About

About InToolbox

InToolbox is a set of 10 free online calculators covering finance, with more publishing every week. Every one of them runs in your browser, needs no account, and works the moment the page finishes loading.

What the site is trying to be

Most calculator sites are built to hold advertising. The formula is three input boxes, a result, four hundred words of padding, and a page that takes eight seconds to become usable. InToolbox is an attempt at the opposite trade: the tool sits at the top of the page and works immediately, and the explanation underneath it exists because the question genuinely needs one, not because a word count had to be hit.

Each page does one job. A tool that could plausibly cover five related problems is usually split into five pages instead, because a single-purpose calculator with three fields is faster to understand than a general one with fifteen fields and a mode selector. The full index lists everything by category, and related tools are cross-linked at the bottom of each page so you can move sideways when the calculator you opened turns out not to be the one you needed.

Who builds it

InToolbox is a small independent project, not a content agency and not a marketing operation. There is no editorial pipeline behind it, which has an obvious downside (things take a while) and one real upside: every formula on the site was implemented by someone who had to make it produce correct numbers in working code, rather than by a writer paraphrasing another website.

The project is deliberately run without personal bylines. What is offered instead of a name is something more checkable: each page states the source it follows, says where a quantity is genuinely contested, and the arithmetic is covered by tests pinned to published worked examples. If a number here disagrees with an authority you trust, that is a bug worth reporting, and the contact page is monitored for exactly that.

How a calculator gets built

Work on a new tool starts with the definition rather than the interface. A surprising number of everyday quantities have more than one accepted formula, or a formula that differs by country, standard, or year, so the first step is finding a primary source: a published standard, a regulator's own worked example, a textbook, or the reference implementation a field actually uses. Where a quantity is genuinely contested, the page says so instead of quietly picking a side.

The arithmetic then goes into a plain TypeScript function that takes numbers and returns numbers, with no user interface attached. That separation is deliberate: a function with no dependency on the screen can be tested directly, and it is the part that has to be right. The interface is assembled afterwards from a small set of shared components, which is why calculators across the site look and behave alike. Learning one page teaches you the rest.

How the numbers are checked

Calculation logic is covered by automated tests that run before every deployment. The useful cases are rarely the ordinary ones: the tests concentrate on boundaries where implementations tend to break, such as zero, negative input, values that should be rejected outright, unit conversions in both directions, and the exact points where a category label changes. Where an authority publishes a worked example, that example becomes a test case, so the site is pinned to the published answer rather than to our own reading of the formula.

Results are also spot-checked by hand against official calculators and reference tables where those exist. Rounding is handled at the last possible moment: the arithmetic runs at full floating-point precision and only the displayed value is rounded, which is why a result may differ in the final decimal place from a calculator that rounds at every intermediate step.

None of this makes the site infallible, and it is worth being blunt about the limits. These are calculators, not professional advice. A mortgage page cannot know your lender's day-count convention, a health page cannot examine you, and a tax-adjacent page cannot know your circumstances. For decisions that carry money, health, or legal consequences, use the output as a sanity check and talk to someone qualified.

Why everything runs in your browser

There is no application server. Pages are pre-rendered to static HTML at build time and served from a content delivery network, and the calculation happens in JavaScript inside your tab. That decision follows from the nature of the input: salaries, debts, weights, and test results are not things anyone should have to send to a stranger's server to get a number back.

The same design makes the site fast, because no round trip stands between typing and seeing a result, and it makes most pages work with the network disconnected once loaded. Four calculators are exceptions, because they need live exchange rates, coin prices, or astronomical data, and those call a third-party API directly from your browser. They are named individually in the privacy policy along with exactly what each request contains.

Why there is nothing to sign up for

Accounts exist to hold data, and this site holds none, so an account would be a login screen guarding an empty room. Pinned tools and recent calculations are kept in your browser's local storage on your own device, which covers the only thing an account would plausibly be for. The cost is that preferences do not follow you between devices. That seems a fair price for never having to remember a password or receive an onboarding email.

How the site is funded

At the time of writing InToolbox carries no advertising and sells nothing. Running costs are low because the site is static. The plan is to cover hosting with display advertising, which is why the privacy policy already describes advertising cookies as a section that switches on when ads do. There will be no paid links inside explanatory content, no affiliate arrangements dressed up as recommendations, and no paywalled calculators.

How to report an error

Corrections are the most valuable message the site receives. If a result looks wrong, send the page URL, the exact values you entered, the number you saw, and the number you expected, ideally with the source you are comparing against. Reproduction beats description: with the inputs in hand a bug can usually be confirmed in minutes. Typos, unclear wording, broken layouts, and accessibility problems are all worth reporting too. The contact page has pre-filled email templates for each of those.

When a calculation error is confirmed it gets fixed first and explained second, and the fix arrives with a test that stops the same mistake from coming back.