Methodology

Principles

• Every factual default (a rate, limit, levy, exemption or benchmark) comes from a dated public source, shown next to the number and listed below.
• We keep a copy of each source file as downloaded and record its SHA-256 fingerprint, so any figure can be traced to the exact document.
• Values a source doesn't provide aren't invented. Inputs that are assumptions, not facts, are labeled as assumptions you can change.
• Calculations run in your browser. Results are rounded only for display.
• Every calculator is unit-tested against hand-worked examples, including published examples from the Oklahoma Tax Commission and the Oklahoma County Assessor.

Data sources

Notes on the sources: the Oklahoma Tax Commission's 2024 Ad Valorem Statistics is the latest edition published as of October 8, 2026; no 2025 edition was found. OHFA's income-limit exhibits are labeled 2025 and were released September 1, 2025. The DREAM Freddie Mac exhibit labels its price line “2024 Purchase Price Limits” although the exhibit is dated 09/01/2025; the $453,100 figure matches OHFA's program page. In the Oklahoma County levy table, code 852 (published total 111.30, parts 111.85) doesn't add up; we use the published total. The OTC rule for the additional homestead exemption says $30,000; the Oklahoma County Assessor's page says $30,000 in one place and $25,000 in another.

How often figures are updated

OHFA posts rates around 10:00 a.m. Central on business days. Freddie Mac publishes PMMS on Thursdays at noon Eastern. FHFA and HUD announce next year's loan limits in late November and December. Counties certify levies in November, and the Tax Commission publishes its statistics book afterward. OHFA releases new income-limit exhibits around September. LendingTree updates its insurance study about once a year.

OHFA rates, Freddie Mac's survey and the FHFA and HUD limit files are refreshed by script. Each refresh downloads the source with an identifying user agent, saves a dated copy with its fingerprint, parses it, and compares it with the figures already published. If a rate moves more than 0.75 percentage points, any limit or assistance percentage changes, or the source no longer parses the way it should, nothing is published until a person checks the source. OHFA's program page is checked weekly for changed limits or new exhibits; county levies, the Tax Commission's statistics and OHFA's exhibits are updated by hand when new editions come out. Each page's “Data as of” date shows the latest pull, and the dot turns from green to amber if that date is older than the source's normal update cycle. Current pulls: Freddie Mac week of Oct 8, 2026; OHFA rates 10:00am - October 8, 2026.

Formulas

Mortgage payment

M = P × r / (1 − (1 + r)^−n)
Monthly payment = M + property tax ÷ 12 + insurance ÷ 12 + PMI or FHA MIP + HOA

• Amortization. Each month, interest = balance × r and principal = M − interest. The last payment is trimmed so the balance ends at exactly zero. At a 0% rate, M = P ÷ n.
• FHA MIP. Upfront premium 1.75% of the base loan. Annual premium from HUD Mortgagee Letter 2023-05 by term (over 15 years or not), base loan versus the national conforming limit ($832,750 in 2026), and LTV = base loan ÷ price. Over 15 years at or under the limit: 0.50% up to 95% LTV, 0.55% above. We show annual rate × base loan ÷ 12; HUD charges it on each year's average outstanding balance, so the real amount declines slowly.
• PMI estimate. Conventional loans above 80% LTV: loan ÷ $100,000 × $50, the midpoint of Freddie Mac's published range of $30–$70 a month per $100,000. It's an estimate; you can change it.
• Mortgage registration tax. ⌈loan ÷ $100⌉ × $0.10 + $5.00 (68 O.S. § 1904, mortgages of five years or more; any remaining fraction of $100 counts as a full $100). Computed in whole cents.
• Defaults. Rate: Freddie Mac PMMS for the chosen term. Insurance: LendingTree's Oklahoma average. The $250,000 price and 10% down payment are just a starting point.

Property tax

Assessed = V × A
Taxable = max(0, Assessed − E)
Tax per year = Taxable × M ÷ 1,000

Checked against the Oklahoma County Assessor's published example ($50,000 market value, homestead exemption, $100 per thousand = $450) and against a $300,000 home in district 200 ($3,932.80 a year).

