact scheduled payment every month for the full term. If you want to see the effect of paying extra, the loan calculator on this site has an extra payment field and rebuilds the schedule around it.
The term is entered in whole years and converted to months, and the rate is treated as fixed for the entire term. Adjustable rate loans, loans with an interest only period, balloon structures, and any loan whose payment changes partway through are outside what this table can represent. The schedule also assumes payments arrive exactly on schedule; late payments accrue additional interest that the model does not show.
The mistake people make reading an amortization table
The first mistake is assuming that half the term means half the balance is repaid. On a 30 year loan at 6.5 percent, the balance at month 180 is still above 175,000 out of 250,000, roughly 70 percent of the original amount. Amortization is not linear, and intuition built on straight line thinking will be wrong by a wide margin.
The second is misreading the total interest figure as avoidable waste. It is the cost of the term you chose. The way to reduce it is to shorten the term, which raises the payment: the same 250,000 at 6.5 percent over 15 years costs about 2,178 a month and roughly 142,000 in interest instead of 318,861.
The third is expecting the lender's schedule to match this one exactly. Servicers round each payment to the cent, some apply payments on the day received rather than the due date, and the first period is often an odd length depending on the closing date. Cumulative differences of a few dollars over thirty years are normal and do not indicate an error on either side.