Tool · Loan
Loan repayment calculator
Plain amortization math for mortgages, auto loans, and student debt. The chart makes one thing visible that the monthly-payment number alone obscures: how slowly the principal moves in the first few years of a long loan, and how much of every dollar at that stage is going to interest.
Monthly payment
$2,528.27
360 payments over 30 years
Total interest
$510,178
56.1% of total payments
Total paid
$910,178
$400,000 principal + $510,178 interest
How this is calculated
Monthly payment uses the standard amortization formulaP × r(1+r)n / ((1+r)n − 1)where P is the loan amount, r is the monthly rate (annual / 12), and n is the total months. Each month, interest is computed on the outstanding balance, the rest of the payment reduces principal, and the balance carries to the next month.
Cumulative interest climbs steeply early — when the balance is highest — and tapers off as the principal pays down. The chart shows both the balance curve (solid) and total interest paid (dashed); the gap between them at any year tells you how much you've reduced principal so far.
Property tax, insurance, PMI, HOA dues, and other carrying costs are not included — this is the principal-and-interest payment only. Variable-rate loans, ARMs, balloon payments, and pre-payment penalties are not modelled.
Illustrative calculation. Confirm any actual loan terms with your lender's amortization schedule — small rate-rounding and day-count differences can shift the headline payment by a few dollars.
Principal-and-interest only. Taxes, insurance, PMI, HOA dues, and closing costs are not modelled. Use your lender's official amortization schedule for the legally binding numbers.
PlainIndex publishes data and editorial commentary — nothing here is personalized financial advice.