Mortgage APR Calculator with Extra Payments

Amortization math uses the standard fixed-rate loan formula to compute the scheduled periodic payment and the remaining balance after a given number of payments. When extra principal is applied, we recalculate the number of remaining payments assuming the same scheduled payment amount plus the extra principal contribution.

For recurring extra payments we estimate payoff using the closed-form solution for the number of payments when payment differs from the original amortizing payment. For a one-time extra we reduce outstanding principal at the chosen payment number and then compute remaining payments on the reduced principal.

APR as defined by consumer regulation (TILA) is a legal disclosure that may include finance charges and certain fees and is solved using root-finding to equate present values. This tool reports an approximate effective annualized interest based on interest paid divided by time to payoff; it is not a regulation-compliant APR calculation.

Example 1: $300,000 loan at 4.0% for 30 years. Scheduled monthly payment is computed; adding $200 extra/month shortens the payoff and reduces total interest. The calculator reports months saved and interest saved.

Example 2: Same loan, make a $5,000 one-time principal payment at payment number 12. The calculator estimates the new remaining schedule, interest saved and an annualized effective interest rate approximation.