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How we check our calculators

Every result comes from a plain formula, and every formula is tested against an answer worked out separately.

How is each result tested?

Each of the 49 calculators has automatic tests that compare its result with a known answer. For example, a ₹10,00,000 loan at 9% over 20 years must give an EMI of ₹8,997, and 70 kg at 175 cm must give a BMI of 22.9. The tests also check that every calculator page has its formula, a worked example, a set of questions and answers, and links to related tools, and that no page shows an empty or invalid number.

A second set of tests opens every page in a real browser, types into the calculators, and checks the answers and that nothing breaks. The tests run again on every change, and a change cannot go live unless they pass.

How are the guides and tables checked?

The 50 how-to guides quote many figures. A test recalculates each of them with the calculators themselves and fails if a guide says something the calculator does not. The reference tables under each calculator, such as the EMI on ₹1 lakh or CGPA to percentage, are not typed in: every cell is worked out by the calculator when the site is built.

Where do the formulas come from?

The 23 finance calculators use standard textbook formulas: the reducing-balance EMI formula, compound interest, the future value of regular investments, CAGR and simple interest. The 7 health calculators use published equations: BMI with the WHO categories, Mifflin-St Jeor for BMR, the U.S. Navy method for body fat, the Devine and Robinson formulas for ideal weight and Naegele’s rule for due dates. Each page shows its basis and links to a reference. The water intake calculator uses a rule of thumb that is not an official guideline, and says so.

Several of these are general references, not a lender’s or a regulator’s rulebook. A bank’s own schedule can differ by a few rupees because of rounding and dates.

What can the calculators not tell you?

They are estimates. Investment calculators assume a steady return that real markets do not give. Loan calculators leave out processing fees, insurance and taxes. Health calculators use averages and are not medical advice. None of this is financial or medical advice, so check the terms of any loan, deposit or plan with your lender, bank or adviser, and talk to a doctor or dietitian about your own health.

Tax and savings-scheme rules change with each Budget and notification, so those calculators are built only from the current slabs, rates and limits read from the official source, and each page shows the financial year and the date the rules were read.

When was this last reviewed?

The formulas, sources and wording of the finance and health pages were last reviewed on . That date is shown on each of those pages.

Found a mistake?

Please tell us, with what you typed and what you expected, or write to [email protected]. iDuckCalculator is run by Basic App Company; see About.