Generate loan amortization schedules with stacked area charts showing principal vs interest breakdown over the loan lifetime.
This tool takes a loan amount, annual interest rate, and term, then computes the fixed periodic payment using the standard amortizing-loan formula and walks that payment forward period by period. Each row splits the payment into interest (the balance times the periodic rate) and principal (the remainder), then reduces the balance accordingly; the results feed a stacked area chart so the principal and interest bands are visible across the whole term. What surprises most users: even though the payment amount never changes, the interest portion is largest in the very first period and shrinks every period after that, since it's always computed on a declining balance. How early principal overtakes interest depends on the rate and term - on a 30-year mortgage that crossover can take years, but on a short-term loan like the example below, principal already exceeds interest in month 1.
A concrete input and expected output from the current implementation.
Input
$10,000 loan, 12% annual interest rate, 12-month term, monthly payments
Expected output
Monthly payment: $888.49. Month 1: interest $100.00, principal $788.49, ending balance $9,211.51. Total interest paid over the 12 months: $661.88.
The periodic rate is 12%/12 = 1% per month, so month 1 interest is 1% of the full $10,000 balance; the fixed payment minus that interest is the principal reduction, and summing all 12 payments minus the original $10,000 principal gives the total interest.
The payment is solved once, up front, from M = P*r(1+r)^n / ((1+r)^n - 1), where P is the principal, r is the interest rate per period, and n is the number of periods. That single value is then held constant for every row in the schedule, exactly as a standard fixed-rate installment loan works.
For each period the tool multiplies the current balance by the periodic rate to get that period's interest, subtracts it from the fixed payment to get principal, then subtracts principal from the balance to carry forward. The resulting principal and interest series are stacked into an area chart so the shifting split is visible across the loan's lifetime.
Interest for a period is the periodic rate applied to the remaining balance, and the balance only goes down, so the interest charge shrinks every period. Since the total payment is fixed, whatever interest no longer takes is automatically credited to principal - that's why the principal share grows as the interest share shrinks.
Not from the loan amount, rate, and term fields described here - those alone produce the standard fixed-payment schedule. If the tool doesn't expose a separate extra-payment or lump-sum field, it isn't modeling early payoff, and you'd need to recompute a shorter schedule manually to see that effect.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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