Mortgage planning sources and verification notes.

How this site uses sources

True Cost Mortgage Calc uses transparent formulas and user-entered assumptions to estimate mortgage affordability. Some values, such as loan amount and interest rate, can be calculated directly from user inputs. Other values, such as property tax, insurance, PMI, HOA dues, utilities, and maintenance, require outside verification because they depend on the borrower, property, location, lender, insurer, and local rules.

The site’s purpose is to organize those assumptions in one place. A calculator result is strongest when the user replaces rough estimates with lender documents, tax records, insurance quotes, HOA documents, inspection findings, and local utility information.

Primary documents to verify before relying on a scenario

• Loan Estimate: Confirms loan amount, rate, APR, points, fees, projected payment, escrow assumptions, and cash to close.
• Closing Disclosure: Confirms final transaction costs before closing.
• Property tax record: Helps identify current tax bill, assessed value, exemptions, and local tax history.
• Homeowners insurance quote: Provides a property-specific premium estimate and coverage assumptions.
• HOA documents: Identify dues, reserve health, special assessments, covered maintenance, and owner responsibilities.
• Inspection report: Helps translate property condition into repair and maintenance reserves.
• Utility history: Helps estimate electricity, gas, water, sewer, trash, internet, and seasonal costs.

Rate Watch source

The Rate Watch banner stores national average mortgage-rate references in assets/rate-watch.json. National averages are useful for planning, but they are not personal quotes. Actual rates vary by credit profile, lender, location, property type, loan program, points, fees, lock timing, down payment, and market conditions.

Users should replace the default rate with a current lender quote before making a decision.

• Freddie Mac Primary Mortgage Market Survey: https://www.freddiemac.com/pmms
• FRED mortgage-rate data release: https://fred.stlouisfed.org/release?rid=190

Assumptions that need local verification

Property taxes can vary by city, county, school district, assessed value, exemptions, and reassessment rules. Insurance can vary by property condition, roof age, replacement cost, claims history, location risk, and coverage choices. PMI can vary by loan program, credit score, down payment, lender, and borrower profile. Maintenance can vary by home age, square footage, climate, system condition, and local labor costs.

For these reasons, the calculators use estimates for planning, not final answers. A realistic estimate should become more accurate as the buyer gathers documents.

Why ZIP code alone is not enough for property tax

A ZIP code can be useful for a rough early estimate, but it does not always match a tax jurisdiction. Multiple cities, school districts, special assessments, and exemption rules can exist within one ZIP code. The safest property-tax estimate comes from local property records or an address-level property source. Until that information is confirmed, users should run a conservative tax scenario.

Calculator formula transparency

The fixed-rate payment formula is described on the methodology page. The site also explains how true monthly housing cost, front-end DTI, back-end DTI, leftover cash, reserve gap, refinance breakeven, and escrow shock are estimated. These formulas are visible so users can understand the result and challenge the assumptions.

How to replace estimates with confirmed values

When a buyer first uses the calculator, some numbers may be rough estimates. That is normal early in the process. As the purchase becomes more serious, each estimate should be replaced with a stronger source. Interest rate and closing costs should come from lender documents. Insurance should come from a property-specific quote. Taxes should come from local property records and local rules. HOA dues should come from association documents. Maintenance reserves should be adjusted after inspection.

The site is most useful when it becomes a worksheet for collecting better numbers. A calculation that begins as a rough screen can become a more reliable planning scenario as each assumption is verified. This process also helps the user identify which number is creating the most risk.

Examples of weak versus strong assumptions

Using stronger assumptions does not guarantee a perfect result, but it reduces the chance that the calculator understates the cost of ownership.

Document hierarchy for better estimates

Not every source has the same reliability. A generic online average can help with early screening, but a property-specific quote is stronger. A prior tax bill can help, but local reassessment rules are stronger. A seller-provided utility estimate can help, but utility history for the actual property is stronger. A general maintenance rule can help, but inspection findings and contractor estimates are stronger.

When two sources conflict, the more property-specific and current source usually deserves more weight. The calculators are designed to accept updated inputs as better information becomes available. This lets users move from rough screening toward a more practical buying or refinancing decision.