Methodology and Data Sources
This page names the actual data behind the numbers on this site, so you can check any figure at its origin rather than taking ours on trust. If a page here disagrees with the source it cites, the source is right and we have a bug to fix. Please tell us.
The rule sources: where the rules themselves come from
Explanations of how a rule works are checked against the body that sets or enforces it:
| Topic | Primary authority |
|---|---|
| Federal taxes, retirement account rules and limits | Internal Revenue Service |
| Consumer credit, debt collection, mortgages, banking practices | Consumer Financial Protection Bureau |
| Investing basics, fund structures, fraud | SEC and Investor.gov, FINRA |
| Deposit insurance | FDIC |
| Social Security, Medicare premiums | SSA, Medicare.gov |
| Workplace benefits, wage and hour rules | Department of Labor |
The running reference list lives on the Sources page, and individual guides cite the specific pages they rely on in a “Sources and further reading” section.
The data feeds: where the numbers come from
Where this site uses live or recent economic data, it comes from these public sources, all of them official or widely used reference feeds:
- FRED (Federal Reserve Bank of St. Louis) for benchmark economic series such as the federal funds rate, the consumer price index, and the average 30-year mortgage rate. FRED is the Federal Reserve’s own public data service. fred.stlouisfed.org
- U.S. Bureau of Labor Statistics (BLS) for inflation and price data, specifically the Consumer Price Index series. bls.gov
- U.S. Treasury Fiscal Data for average interest rates on Treasury securities. fiscaldata.treasury.gov
- Frankfurter (European Central Bank reference rates) for currency exchange rates. frankfurter.app
Every figure drawn from these feeds is attributed on the page that uses it, with the date it applies to.
How we handle numbers that go stale
This is the single biggest accuracy risk on a personal finance site, so we handle it deliberately:
- Prefer the mechanism over the number. Explaining how a threshold works stays true for years. Quoting the threshold itself goes out of date every January. Where the mechanism is enough, we explain the mechanism.
- Date every figure that changes. When a specific number genuinely helps, it is labelled with the year it applies to, for example “the FDIC insures $250,000 per depositor, per insured bank, per ownership category”.
- Link the live source. For figures that change annually, we point to the official page carrying the current number rather than asking you to trust a static copy of it.
- Never guess. If we are not confident in a figure, it does not go on the page. An article that says “check the current limit here” is more useful than one that states a plausible but wrong number.
How the calculators and decision tools work
The 16 tools on this site run entirely in your browser. No financial numbers you type are sent to The Penny Plan, stored by the tools, or attached to an account: the arithmetic happens on your device.
Each calculator page shows the formula it uses in a “How the math works” section, so the tool is not a black box. The mortgage, loan, and savings tools use the standard amortization and future-value formulas; the paycheck and salary tools use simplified federal brackets and flat FICA, and they say so explicitly on the page, because a simplified estimate presented as an exact figure would be misleading.
The calculators are educational estimates, not quotes, offers, or professional tax or financial advice. Real terms depend on your lender, your employer, your state, and your full situation.
Money Priority Planner method
The Money Priority Planner combines four calculations that are usually separated:
- Monthly cash-flow margin equals take-home income minus essential expenses and required debt minimums.
- Emergency runway equals liquid emergency savings divided by one month of essential expenses and required debt minimums.
- Illustrative annual debt cost equals the current balance multiplied by APR. This is a simple snapshot, not a payoff forecast, because the balance should fall as payments are made.
- Possible unclaimed employer match equals the plan’s maximum match under the inputs minus the match captured at the current contribution rate. The planner cannot read an employer’s plan document, vesting schedule, eligibility rules, or payroll treatment, so the result is explicitly labelled an estimate to verify.
The planner uses a three-month emergency reserve as its base comparison and adds one month for each user-selected risk factor: variable income, a one-earner household, and dependents, capped at six months. That is a transparent risk illustration built around the common three-to-six-month range, not a claim that the resulting target is correct for every household.
The 30-day map is a decision aid, not an optimization engine. It first flags a negative monthly margin, then a gap below one month of essential obligations, then a possible unclaimed employer match, and finally the trade-off between a fuller reserve and extra debt payments. It explains which condition triggered the stage so the reader can disagree with the sequence and still use the calculations.
Validation and limits
Calculator functions are tested against known scenarios and edge cases before release, including zero-interest loans, payments that do not cover monthly interest, empty optional fields, negative cash flow, fully funded reserves, and retirement contributions above the employer-match cap. The site displays model assumptions beside the output because a calculator that hides its limits can create false precision.
Editorial standards
How pages are researched, reviewed, and corrected is documented separately in our Editorial Policy, including how AI tooling is used and where human responsibility sits. The Disclaimer is the controlling statement of what this site is and is not.