How we build and test our tools

ZoolTools publishes 114 calculators and converters plus 16 explanatory guides. Anyone can put a form and a number on a page; the harder problem is being right, and staying right as tax rules and statutory rates change. This page documents exactly how we do that, so you can judge the output rather than trust it blindly.

One formula, two apps, one set of test vectors

Each tool’s maths lives in a single shared module — not in the page you are looking at. Alongside it sits a file of canonical test vectors: hand-authored input/expected-output pairs written from the published rule or formula, never copied from what our own code happens to print. That distinction matters. If we generated expected values from the code, the tests would only prove the code agrees with itself.

Both the website and our mobile app run against that same vector file in their own test suites. A release ships only when both pass, which is why a calculation you do here matches the app to the last decimal. When a vector and the code disagree, we treat the vector as the contract and fix the code.

Where the formulas come from

• Loans and mortgages use the standard amortising-payment formula, with interest accrued on the reducing balance — the same convention lenders use for EMI and monthly payment schedules.
• Deposits and investing tools state their compounding convention explicitly (monthly, quarterly or annual) and whether contributions are treated as arriving at the start or the end of each period, because that choice visibly changes the result.
• Tax calculators implement published slab structures, standard deductions, rebates and cess for a specific financial year, which each page names.
• Health tools use the named published equations — Mifflin-St Jeor for BMR, the US Navy circumference method for body fat, the Devine formula for ideal weight, WHO cutoffs for BMI categories — and each page says which one it runs.
• File and developer tools describe the actual technique in use, whether that is a canvas re-encode, a PDF page copy, or a browser cryptographic hash.

How we handle rules that change

Tax slabs, provident-fund interest rates, TDS thresholds and indirect-tax rates change, sometimes mid-year. We deal with that in three ways. Every figure that depends on a period is labelled with the year it applies to rather than presented as timeless. Where a tool models a simplified version of a rule — for example a flat rate instead of a full surcharge and marginal-relief ladder — the page says so plainly rather than implying completeness. And where a recent reform changed the picture, we keep the older rate visible when it is still needed for checking historical invoices or filings.

If you find a figure that has moved on, tell us on the contact page — the source and the date you saw is enough for us to act on.

Worked examples are checked, not illustrative

Most tool pages include a worked example: real inputs, real intermediate steps, a real answer. Those numbers are produced by running the tool’s own code and are re-checked when the code changes, so the arithmetic in the prose and the arithmetic in the calculator cannot drift apart. Rounding is described where it affects the result — for instance when a monthly payment is computed unrounded and only displayed to two decimals, so a payment-times-months total differs from the shown total by a few cents.

Privacy is an engineering decision, not a promise

Image, PDF and text tools run entirely inside your browser. Your files are never uploaded, because there is no upload step to speak of — the processing happens on your device and the result is written straight back to it. That is why these tools work with a slow connection and why we can make the claim at all. Read the privacy policy for what the site itself collects (analytics and advertising, both disclosed there).

What we are not

These are information tools. Finance and tax outputs are estimates for planning and are not tax, investment or legal advice; for a filing, a loan agreement or a large decision, confirm the numbers with your lender, a qualified accountant or the relevant official source. Health outputs are general guidance, not a diagnosis, and are no substitute for a clinician who can see your full history. Our disclaimer states the limits in full.

Who publishes this

ZoolTools is built and operated by Ujjwal Technolabs. We are a small independent team; there is no sponsored placement in any calculator, and no result changes based on who is paying. The site is funded by advertising, which is why you may see ads around the content — never inside a result, and never dressed up as a button. Corrections, tool requests and bug reports all reach us at ujjwaltechnolabs@gmail.com.