Methodology

How DollarDrill
Actually Calculates

Every formula we use, every assumption we make, every source we rely on — and what each tool deliberately leaves out. If you can't check our work, you shouldn't trust it.

Why this page exists

DollarDrill aggregates standard, published financial mathematics and figures from primary sources — federal agencies and the institutions that actually publish the underlying data — into free calculators and plain-language guides.

That is the whole claim. We are not telling you that a certified planner reviewed your situation. We are telling you that the math is standard, the inputs are sourced, and you can verify every one of them. This page is where you check.

"If a number on this site can't be traced to a formula we've stated or a source we've named, it shouldn't be on this site."

Who writes this

DollarDrill is written and maintained by a small independent team, published under the organization's name rather than an individual byline. We do not attribute articles to invented experts, and we do not claim credentials we don't hold.

Where a claim requires expertise we don't have — the tax treatment of an insurance product, the text of a federal rule — we cite the agency that does hold it, link to the source, and quote it rather than paraphrasing it into something it doesn't say.

The formulas

Amortized loan payment — Loan & Mortgage, Home Affordability

The standard amortization formula. M is the monthly payment, P the principal, i the monthly rate (annual rate ÷ 12), and n the number of monthly payments.

M = P × i / (1 − (1 + i)−n)
Does not account for: property taxes, homeowner's insurance, PMI, HOA dues, points, origination fees, or any rate that changes after closing — unless the tool asks you for that figure explicitly.

Simple interest — Simple Loan

Interest accrues on the original principal only, never on accumulated interest.

I = P × r × t
Does not account for: fees rolled into the balance, penalty rates after a missed payment, or compounding — which is the entire point of the comparison.

Compound growth with regular contributions — Investing

Future value of a lump sum plus a series of level contributions, where i is the periodic rate and n the number of periods.

FV = P(1 + i)n + PMT × [((1 + i)n − 1) / i]
Does not account for: fund fees, taxes on gains or dividends, sequence-of-returns risk, or the fact that real markets do not return a constant percentage every year. A constant-rate projection is a teaching tool, not a forecast.

The 28/36 rule — Home Affordability

A long-standing underwriting guideline, not a law. Housing costs (principal, interest, taxes, insurance) capped at 28% of gross monthly income; total monthly debt obligations capped at 36%.

Max housing payment = 0.28 × gross monthly income Max total debt = 0.36 × gross monthly income
Does not account for: your actual cost of living, childcare, medical costs, or the difference between gross and take-home pay. It is a lender-side heuristic; we show it because lenders use it, not because it describes your life.

Paycheck estimate — Salary & Paycheck

Gross pay less pre-tax contributions, then federal income tax on the remainder using the current-year standard deduction and marginal brackets, plus Social Security (6.2% up to the annual taxable maximum) and Medicare (1.45%, with the 0.9% Additional Medicare Tax above the statutory threshold), plus a state estimate.

Important simplification: state income tax is applied as a single flat rate per state, shown next to the state name in the selector. Most states use graduated brackets, local income taxes exist in some cities, and several states tax differently by filing status. Treat the state line as an approximation, not a filing figure. Federal figures reflect the tax year noted below.

Where the numbers come from

Anything that is not arithmetic comes from a named primary source. We cite the agency that publishes the data, not a site summarizing it.

FigureSourceCurrent value used
30-year / 15-year fixed mortgage ratesFreddie Mac Primary Mortgage Market Survey[4]6.71% / 6.04% — week of 3 Sep 2026
Mortgage rate historyFRED, Federal Reserve Bank of St. Louis, series MORTGAGE30US[5]Full weekly series since 1971
Prime rate (HELOC and variable-rate pricing)Federal Reserve H.15 Selected Interest Rates[6]6.75% — 3 Sep 2026
Federal standard deduction and bracketsIRS tax inflation adjustments for tax year 2026[7]$16,100 single / $32,200 married filing jointly
Social Security taxable maximumSocial Security Administration, Contribution and Benefit Base[8]$184,500 for 2026
Wages by occupation and metro areaBLS Occupational Employment and Wage Statistics[9]May 2025 estimates, released 15 May 2026
Median household incomeU.S. Census Bureau, Income in the United States: 2024[10]$83,730 (2024)
Consumer lending rules and definitionsConsumer Financial Protection Bureau[1]Cited per claim
Tax treatment of insurance productsIRS Publication 525[11]Cited per claim

How often we refresh

Rate-linked figures go stale in a way that makes content wrong, not just dated. Our refresh cadence:

DataPublisher cadenceOur review
Mortgage rates (PMMS)Weekly, ThursdayMonthly, or on any move greater than 0.25 points
Prime rate (H.15)Daily / on Fed actionAfter each FOMC decision
Federal tax parametersAnnually, autumn release for the following yearAnnually, before 1 January
BLS wage dataAnnuallyWithin 60 days of release
Census income dataAnnually, SeptemberWithin 60 days of release

Every guide that depends on a rate carries a "figures current as of" line. If you find one that doesn't, that is a bug — tell us.

