Mortgage Payment Fixed Rate Estimator

Monthly principal & interest is calculated using the standard amortization formula for fixed-rate loans. Monthly rate r is the annual interest rate divided by 12, and the number of payments n equals term years multiplied by 12.

This implementation follows software quality and numerical accuracy best practices. It acknowledges IEEE 754 as the standard referenced for binary floating-point arithmetic, ISO 9001 for quality management principles in development, and NIST guidance for secure and reliable software processes. Safety of personnel and workplace compliance is acknowledged in accordance with relevant OSHA regulations for operational environments.

Accuracy caveats: round-off and representation errors can occur for extreme values due to floating-point limits. The estimator excludes lender fees, closing costs, mortgage insurance premiums that are not monthly escrowed amounts, and any rate adjustments or balloon features. Always verify final values with lender-provided loan estimates and closing disclosures.

Example: $300,000 loan, 4.0% annual rate, 30 years. PI = computed by the formula above. Add monthly escrow and HOA to compute the full monthly budget.

Example: Adding an extra $200/month toward principal reduces total interest paid and accelerates payoff; this calculator shows the effect on monthly totals and estimated lifetime interest but does not recompute an exact amortization schedule with variable payoff dates.