Finance
Mortgage Calculator with Extra Payment & Bi-Weekly Mode
See your monthly payment, total interest, and exactly how many years extra principal saves. Compare scenarios A vs B side-by-side.
How this is calculated
The standard amortizing mortgage formula:
M = P · r(1+r)^n / ((1+r)^n − 1)
where P is principal, r is the monthly rate (APR ÷ 12), and n is the number of payments (years × 12).
Why extra payments save so much
On a 30-year loan, almost two-thirds of your first decade's payments are interest. Every extra dollar applied to principal reduces the balance the rest of the loan accrues interest on — so $100/month early in the loan can save tens of thousands of dollars over the life of the loan.
Bi-weekly = one extra payment
26 half-payments equal 13 monthly payments — automatically. Switch to bi-weekly above to see the impact.
Frequently asked
- On a typical 30-year, $400k loan at 7%, an extra $100/month saves about $80k in interest and pays the loan off ~4 years early. Run your own numbers above.
- Yes — 26 half-payments per year = 13 monthly payments. The trick is that lenders must apply each half-payment immediately, not hold it.