What DollarDrill is not

Not financial advice. Nothing here is personalized to your situation, because we do not know your situation. A calculator output is arithmetic, not a recommendation. For decisions that matter — a mortgage, a rollover, a tax position — talk to a licensed professional who can see your full picture.

Not a lead generator. We do not sell your information, because we do not collect it. No account, no email gate, no "unlock your results." Every calculator runs entirely in your browser; the numbers you type never leave your device.

Not free of commercial interest. The site carries advertising. Ads are what pay for it. They do not influence which formulas we use or what our guides conclude, and we do not accept payment for editorial placement or recommendations.

Not complete. Every model on this site simplifies. We have tried to say exactly where, above, rather than let the precision of a dollar figure imply a certainty the math doesn't have.

Corrections

Journals publish errata. Almost no calculator site does. We will.

If a number here is wrong, a formula is misapplied, or we have cited a source for something it does not actually say, email our contact page and tell us which page and which sentence. We will check it against the primary source, fix it, and note the correction and its date on the page rather than quietly editing it.

This applies to the tools as well as the writing. If a calculator returns a figure you can't reproduce by hand from the formulas above, that is a bug worth reporting.

Corrections log

DatePageCorrection
8 Sep 2026Home affordability guidesRemoved the implication that a 43% debt-to-income ratio is the current federal qualifying standard. The CFPB's December 2020 General QM final rule removed the 43% DTI limit and replaced it with price-based thresholds, effective 1 October 2022.[2][3] 43% now describes a common lender guideline, not a rule.
8 Sep 2026Three 2026 guidesReplaced an individual byline that did not correspond to a real, identifiable author with organizational authorship. Nothing about the underlying content changed; the attribution was not accurate and has been corrected.

Last reviewed: 8 September 2026. Rate figures on this page current as of the dates shown in the table above.

References

Primary sources this site is built on. Every one is a public document you can read yourself.

  1. Consumer Financial Protection Bureau. What is a debt-to-income ratio? consumerfinance.gov. Accessed 8 Sep 2026.
  2. Consumer Financial Protection Bureau. Qualified Mortgage Definition under the Truth in Lending Act (Regulation Z): General QM Loan Definition. consumerfinance.gov. Accessed 8 Sep 2026.
  3. Federal Register. General QM Loan Definition; Delay of Mandatory Compliance Date, 86 FR 22844 (30 April 2021). federalregister.gov. Accessed 8 Sep 2026.
  4. Freddie Mac. Primary Mortgage Market Survey. freddiemac.com/pmms. Accessed 8 Sep 2026.
  5. Federal Reserve Bank of St. Louis (FRED). 30-Year Fixed Rate Mortgage Average in the United States (MORTGAGE30US). fred.stlouisfed.org. Accessed 8 Sep 2026.
  6. Board of Governors of the Federal Reserve System. H.15 Selected Interest Rates. federalreserve.gov. Accessed 8 Sep 2026.
  7. Internal Revenue Service. IRS releases tax inflation adjustments for tax year 2026. irs.gov. Accessed 8 Sep 2026.
  8. Social Security Administration. Contribution and Benefit Base. ssa.gov. Accessed 8 Sep 2026.
  9. U.S. Bureau of Labor Statistics. Occupational Employment and Wage Statistics, May 2025. bls.gov/oes. Accessed 8 Sep 2026.
  10. U.S. Census Bureau. Income in the United States: 2024 (P60-286). census.gov. Accessed 8 Sep 2026.
  11. Internal Revenue Service. Publication 525, Taxable and Nontaxable Income. irs.gov. Accessed 8 Sep 2026.
  12. U.S. Securities and Exchange Commission. Compound Interest Calculator, Investor.gov. investor.gov. Accessed 8 Sep 2026.

Found something wrong?

Tell us the page and the sentence. We check it against the source, fix it, and log the correction above.

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