Methodology

Sourcing

Every number on this site traces to a primary source. Our principle is: if we can't cite it, we don't publish it.

• Mortgage rate benchmarks — Federal Reserve H.15 weekly release, published Mondays. federalreserve.gov/releases/h15
• State income tax brackets— each state's department of revenue (linked per state on its detail page). Reconciled monthly.
• Property tax median effective rates — Tax Foundation 2025 state-level effective rates, cross-referenced with US Census Bureau ACS 5-year median property tax / median home value.
• PMI tiering — FNMA Form 4030 (Mortgage Insurance Required) guidance for conventional conforming loans. Tiers: 0.50% (LTV 80-85%), 0.75% (85-90%), 1.00% (90-95%), 1.25% (LTV ≥ 95%).
• PMI auto-removal — Homeowners Protection Act of 1998 (12 U.S.C. § 4901-4910): automatic termination at 78% LTV based on the original amortization schedule.

Computation

The mortgage amortization formula is the standard closed-form expression:

M = P · r(1 + r)ⁿ / ((1 + r)ⁿ − 1)

where P is the loan principal, r is the monthly interest rate (annual rate ÷ 12 ÷ 100), and n is the number of monthly payments. Zero-rate loans are handled as straight-line principal: M = P / n.

Float arithmetic can produce sub-cent drift over 360 iterations. The accumulated error is at most a few cents on a typical loan and does not affect monthly payment display. We may port to arbitrary-precision arithmetic in a future version if cents-accurate reconciliation becomes a user request.

What we do not model

• Closing costs (typically 2-5% of the loan)
• Discount points / rate buy-downs
• Local transfer taxes
• Local municipal income tax (NYC, Yonkers, Philadelphia, San Francisco, etc.)
• State-level deductions, exemptions, and credits
• FHA MIP / VA funding fee (these are separate from conventional PMI)
• Adjustable-rate mortgage rate-reset behavior (planned for v0.2)

Update cadence

Mortgage rate benchmarks: weekly. State tax brackets: reconciled monthly, with corrections published within seven days of statutory changes. Methodology changes: documented in the changelog with an explanation.

Corrections

Found an error? Tell us via the contact page. Confirmed corrections are logged in the public changelog with the date and the source.