Look at the first line of any 30-year mortgage’s amortization schedule and it can feel like a bad deal: on a $250,000 loan at 6.5%, the very first payment of $1,580.17 sends about $1,354 to interest and only $226 to principal. Fifteen years later, that split has nearly reversed. Nothing has gone wrong — this is exactly how an amortized loan is built to work, and the schedule shows why once you know what to look for.
Why the split moves
Interest for any given month is calculated on the balance still owed, not on the original loan amount. Early on, the balance is close to the full amount borrowed, so the interest portion is close to its maximum. Every payment reduces the balance slightly, which slightly reduces next month’s interest, which means slightly more of next month’s fixed payment goes to principal instead. The payment amount itself does not change over a fixed-rate loan’s life — only the split between interest and principal inside it moves, and it moves in one direction only: toward principal, every single month.
The loan amortization calculator builds this table in full and draws the balance curve alongside it, so the crossover point — where a payment starts sending more to principal than to interest — is visible rather than something you have to compute by hand.
What this means for extra payments
Because interest is calculated on the remaining balance, an extra payment made early in the loan avoids more future interest than the identical extra payment made later — it is removed from the balance while that balance still has the most years left to accrue interest against. This is the same compounding mechanism from how compound interest works, running as a discount instead of a gain: a dollar of principal paid down early is worth more, in total interest avoided, than a dollar paid down late.
The CFPB’s own check figure
$100,000 at 4% over 30 years comes out to approximately $477 a month by the standard amortization formula — a figure the CFPB itself publishes, which is why it doubles as a test case rather than only a citation. If a calculator or spreadsheet gives you something wildly different for a similar loan, that is the number to sanity-check against first.
Reading the table itself
- The payment column never changes on a fixed-rate loan — that consistency is the entire point of amortization, and it is what a rate change or a variable-rate loan gives up.
- The balance column falls slowly at first, then faster — not because payments grow, but because more of each unchanged payment is reaching principal as time passes.
- The interest column is the mirror image of the principal column at every single row, because together they always sum to exactly the payment.
Why refinancing resets the clock in a way that can cost more than it looks
Because the interest portion of a payment is largest at the start of a loan’s life, refinancing — taking out a new loan to replace an existing one, even at a genuinely lower rate — restarts that front-loaded interest pattern. A borrower ten years into a thirty-year mortgage has already worked through a decade of the highest-interest, lowest-principal payments; refinancing into a new thirty-year term moves them back to year one of that same pattern, on a new schedule. A lower rate can still be a net win, but it is a different question from “is the new rate lower,” and it depends on comparing total interest paid under both paths to the same finish line, not just comparing the two monthly payment amounts.
This is exactly the kind of comparison the amortization table is built for: running both the existing loan’s remaining schedule and a proposed refinance’s full schedule side by side shows the actual total interest difference, rather than relying on the monthly payment difference alone, which can look more favorable than the full picture supports.
What “points” actually buy on the table
Loan points — an upfront fee paid to reduce the interest rate — show up in the amortization table as a lower interest column throughout the entire schedule, in exchange for a lump sum paid at the start. Whether points are worth it depends entirely on how long you expect to hold the loan: the monthly savings from a lower rate take time to add up to more than the upfront cost, and if the loan is paid off or refinanced before that break-even point, the points were a net loss. Running the amortization table with and without points, and comparing cumulative interest paid at several possible payoff horizons rather than just the full loan term, is the only way to answer this for a specific loan rather than relying on a rule of thumb.
How to use this
If you are deciding whether an extra payment is worth making, the table answers it more usefully than instinct does: it tells you exactly how much interest that specific extra payment, made in that specific month, avoids over the rest of the loan — not a rule of thumb, the actual number for your actual loan.
