The calculator starts with the standard loan payment formula to find your fixed monthly payment. From there it builds the amortization schedule month by month: each month's interest charge is the current outstanding balance multiplied by the monthly interest rate. The interest is subtracted from your fixed payment, and what remains reduces the principal. The new (lower) balance becomes the starting point for next month's calculation.
This is why early payments in a long loan are dominated by interest. In month one, interest is calculated on 100% of your original loan balance. In month two, it's calculated on a slightly smaller balance. The shift is gradual but persistent — by the midpoint of a typical 20-year loan, each payment is roughly split between interest and principal, and by the final years, nearly all of each payment goes to principal.
The amortization schedule makes the true cost of the loan transparent. It also shows you precisely what the outstanding balance is at any given point, which is useful if you're considering refinancing, selling an asset, or evaluating a prepayment.
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