About this tool
Early Payoff & Extra Payment Calculator is a free, in-browser tool that shows how much interest you save — and how many months you cut — by adding a recurring extra payment, a one-time lump sum, or switching to biweekly payments on any loan or mortgage.
It simulates two amortization schedules month by month: the baseline with your current payment, and a new one with the lump sum applied up front and the extra amount added to every payment. Biweekly mode models 26 half-payments a year, which works out to roughly one extra full payment annually.
Use it to answer questions like "what does an extra 200 a month do to my 30-year mortgage?" — the payoff time, months saved and interest saved update live, all computed locally with nothing uploaded.
Frequently asked questions
How is the biweekly option modeled?
Paying half your monthly payment every two weeks means 26 half-payments — 13 full payments — per year. The tool models this as your monthly payment plus one-twelfth extra each month, a close approximation of the true biweekly schedule.
Why do I get a 'payment too small' error?
If your monthly payment does not exceed the first month's interest (balance × annual rate ÷ 12 ÷ 100), the balance never falls and the loan can never be paid off. Increase the payment to get a result.
When is the lump sum applied?
Immediately, before the new schedule starts, so it stops accruing interest from month one. Applying it later would save slightly less.
What rate convention is used?
The rate is an annual percentage compounded monthly (annual % ÷ 12 ÷ 100 per month), the standard convention for mortgages and installment loans. All simulation happens in your browser.