How a schedule is built
Each month, interest is charged on the outstanding balance at one twelfth of the annual rate. The scheduled payment is fixed for the life of the loan, so the interest is subtracted first and everything left over reduces the principal. Early on the balance is at its largest, which is why most of the early payments go to interest; only later does the principal share take over.
On a $320,000 loan at 6.5%, month one charges roughly $1,733 of interest, so only a small part of a $2,023 payment touches the principal. That is why the interest column shrinks and the principal column grows from row to row even though the payment never changes. An amortization schedule just unfolds that curve month by month, keeping every row consistent with the running balance.
Extra payments change the tail of the schedule
Every extra dollar of principal removes all future interest on that dollar, which is why even a modest monthly extra at the start of a 30-year loan can pull years off the term. The schedule shortens at the end rather than the beginning, because the extra money attacks a balance that would otherwise keep accruing interest far into the future.
This page lets you add extra principal every month and, separately, once a year. The calculator applies an extra payment after the scheduled payment each month, so the balance falls sooner, the payoff month moves up, and the interest saved is shown right beside the time saved. Because an extra dollar early avoids interest for every remaining month, it is worth far more in year one than in year twenty.
Yearly totals and the CSV export
The page also groups the schedule by year, so you can see the interest and principal paid in each calendar year — handy when you need the interest figure for records at income-tax time, or want to watch the principal column build. Twelve monthly rows compress into a single yearly line, which makes the underlying trend easier to read.
The first one hundred and twenty payments are previewed on the page, and the download button exports the complete schedule as a CSV with every row and the running balance, ready for a spreadsheet, an accountant or your own records. Exporting is a convenience, not a requirement: the schedule itself is readable on the page, and the full table is one click away.
What the schedule is and is not
A schedule is a model of your loan, not a statement from it. Real payments can differ by a few cents or a few dollars because of payment timing, rounding conventions, escrow amounts and bank fees, none of which are in this arithmetic. Balances here are carried at full precision and rounded only for display, so the printed rows reconcile with the totals.
The value of the schedule is the shape of the loan — when the balance stops shrinking slowly, what an extra payment really removes, and how much total interest a term costs. Those answers are exact for the numbers you enter, and the yearly summary shows them at a glance. For exact figures on your own loan, the lender statement remains the authoritative record.
Where the money goes in year one
In the first year of a long loan almost every dollar of the payment is interest. That is not a trick of the lender: interest is charged on the balance, the balance is at its largest at the start, so the same payment buys far less principal in month one than in month two hundred.
The yearly view is the fastest way to see the crossover. Watch the interest column fall and the principal column rise year by year; the point where they meet is where the loan starts to pay itself down quickly, and on a 30-year mortgage at a typical rate that point arrives later than most borrowers expect.
That is what makes extra principal so effective early and so unremarkable late. A dollar paid in year two removes thirty years of interest on itself; the same dollar paid in year twenty-eight removes two. The two extra-payment fields exist to price that difference on your own numbers.
Danfio