Loan Payment Calculator

Type the loan amount, annual interest rate, and term in years, and the calculator returns the monthly payment, total interest paid over the life of the loan, and a full amortization table showing principal vs interest for every payment.

The amortization table is where the long-term cost becomes obvious. On a 30-year mortgage at 7%, the first payment is roughly 80% interest and 20% principal. It takes around 20 years before principal overtakes interest in each payment. This is why making extra principal payments early saves so much more than the same dollar paid late.

For adjustable-rate or interest-only loans, this calculator's fixed-rate model isn't right. It assumes a constant interest rate and full amortization — the standard mortgage, auto loan, and personal loan structure.

The formula: monthly_payment = P × (r(1+r)^n) / ((1+r)^n − 1), where P = principal, r = monthly rate (annual / 12), n = total months. The amortization table iterates: each month, multiply remaining balance by r to get interest portion, subtract from monthly payment to get principal portion, then reduce balance. Repeat until balance hits zero. The shape of the table is exponential — early months are nearly all interest, late months nearly all principal, and the crossover happens late in the term.

Worked example: $300,000 mortgage at 7% over 30 years. Monthly payment: $1,996. Total paid over 30 years: $718,527. Total interest: $418,527 — more than the original loan. First payment: $1,750 interest + $246 principal. Last payment: $11 interest + $1,985 principal. The first 10 years pay down only $42,000 of principal. The last 10 years pay down $174,000. That's why an extra $200/month in years 1-3 saves dramatically more than $200/month in years 25-27 — early extra payments hit a balance that's still earning massive interest.

Where the model breaks: variable-rate loans (ARMs) where the rate adjusts every 1, 5, 7, or 10 years. Interest-only loans where you pay no principal for the first N years. Negative amortization loans where unpaid interest gets added to principal. Balloon loans where most of the principal is due at term-end. None fit a fixed-rate amortization formula. For ARMs specifically: model the worst case (cap rate × max term length) before signing — what looks affordable at the teaser rate may not be at the post-reset rate.