US Calculator Methodology
Last updated 25 July 2026
Every US calculator on Fiscalgrove is built from the same underlying amortizing-loan formula lenders use, plus the specific regulatory or lending-convention data relevant to that calculator. This page summarizes the formulas and sources; each calculator's own page also explains its formula and worked example in context.
The core mortgage formula
Principal & interest is calculated as: M = P × r × (1+r)^n ÷ ((1+r)^n − 1), where P is the loan amount, r is the monthly interest rate (annual rate ÷ 12), and n is the number of monthly payments (term in years × 12). A US monthly housing payment is conventionally PITI — Principal, Interest, property Taxes, and homeowners Insurance — plus PMI when applicable and HOA dues where relevant. All calculations run on full, unrounded figures and round only for display, so amortization schedules don't drift.
Regulatory and convention-based figures
- PMI removal thresholds (78% automatic termination, 80% borrower-requested cancellation, both versus the ORIGINAL home value) come directly from theHomeowners Protection Act of 1998, as summarized by the Consumer Financial Protection Bureau.
- The 28/36 DTI rule (front-end housing payment ≤28% of gross income, back-end total debt ≤36%) is a long-standing conventional-lending guideline, not a hard legal cap — many lenders will go higher with strong compensating factors, which we note as context alongside the strict guideline.
- State property tax rates are compiled from Tax Foundation effective-rate data and cross-checked against multiple current secondary compilations; they are STATE AVERAGES only; actual rates are set at the county/municipal level and can vary substantially within a state.
- PMI cost estimates, closing cost ranges, and points-to-rate reduction ratios are lender-set and vary by product — we use commonly cited industry ranges, all editable, and label them clearly as estimates.
Editable defaults
Every default value used in a calculation (property tax rate, PMI rate, points reduction ratio, DTI thresholds, and so on) is visible and editable in the calculator itself — nothing is a hidden assumption.