Methodology
How we make sure the math is right.
Every calculator on Calculator Online goes through the same six-step review before it ships. Here is what that looks like in practice.
The premise
A wrong answer is worse than no answer.
Most calculator sites prioritize speed-to-market over correctness. A formula gets typed in, a result appears, and that's the end of the editorial process.
We take the opposite approach. Before we write a single line of code, we identify the authoritative source for the formula. We reproduce the math by hand. We compare against established tools. A second person reviews the work. Then, and only then, does the page go live.
When something changes (a new WHO threshold, an updated equation, a regulatory shift) we update the page and bump the last-reviewed date visible at the bottom.
Six-step review
Every calculator follows this checklist before going live.
Source the primary reference
For every formula, we identify the authoritative source, the peer-reviewed paper, the standard-setting body (WHO, NIH, ECB, IMF, NIST), or the canonical textbook. No formula goes live without a citation.
Reproduce by hand
Before any code is written, we walk through the formula with at least one worked example by hand. If the formula has edge cases (division by zero, negative inputs, domain restrictions), we document them.
Write the explanation
The plain-English explanation, the variable definitions, and the worked example are written together with the calculator. They are part of the deliverable, not an afterthought.
Cross-check against trusted tools
Results are compared against established calculators (NIH calculators for health, ECB rates for currency, financial regulator tools for loans). Any discrepancy is investigated before publication.
Editorial review
A second person reviews the math, the explanation, and the page UX before publishing. The reviewer is named in the page footer alongside the last-updated date.
Schedule review cycles
Formulas that depend on regulations, standards, or rates are scheduled for periodic review. The last-updated date on every page reflects the last time this happened.
Sources we cite
The primary references behind our most-used calculators.
Each calculator page includes a Sources block linking to the authoritative reference. Here are a few examples.
| Calculator | Primary source |
|---|---|
| Body Mass Index | WHO BMI Classification (1998, updated 2004) |
| Basal Metabolic Rate | Mifflin-St Jeor equation, Am. J. Clin. Nutr. (1990) |
| Body Fat Percentage | U.S. Navy circumference method, BUMED |
| Compound Interest | Standard A = P(1 + r/n)^(nt) |
| Currency Conversion | European Central Bank reference rates (live, via frankfurter.app) |
| Mortgage Payments | Standard amortization formula M = P·r(1+r)ⁿ / ((1+r)ⁿ − 1) |
What calculators can't do
The math is right. The advice is your own.
A calculator applies a formula to the inputs you give it. It does not know your tax jurisdiction, your medical history, your specific contract terms, or your individual goals.
For consequential decisions, buying a home, changing your diet, restructuring debt, use our calculators to estimate the shape of the answer, then talk to a qualified professional (a financial planner, a doctor, a tax accountant) who can apply the math to your specific situation.
We don't replace professional advice. We make it easier to walk into a professional conversation already understanding the basics.
Help us improve
Find a math error? We want to hear about it.
Math errors are the highest-priority issue we triage. Include the calculator, the inputs, and what you expected the result to be.