How we earn the right to show a number

Most calculator sites keep their formulas invisible. TrueCostHousing publishes this page so any number produced by any calculator can be traced back to a stated rule rather than taken on faith. If a result here disagrees with a lender's, agent's, or contractor's estimate, this page is where to check which assumption caused the difference — and the about page covers why the site was built this way in the first place.

1. Source: reference figures require a named, dated source; otherwise they are clearly labeled editable examples.
2. Locality: locally variable numbers stay user-entered without authoritative local evidence.
3. Recency: sourced figures carry a verification date and review cycle.
4. Testing: assertions, boundary cases, and randomized checks must pass.
5. Limits: every tool explains what it cannot determine.

TrueCostHousing separates arithmetic from assumptions. A formula can be exact while its inputs remain uncertain, so every locally variable figure stays editable.

• Fixed-rate mortgage payment: principal multiplied by the monthly rate and amortization factor. Taxes, insurance, HOA, PMI, and maintenance are added only when a tool explicitly asks for them.
• Debt-to-income: selected monthly obligations divided by gross monthly income. The real-life burden ratio is deliberately labeled separately from lender-style DTI.
• Cash to close: down payment plus entered loan, title, inspection, prepaid, escrow, moving, repair, and reserve amounts, minus credits and deposits already paid.
• Net operating income: effective rental income minus operating expenses before debt service. Financing is subtracted afterward when calculating cash flow.
• Cap rate: annual net operating income divided by property price. It is a screening ratio, not a forecast.
• Scenario comparisons: rent vs. buy, build vs. buy, stay vs. sell, and sell vs. rent show differences under user-entered assumptions; they do not issue verdicts.

Each calculator page explains its exact inclusions and exclusions. Corrections can be sent through the contact page.

The current calculation engines passed deterministic boundary tests and a large batch of randomized scenario checks before this release. Tests cover zero-interest loans, negative and positive scenario changes, high and low percentages, balance limits, sign handling, and reconciliation between displayed subtotals and totals.

A passing test does not make an entered assumption true. It establishes that the published engine performs the stated arithmetic across the tested range. When a formula, field, or displayed result changes, its tests must be run again before publication.

1. No invented ZIP precision: a ZIP code never creates a tax, insurance, HOA, utility, repair, or construction quote.
2. User-entered local costs: material local figures remain editable until the user replaces them with an authoritative document or quote.
3. Named sources: explanatory guides identify the responsible public source and review date when they summarize a rule or document.
4. Visible uncertainty: scenarios and planning ranges are labeled as assumptions rather than predictions.
5. Privacy: calculations run in the browser and calculator values are not transmitted as analytics events.

TrueCostHousing is built and maintained independently — see the about page for more on who runs the site and why. Because no advertiser, lender, or brokerage is involved in setting a default value or a formula, a mistake found here is corrected on its own timeline, not a sales one.

Sourced figures are checked against their named source at least once a year, or sooner when the underlying data changes; the review date shown next to a source reflects the most recent check, not the date the page was first written. Formula changes always ship with the re-run tests described above before anything goes live. If a figure, link, or calculation looks outdated or wrong, report it through the contact page — corrections are the fastest way this methodology stays accurate.