How we build and verify these tools

Why this page exists

A calculator is a claim. Type in a number, get a number back, and you are trusting that whoever built the thing did the maths correctly and used the right rate. Most calculator sites never tell you where their numbers came from, who wrote the page, or what happens if they're wrong.

This page answers those questions for all 33 tools on ToolsNook. It is deliberately specific. If any part of it turns out not to be true, that is a mistake worth reporting, and there are instructions at the bottom for doing exactly that.

The four principles

1. Primary sources, not secondary ones. A VAT rate comes from HMRC, not from another blog quoting HMRC. Where a figure has an issuing authority, we cite that authority and link to it.
2. Show the formula. Every calculator page states the formula it uses in plain text. You should be able to check our arithmetic on paper.
3. Say what we don't know. Tax treatment varies by jurisdiction and circumstance. Health thresholds are population-level screening tools, not diagnoses. Where a tool has real limits, the page says so rather than projecting false confidence.
4. Human accountability. One named person is responsible for every page on this site. That is Asad Janjua, and his contact address is at the bottom of this page.

How a formula gets onto the site

1. Definition. The formula is taken either from the issuing authority (for regulated figures like VAT and BMI) or from a standard, uncontested definition (for arithmetic like compound interest or amortisation).
2. Worked examples. We collect worked examples with known answers — from the source authority where one publishes them, or constructed by hand where none does.
3. Implementation. The formula is written in JavaScript that runs entirely in your browser.
4. Testing. The implementation is run against every worked example before the tool goes live. If the code and the example disagree, the code is wrong until proven otherwise.
5. Edge cases. Zero, negative, and out-of-range inputs are checked so the tool fails visibly rather than returning a confident wrong answer.

Two specific traps we test for, because they're the most common errors in this category: margin versus markup (a 50% markup is a 33.3% margin, and mixing them up silently under-prices everything), and inclusive versus exclusive date counting (the source of nearly every "off by one" bug in date arithmetic).

Where each tool's numbers come from

Tools not listed below implement uncontested arithmetic — percentage change, unit conversion, amortisation — where there is no rate or threshold to source. Those pages state their formula directly.

Our AI disclosure

We use AI, and we would rather say so plainly than have you find out.

For the written explanations that sit beneath each calculator: these are drafted with the assistance of AI language models, then edited, fact-checked and published by Asad Janjua. Every factual claim — a tax rate, a health threshold, a statutory interest figure — is checked against the source authority before publication. Google's publisher policies permit AI-assisted content where there is genuine human oversight, and that oversight is a person with a name, not a rubber stamp.

For the five AI writing tools (business name, product description, bio, cover letter, email subject line): these send your input to a third-party large language model through our server and return its response. Their output is generated on demand, is not reviewed by anyone, and should be treated as a first draft rather than a finished document.

Where your data goes

Every calculator on this site runs in your browser. The figures you enter into the VAT calculator, the BMI calculator or the invoice generator are never sent to us, because there is nowhere for them to be sent — there is no database behind these tools.

There are exactly two exceptions. The currency converter fetches live rates from ExchangeRate-API when the page loads; it sends no personal data. The AI writing tools transmit whatever you type into them to a language model provider in order to generate a response. Both are described in full in the privacy policy.

Updates and corrections

When pages get updated

Rate-dependent pages — VAT, tax, statutory interest — are reviewed when the issuing authority changes the figure, and at minimum once a year. Every tool page carries a Last updated date at the foot of the page. That date reflects the last time the content was substantively reviewed, not the last time a stylesheet changed.

How to report an error

Email hello@toolsnook.com or use the contact form. Tell us the tool, the inputs you used, and what you expected. Every message goes to Asad directly.

Confirmed calculation errors are fixed as a priority and the page's last-updated date changes to reflect the correction. We would rather be told than be wrong quietly.

Limitations — please read this one

Financial tools. Tax, VAT and payroll treatment depend on your jurisdiction, your registration status, and facts specific to your situation. A calculator cannot know those. Use these tools to understand the shape of a number, then speak to a qualified accountant before acting on it.

The BMI calculator. BMI is a population-level screening measure. It does not distinguish muscle from fat, and it does not account for age, sex, ethnicity or fat distribution. Athletes are routinely classified as overweight by it. It is not a diagnosis and it is not a measure of your health. If you have concerns about your weight, speak to a doctor.

Exchange rates. The rates shown are indicative mid-market rates. No bank will give you them.

We would rather you leave this site with an accurate sense of what a number does and does not tell you than with false confidence.