Mortgage Calculator · Manual
How to Use the Mortgage Calculator
Enter a fixed-rate mortgage scenario, add optional monthly housing costs, inspect amortization, and test extra principal without sending the scenario to a calculation server.
1. Enter the purchase assumptions
Start with Home price, choose whether Down payment is a percent or dollar amount, then enter the annual Interest rate and Loan term in whole years. Start month is optional and only labels the payoff month.
2. Add housing-cost assumptions when useful
Property tax and homeowners insurance are entered as annual amounts and divided by 12. HOA and mortgage insurance are entered as monthly amounts. The app does not infer these values or decide whether they apply.
3. Calculate and read the result
The headline is the estimated monthly housing payment. Principal & interest is shown separately from tax, insurance, HOA, and mortgage-insurance assumptions. Total interest and payoff time come from the full amortization schedule.
4. Test extra principal
Extra principal / month applies every payment. Extra principal / year follows the preserved legacy cadence at payment 1, 13, 25 and so on. One-time extra principal applies with payment 1. The final extra payment is capped so the modeled balance cannot become negative.
5. Keep a reviewed copy
Copy result, download the amortization CSV, print or save as PDF, or use TXT, Markdown, JSON, HTML, DOCX, TSV, or ZIP Mortgage Kit. Exports contain financial assumptions, so review them before sharing.
Validation and limits
Required fields do not silently become zero. Plain decimal input is required; malformed numbers, negative costs, an amount down payment above the home price, invalid terms, and non-finite scenarios are rejected. This app is a planning calculator, not a lender quote or financial, tax, or legal advice.