Generate a monthly mortgage amortization schedule and compare total interest and payoff time with an extra-principal plan.
The Mortgage Amortization Schedule Calculator validates every input, normalizes monthly and annual amounts, and calculates monthly principal, interest, balance, extra payments, and payoff time from one consistent scenario.
Monthly rate r = annual percentage rate / 1200; n = years × 12. Scheduled M = P × r / (1 − (1+r)^−n), or P/n at zero interest. Month t interest = previous balance × r; principal paid = payment − interest; next balance = previous balance − principal paid. Extra principal accelerates this recurrence.
For $320,000 at 6.5% over 30 years, the scheduled payment is about $2,022.62. First-month interest is $1,733.33, leaving $289.29 principal. Adding $250 makes first-month principal about $539.29; the final payment is capped at the remaining balance plus interest.
Content and calculation reviewed:
Understand the calculation
A fixed-payment loan is an annuity: the discounted value of all future payments equals the amount borrowed today. Summing that geometric series gives the monthly-payment equation. Match the rate and time unit: monthly payments use an annual nominal rate divided by twelve and a term counted in months.
P = Σ[t=1…n] M/(1+r)^t M = Pr / (1 − (1+r)^−n) Iₜ = Bₜ₋₁r; Bₜ = Bₜ₋₁ + Iₜ − paymentₜ
The detailed guide adds variable definitions, a reproducible example, a practice question and model limitations.