Mortgage Amortization Calculator
Generate a complete month-by-month amortization schedule for any mortgage, with optional extra payments to see how much faster the loan pays off and how much interest you save.
How to use the Mortgage Amortization Calculator
- Enter your inputs into the Mortgage Amortization Calculator above.
- Results update instantly as you type — no submit button needed.
- Adjust any value to see how the result changes in real time.
The amortization schedule logic
Each month: Interest_k = Balance_(k−1) × monthly rate · · · Principal_k = Payment + Extra − Interest_k · · · Balance_k = Balance_(k−1) − Principal_k
The schedule recursively applies these three operations from month 1 until the balance reaches zero. Any extra payment in a month immediately reduces the balance and changes all subsequent interest charges.
Worked example
$350,000 mortgage at 6.5% for 30 years: month 1 interest $1,896, principal $317. Adding $200/month extra: original payoff in 360 months becomes 304 months, total interest drops from $446,400 to $341,200 — savings of $105,200.
Frequently asked questions
How should I send extra principal payments?
Specify in your payment that the extra is "applied to principal." Some servicers default to "future payment" — your balance doesn't fall and you don't save interest.
Are biweekly payments worth it?
Biweekly half-payments produce 26 half-payments per year = 13 full monthly equivalents. That extra monthly payment per year can shave 4–6 years off a 30-year mortgage. Some servicers charge fees for this.
When are the savings biggest?
Early in the loan, when interest is the largest share of each payment. A $200 extra payment in year 1 saves far more interest than the same payment in year 25.