STRATEGY PAPER

How DwellQ Works: Engine, Data Sources, and Formulas

Every number has a source. Every formula is verifiable.
Last reviewed March 2026 · DwellQ Research7 SOURCES

Key Findings

01Net worth comparison framework: models both paths month-by-month, not just payments
02Renter is modeled as an active investor—down payment + monthly surplus goes into a portfolio
03Monthly simulation captures PMI drop-off, ARM resets, tax reassessments, and amortization
04Tax engine compares itemized vs standard deduction each year, applies SALT cap per 2025 law
05Data from FRED (rates), FHFA (appreciation), Tax Foundation (property tax), III (insurance)
0650+ input variables—every formula is visible via Show the Math on each tab

The Net Worth Comparison Framework

DwellQ compares two parallel financial futures: renting (investing the capital you would have used as a down payment, plus any monthly surplus) versus buying (building equity through mortgage payments, appreciation, and tax benefits, minus all ownership costs). Both paths are simulated month-by-month over the full holding period. The result is a net worth comparison at every point in time, showing when (or whether) one path overtakes the other.

Month-by-Month Simulation

Unlike simple calculators that use annual averages, DwellQ models each month individually. This matters because mortgage amortization, PMI drop-off, ARM rate resets, property tax reassessments, and investment compounding all happen on specific timelines. Monthly simulation captures: principal vs interest split on every payment, exact PMI removal month (when LTV reaches 78%), ARM rate adjustments at specified intervals, property tax reassessment at annual boundaries, rent growth compounding, and investment portfolio growth including monthly contributions from the renter’s surplus.

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The Renter’s Portfolio

DwellQ models the renter as an active investor, not a passive consumer. The renter starts with the full down payment amount invested in a diversified portfolio (default: 7% nominal annual return). Each month, if the renter’s total housing cost (rent + renter’s insurance + utilities) is less than the buyer’s total cost (mortgage + property tax + insurance + maintenance + HOA), the surplus is added to the portfolio. If renting costs more, the deficit is withdrawn. This dynamic monthly tracking is what most calculators omit entirely.

Tax Intelligence (Q+)

The Q+ premium tier models tax implications at every level: mortgage interest deduction (only the portion above the standard deduction threshold), property tax deduction (subject to the SALT cap, which varies by filing status and year per OBBBA 2025 provisions), PMI deductibility (income-phased), state income tax rates, and capital gains tax at exit (with the $250K/$500K home sale exclusion for buyers). DwellQ compares itemized deductions against the standard deduction each year and only applies the larger benefit—the same logic the IRS uses.

Data Sources

Mortgage rates: Federal Reserve Economic Data (FRED), updated weekly via API. Appreciation rates: FHFA House Price Index (Purchase-Only), state-level quarterly data. Property tax rates: Tax Foundation and ATTOM Data, 50 states + DC. Insurance rates: Insurance Information Institute (III) and NAIC, 50 states + DC. Standard deduction: IRS Publication 501. SALT cap: One Big Beautiful Bill Act (OBBBA, H.R. 1, 2025). S&P 500 returns: FRED Total Return Index, used for default investment return assumption. All sources are publicly available and cited in every calculation.

What DwellQ Does Not Do

DwellQ produces scenario-based projections, not predictions. It does not forecast future home prices, interest rates, or market movements. It does not account for individual tax situations beyond the inputs provided (consult a CPA for tax advice). It does not include location-specific factors like flood risk, HOA special assessments, or local rent control unless manually adjusted by the user. The value of the tool is in modeling the complete set of relevant variables and letting you test sensitivity—not in claiming to know the future.

THE BOTTOM LINE
DwellQ is a financial modeling engine, not a financial advisor. Every input has a default sourced from public data, every formula is documented, and every result can be stress-tested by adjusting assumptions. The goal is not to tell you what to do—it’s to show you what the math says under the conditions you specify.

Frequently Asked Questions

How accurate is DwellQ?+
DwellQ is as accurate as your inputs. It models the complete set of financial variables that determine the rent-vs-buy outcome. Its value is in comprehensiveness and transparency, not in predicting the future. Run multiple scenarios with different assumptions to understand your range of outcomes.
Where does DwellQ get its data?+
Mortgage rates from the Federal Reserve (FRED), appreciation from the FHFA House Price Index, property tax rates from the Tax Foundation, insurance rates from the Insurance Information Institute, and tax brackets from the IRS. All sources are publicly available and updated periodically.
Why does DwellQ show different results than other calculators?+
Most calculators omit opportunity cost of the down payment, underestimate selling costs, oversimplify tax treatment, and ignore maintenance. DwellQ includes all of these. The result is often a less favorable picture for buying in the first 5–7 years, which aligns with academic research on the topic.
Is DwellQ financial advice?+
No. DwellQ is a financial education tool that produces scenario-based projections. It does not replace professional financial, tax, or legal advice. Always consult qualified professionals before making major financial decisions.
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RELATED ANALYSIS
Why Most Rent vs Buy Calculators Get It WrongTax Implications of Buying vs RentingUnderstanding Break-Even Time in Housing Decisions
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METHODOLOGY
DwellQ research uses a net worth comparison framework. Both paths—buying (building equity minus all ownership costs) and renting (investing the down payment plus monthly surplus)—are modeled month-by-month over the full holding period. Assumptions are documented, sensitivity-tested, and sourced from publicly available data. This is scenario analysis, not financial advice.
SOURCES & REFERENCES
  1. Federal Reserve Bank of St. Louis. FRED: 30-Year Fixed Mortgage Rate, S&P 500 Total Return Index.[fred.stlouisfed.org]
  2. Federal Housing Finance Agency. House Price Index (Purchase-Only), Quarterly.[fhfa.gov/data/hpi]
  3. Tax Foundation. State and Local Property Tax Rates by State.[taxfoundation.org]
  4. Insurance Information Institute. Homeowners Insurance Facts and Statistics.[iii.org]
  5. IRS. Publication 501: Standard Deduction. Publication 936: Mortgage Interest Deduction.[irs.gov]
  6. One Big Beautiful Bill Act, H.R. 1, 119th Congress (2025). SALT Cap Provisions.
  7. Beracha, E. and Johnson, K.H. ‘Lessons from Over 30 Years of Buy vs Rent Decisions.’ Real Estate Economics, 40(2), 2012.