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    How We Source Our Formulas

    A calculator is only as reliable as the method behind it. This page explains which kinds of sources we use, gives concrete examples from current calculators and lists the limits of those sources.

    How We Choose a Source

    For each calculator we look for the publication that defines the method or the number being used, and prefer it over summaries. When a formula depends on a law, a tax year or a measurement standard, the source must name that rule or year.

    Government agencies and official rules

    Examples: IRS Revenue Procedures for federal tax brackets, the Social Security Administration for the wage base, CFPB material on APR and Loan Estimates, FDIC material on deposit insurance.

    Standards and reference bodies

    Examples: NIST for physical constants and SI units, WHO and CDC for BMI categories, ACOG guidance for estimating a pregnancy due date.

    Established textbook methods

    Examples: the fixed-rate amortization formula, compound interest, percentage change and standard statistical formulas, which can be verified by hand.

    Published industry conventions

    Examples: the money factor used in vehicle leases or the 28/36 affordability guideline. These are labeled as conventions or guidelines, not rules.

    Worked Example: Take-Home Pay

    The take-home pay calculator uses the 2024 single-filer federal brackets from IRS Revenue Procedure 2023-34 and the 2024 Social Security wage base of $168,600 from the Social Security Administration. Because those figures belong to one tax year, the page says so and lists what is left out: state tax, the standard deduction and pre-tax deductions.

    Limits of Our Sources

    • Dated figures such as tax brackets, wage bases or fee schedules are only correct for the year or version stated on the page.
    • Population formulas (for example BMI or calorie estimates) describe averages and may not fit an individual.
    • Guidelines such as 28/36 or Naegele's rule are simplifications that professionals may adjust.
    • Third-party live data, such as crypto prices, can be delayed or unavailable.
    • Citing a source does not mean that organization has reviewed or endorsed CalcPlanet.

    What We Avoid

    • Presenting an approximation as an exact or official figure
    • Labeling figures with a year that the calculation does not actually use
    • Formulas we cannot trace to a source or verify with a worked example

    If you believe a source is outdated or misapplied, tell us through the contact page. See how we test calculators and the corrections log.

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