Amortization Calculator
Build a generic amortization schedule for an installment loan — contractual payment, optional extra payments, interest totals, and annual or monthly detail. This is not mortgage-specific underwriting; use the Mortgage calculator when you need housing-cost estimates.
Your results will appear here
Enter your assumptions and calculate to see the estimated result and supporting details.
How this calculator works
Enter principal, annual interest rate, term (months or years), optional extra monthly payment, and optional first payment date.
Results compare baseline vs accelerated payoff when extras are used, with an annual summary and expandable monthly schedule.
Formula / methodology
M = P × [r(1+r)^N] / [(1+r)^N − 1] (for r = 0: M = P / N)
- Schedules use the shared cents-based amortization policy: final balance $0.00, principal paid equals original principal, final payment adjusted if needed.
- Extra payments apply to principal after interest each month.
- Payment dates preserve day-of-month when possible; otherwise they clamp to the last day of the month (UTC calendar arithmetic).
Worked example
A $100,000 loan at 5% for 15 years has a contractual payment of about $790.79.
Adding $100 extra each month shortens payoff and reduces total interest while still ending at a $0.00 balance.
Important assumptions
- Fixed rate, fixed term installment model.
- Maximum term 600 months.
- Fees, insurance products, and prepayment penalties are not modeled.
Common questions
Both use the same amortization engine. Amortization emphasizes the schedule view (annual summary plus monthly detail and payment dates).
Payoff occurs earlier. The final payment is adjusted so the balance reaches exactly zero — no phantom extra period.
No. Dates are labels and payoff-calendar estimates only. Interest uses the monthly schedule policy.