Mortgage Calculator: Payments & Amortization
Free, 100% private in-browser mortgage calculator. Calculate monthly payments with PMI, property taxes, home insurance, and HOA fees. Includes interactive amortization schedules and Excel/CSV export.
Interactive WebAssembly Calculation Engine
This tool runs 100% in your local web browser tab. All amortization schedules, debt yield calculations, and tax deferral projections are computed client-side in WebAssembly. Zero bytes are uploaded to remote servers.
Use the interactive form above to adjust parameters, toggle extra principal schedules, stress-test interest rates, and export full reports to Excel (.xlsx) or CSV.
Underwriting Methodology & Mathematical Formulation
Our calculation algorithms strictly adhere to institutional lending conventions, CFPB disclosure rules, and statutory IRS definitions:
- Standard Annuity Formula: Monthly payment M = P * [r(1+r)^n] / [(1+r)^n - 1], calculating exact interest and principal allocations down to the penny.
- Zero-Roundoff Double Precision: Computations execute using standard IEEE 754 floating-point and integer math to match commercial bank loan documents.
- Sensitivity Analysis: Real-time stress testing against interest rate fluctuations, vacancy rate increases, and balloon maturity horizons.
Frequently Asked Questions
What is PITI in a monthly mortgage payment?
PITI stands for Principal, Interest, Taxes, and Insurance. These four components make up your total housing payment. In addition, homeowner association (HOA) fees and private mortgage insurance (PMI) may be included depending on your loan structure.
When does Private Mortgage Insurance (PMI) automatically cancel?
Under the federal Homeowners Protection Act, conventional mortgage lenders must automatically cancel PMI once your principal balance reaches 78% of the original home value, or you can request cancellation once your balance reaches 80% LTV.
How does an extra monthly principal payment save money?
Extra principal payments reduce your outstanding balance faster, which reduces future compound interest. Making an extra $100-$200 monthly principal payment can shave 4 to 6 years off a 30-year mortgage and save $30,000+ in interest.
What is the standard formula for a 30-year fixed mortgage?
Monthly payment M = P * [r(1+r)^n] / [(1+r)^n - 1], where P is loan principal, r is monthly interest rate (annual rate / 12), and n is total number of monthly payments (360 for 30 years).