Methodology & sources

Our pledge: all-in cost first, always

The headline number on every CalculatorByState mortgage calculator is always the all-in monthly payment — principal, interest, property taxes, homeowners insurance, HOA dues (when applicable), and PMI (when applicable). We never lead with a principal-and-interest-only figure the way many calculators do, because that number understates what you'll actually pay every month. The Consumer Financial Protection Bureau has specifically warned that omitting taxes, insurance, PMI, and HOA leads people to underestimate the true cost of homeownership — we built this pledge into the calculator itself (it's enforced in code, not just in copy) rather than leaving it as a design suggestion.

How the calculation engine works

All math lives in a small set of pure, dependency-free TypeScript functions with no AI and no network calls involved — the same functions run for both Novice and Expert mode, so the two views can never drift apart or show different numbers for the same inputs.

• Monthly payment uses the standard fixed-rate amortization formula (M = P[r(1+r)ⁿ] / [(1+r)ⁿ−1]) to compute principal & interest, then adds monthly property tax, insurance, HOA dues, and PMI (when down payment is below 20%; PMI is clearly marked as removable once you reach roughly 20% equity).
• Affordability ("how much house can I afford") inverse-solves the payment calculation against your income, debts, and a debt-to-income ceiling, using the exact same payment function above — so an affordability result and a payment result for the same home price always agree.
• Qualification signal compares your proposed housing payment and total debts to your income using documented industry thresholds: a conservative 28% front-end / 36% back-end guideline, and the 43% back-end ceiling used for a Qualified Mortgage under the CFPB's Ability-to-Repay rule. We also note where FHA and VA loans commonly allow higher back-end DTI with compensating factors. The specific threshold used is always shown in plain language next to your result — never a bare green/yellow/red badge with no explanation.
• Amortization schedule runs the same month-by-month math a lender would use, including support for extra/lump-sum payment scenarios and the resulting interest and time saved.

Before any of this shipped, we tested it against 41 scenarios covering realistic price/rate/term/down-payment/insurance combinations across our launch states, boundary conditions on every DTI threshold, and extra-payment amortization scenarios — each checked against independently-derived expected values, within a documented ±$5/month tolerance for principal & interest rounding. That test suite runs on every change to this codebase; nothing ships if it fails.

Mortgage state data: what it is and how it's sourced

Last updated 2026-08-28

Property tax rates, homeowners insurance averages, transfer tax rules, homestead exemptions, first-time-buyer programs, median home prices, and typical closing-cost ranges are compiled per state from public and industry sources — property-tax comparisons (Tax Foundation, WalletHub), insurance-rate surveys (NerdWallet), official state housing-finance agencies for down-payment-assistance programs (e.g. PHFA in Pennsylvania, TDHCA/TSAHC in Texas, Florida Housing Finance Corporation in Florida), state departments of revenue for transfer-tax rules, and realtor-association / regional research-center data for median sale prices, chosen over smoothed home-value indices because they reflect actual closed transactions. Every first-time-buyer program link points to an official .gov or state-agency .org page — never a lender or an affiliate link.

Every numeric figure carries a sources[] entry with the specific source and the date it was checked — visible on every calculator page in Expert mode, and summarized here. Where sources genuinely disagree (this happens most with homeowners insurance, which varies heavily by coverage tier and region), we say so explicitly rather than picking a number and hiding the uncertainty.

County-level data: for a small, curated set of the largest/most-searched counties per state, we also publish county-specific property tax rates, median home prices, homeowners insurance averages, and FHFA conforming loan limits — sourced the same way as the state figures above (SmartAsset/Ownwell for property tax, local realtor association or Redfin data for median price, Insure.com/Insurify for insurance, FHFA for loan limits). We deliberately have not built this out for every county — a county with no entry below shows an honest "not yet covered" page rather than an estimated number.

Update cadence: state-level figures (tax rates, insurance averages, median prices, program details) are reviewed and refreshed periodically as new official data becomes available — property tax and insurance figures typically shift slowly year-over-year, while program details and median prices are checked more frequently. Every figure's asOfDate tells you exactly how current it is; we do not silently age out stale numbers.

Home insurance state data: what it is and how it's sourced

Last updated 2026-08-28

The Insurance category has its own dataset, researched and cited independently of the mortgage figures above. For every state we publish an average annual premium, the standard deductible, whether a separate catastrophe (wind, hail, hurricane, or named storm) deductible applies and at what percentage, the state's insurer of last resort if it has one, how roof claims are typically settled, and an average rebuild cost per square foot.

Every premium is quoted at the same coverage level — $300,000 of dwelling coverage. This matters more than it sounds: a premium figure means nothing without the coverage it buys, and most published state comparisons quietly mix coverage tiers, which makes their numbers incomparable. Our loader refuses to accept a premium without its tier. Each state's figure is cross-checked against at least two independent sources that both state $300,000 explicitly, with both figures named in the source note — including when they disagree.

A limitation worth stating plainly: the rebuild-cost-per-square-foot figures come from a single cost-data service that publishes bands shared across groups of states, so for most states that number reflects a regional band rather than a state-specific survey. Each source note says which. It is a planning figure, not an appraisal, and the replacement cost calculator says so on the result.

Rental property figures

The Investment category deliberately has no dataset of its own. Cap rate, cash flow, and cash-on-cash return are arithmetic on numbers you supply, and the honest state-level inputs — property tax rate, insurance cost, median price — are the same figures already sourced above. Maintaining a parallel set of citations for the same underlying facts would create two numbers where there should be one. Where a rental analysis shows a state figure, it is the figure documented in the sections above.

The vacancy and management-fee defaults on that calculator are planning conventions rather than sourced statistics, and are shown pre-filled and fully editable. They are defaults because the alternative — blank fields — is how vacancy and capital reserves get left out of an analysis entirely, which is the single most common way a rental appears profitable when it is not.

Where AI is — and isn't — involved

We use an AI model (Claude, from Anthropic) for exactly three things: turning a free-text description of your situation into structured calculator inputs, writing the plain-English (or technical) summary underneath your result, and explaining a term you hover or tap on if it isn't already in our glossary. In every case, the AI is handed the calculator's already-computed output as data and told, explicitly, never to invent, adjust, or independently calculate a number — it only writes text around figures that already exist. If the AI is ever unavailable, every one of these features falls back to a deterministic, non-AI alternative (a plain template summary, a regex-based extractor, or the static glossary) so the calculator itself never stops working.

What this is — and isn't

Every result on CalculatorByState is an estimate based on the information you provide, using publicly available average/typical figures for your state. It is not a loan quote, a pre-approval, a rate lock, or personalized financial or legal advice, and it does not reflect your individual credit profile, a specific property's actual tax bill or insurance premium, or a specific lender's underwriting decision. For an actual quote, speak with a licensed lender; for legal or tax questions specific to your situation, speak with a qualified professional in your state.

CalculatorByState is funded entirely by display advertising placed outside the calculator flow. We do not sell leads, route you to a specific lender, or accept payment to influence a result — the qualification signal, state data, and every number you see are the same regardless of what would monetize better.