Mortgage Extra Payments Calculator with Extra Payments

Core computations use the periodic interest rate r = (annual rate / 100) / payments per year and standard annuity formulas. Number of payments to payoff given a constant payment A is computed from n = -ln(1 - r*B/A) / ln(1+r), where B is the current balance when extra payments begin.

When extras begin after a delay, the remaining balance is computed by evolving the original amortization for k payments and then applying the post-extra payment formula to the remaining principal. The one-time lump-sum scenario reduces the remaining principal at the chosen payment and then recomputes remaining payments.

For engineering rigor and secure handling of numeric operations, implementations should follow recommendations for numerical stability (IEEE 754 floating point considerations), established quality management and process controls (ISO 9001), and security best practices (NIST).

This advanced calculator lets you compare multiple prepayment strategies using standard amortization math. Use the recurring extra mode for steady additional principal and one-time mode for lump-sum impacts.

Accuracy depends on exact loan terms, payment timing and whether your lender applies extras directly to principal or treats them as future payments. Always confirm results with your servicer and review your mortgage contract for prepayment policy and potential penalties.

Example 1: $300,000 loan, 3.75% APR, 30 years, $100 extra per monthly payment. The calculator will show reduced years to payoff and interest saved compared to the baseline.

Example 2: $200,000 loan, 4.0% APR, 15 years, one-time $10,000 prepayment at payment 24. The tool recalculates the remaining payments and interest after the lump sum is applied.