OHFA GOLD and DREAM check

• GOLD income. Government loans: Exhibit H-1A limit for the county, for 1–2 or 3+ people, non-targeted or targeted. Conventional: Exhibit H-1F (one limit per county). Compared with the gross income of all household members 18 and older.
• GOLD price. $349,525, or $427,198 in a targeted area.
• First-time buyer. Required for GOLD unless targeted. Counties Exhibit H-3 lists as all non-targeted are always treated as non-targeted.
• Unknown tract. A value over the non-targeted limit but within the targeted one is “needs a check”, as is a repeat buyer.
• DREAM. Government: $150,000 income, $356,362 price. Conventional: Exhibit H-1D county limit, $453,100 price. Compared with the lender qualifying income.
• Assistance. Posted assistance percentage (3.50%) × total loan amount, where total loan = price − down payment (+ FHA upfront premium if included).
• Credit score and debt-to-income limits are shown, not tested. The result is never stated as approval.

Buydowns and discount points

Fund = Σ 12 × [M(note rate) − M(note rate − k)] over the buydown years
Break-even months = ⌈ points × loan ÷ (M(rate without) − M(rate with)) ⌉
Net at horizon h = (M(without) − M(with)) × 12h − points × loan

Closing costs

Mortgage tax = ⌈loan ÷ 100⌉ × $0.10 + $5.00
Deed stamps = ⌈price ÷ 500⌉ × $0.75 (price > $100; only if the buyer pays)
Prepaid interest = loan × rate × days ÷ 365 (or 360)
Escrow = months × annual amount ÷ 12
Cash to close = max(0, down + all costs − seller credits − OHFA assistance)

Lender, title, appraisal, inspection and recording fees have no default: you enter them from your Loan Estimate. Prepaid-interest days, insurance months and escrow months start at labeled assumptions (15 days, 12 months, 2 and 2 months).

Affordability

Housing limit = min(front% × income ÷ 12, back% × income ÷ 12 − other debts)
Max price = highest price whose full payment ≤ housing limit (bisection)

28/36 is offered as a common guideline (not a lender rule). The FHA option uses the manual-underwrite caps in OHFA's lender training. Because PMI and the FHA premium tier switch on at LTV thresholds, the payment can jump as price rises; the search always keeps the highest price that fits.

Rent vs. buy

Each month: budget = max(owner cost, rent)
Renter investments: start = down + buyer closing costs; += budget − rent
Buyer investments: += budget − owner cost
Growth: monthly rate = (1 + annual)^(1/12) − 1
Year-end net cost = cash put in − (buyer: value × (1 − selling %) − balance + investments; renter: investments)

Rent is always your entry. Rent growth, appreciation, investment return, maintenance and selling cost are labeled assumptions. Income taxes are ignored.

Home insurance

Shows LendingTree's Oklahoma and U.S. averages and your quote. Monthly = annual ÷ 12. Share of payment = (premium ÷ 12) ÷ (your payment before insurance + premium ÷ 12). The optional scaled figure, average × your dwelling coverage ÷ $350,000, is a labeled crude assumption. No county-level premiums are estimated.

Known limitations

• District-level levies are loaded for Oklahoma County only; the other 76 counties use the Tax Commission's 2024 county average.
• Fire protection district levies in Oklahoma County (codes ending in FD) aren't included in the published totals we use.
• Insurance is a statewide average; we have no reliable county-level premium data.
• No official PMI rate table is public; the PMI figure is an estimate.
• VA and USDA loans, the senior valuation freeze and veterans' exemptions aren't modeled yet.
• HUD Fair Market Rents and Census ACS data aren't used yet (they need API accounts), so rent is always your own entry.

Source files and fingerprints

Changelog

• October 8, 2026: first version, with Oklahoma County 2025 levies, OTC 2024 county data, OHFA 2025 exhibits and rates of October 8, 2026, and 2026 loan limits.

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