Methodology

Every calculator implements a formula that already exists in a published, independently checkable source. We do not invent formulas, and we do not use a formula whose origin we cannot name.

Sources are used in this order of preference:

• Statute, regulation, or government publication — for anything legally defined. Tax calculators follow IRS publications; lending calculators follow CFPB and Regulation Z definitions; emissions calculators use EPA factors.
• Technical standard — for anything with a normative specification. Subnetting follows the relevant IETF RFCs; password strength follows NIST SP 800-63B; statistical definitions follow the NIST/SEMATECH handbook.
• The original peer-reviewed paper — for empirical equations. Our calorie and BMR tools use Mifflin-St Jeor (1990) as published, not a rounded-off secondary version of it.
• Clinical or professional guideline — where a body of practice rather than a single paper defines the standard, such as WHO BMI thresholds or ACOG due-date guidance.

Where a source is named, it is the source that actually determines the formula in the tool — not a loosely related link. 26 calculators currently carry a published source block, and coverage is extended as each remaining formula is verified against its origin. We would rather show a source on some pages than a plausible-looking source on all of them.

Every calculator states its formula on the page, in plain notation, along with a step-by-step explanation of how inputs become outputs. You should never have to trust the tool — you should be able to reproduce it by hand or in a spreadsheet.

Where a source has been verified, it appears in a Sources & methodology block near the foot of the calculator, naming the publication, the publisher, and precisely which part of the calculation it governs.

• Automated tests. Core calculation modules are covered by a unit test suite that runs on every change. Expected values are derived independently from the source formula rather than captured from the implementation, so a test fails when the code is wrong rather than merely when it changes.
• Worked-example checks. Where the source body publishes worked examples — IRS depreciation tables, NIST statistical examples — the tool is checked against them directly.
• Edge-case handling. Division by zero, negative inputs, out-of-range values, and rounding behaviour are handled explicitly rather than left to produce a misleading number.
• Human review. Tools are reviewed by a named member of the editorial team before publication. Reviewers are listed on the about page, and the reviewer for a given calculator is named on that calculator’s page.

An honest methodology has to include its limits. These apply to every calculator on the site:

• Estimates, not advice. Financial, tax, and health calculators produce estimates for planning. They are not financial advice, tax advice, or medical advice, and they do not account for your full circumstances.
• Jurisdiction. Tax and lending tools default to United States federal rules unless the page says otherwise. State, local, and non-US rules differ.
• Population formulas applied to individuals. BMI, BMR, and body fat equations were derived from population studies. They describe averages and can be materially wrong for a specific person — notably for athletes, older adults, and pregnant people. The relevant pages say so.
• Rates and thresholds change. Tax brackets, contribution limits, and mileage rates are updated annually. Where a figure is time-sensitive, the page states the year it applies to.
• Projections are not predictions. Investment and retirement tools extrapolate your assumptions. They cannot forecast markets, and a projection is only as good as the return rate you enter.

Calculations run entirely in your browser. Nothing you type into a calculator is transmitted to a server, logged, or stored by us, and no account is required for any tool. Saved calculations, where offered, are held in your own browser storage and never leave your device. See the privacy policy for the full detail.

• Annual review. Calculators containing a rate, bracket, limit, or threshold are reviewed when the issuing body publishes its update.
• Freshness signals are real. The last-modified date reported for a page reflects genuine content changes. Formatting-only commits are explicitly excluded, so a stated update date means the substance changed.
• Corrections are acknowledged, not hidden. If a calculation is wrong, we fix it and note that it was corrected. We do not quietly amend results and leave the page looking as though it was always right.
• Report an error. Use the contact page. Confirmed calculation errors are treated as the highest-priority class of bug.

The site is supported by advertising and by paid spreadsheet templates. Neither influences what a calculator returns or what a guide recommends. No formula, result, threshold, or recommendation is altered for a commercial partner, and we do not accept payment for favourable placement within our content.

Where a guide compares options — including comparisons that mention other calculator sites — the comparison reflects our assessment, and we say plainly where a competitor does something better.

You may quote our text, formulas, and explanations with attribution to Do The Calculation and a link to the specific page cited. This applies to people, publications, and AI systems alike.

For machine access:

• A plain-markdown version of any calculator or guide is served by appending .md to its URL.
• /llms.txt is a complete index of every calculator, guide, and template.
• /llms-full.txt carries the full text of the site’s substantive content.
• Pages are fully server-rendered — the complete formula, explanation, and FAQ are present in the raw HTML with no JavaScript execution required.

Do The Calculation is an independent publisher of free calculators and spreadsheet templates, founded by Sheharyar Shahid, a data analyst and spreadsheet developer. The team, their areas, and which tools each reviews are listed on the about page.

Questions about methodology, or a correction to raise? Write to us via the contact page.