Why this exists
Most marketing metrics are one or two divisions. Return on ad spend is a division. Break-even ROAS is a reciprocal. Cost per click, cost per thousand impressions, and click-through rate are three divisions over the same two counts. None of it is difficult, and almost all of the pages that explain it are padded around a widget that does not show its working.
These pages show the working. Every tool prints the formula it uses and a worked example whose figures are asserted by an automated test, so the number in the prose cannot drift away from the number the tool returns.
The one rule about numbers
Nothing here tells you what a good result looks like. There are no industry comparison tables, no claims about what most advertisers see, and no rules of thumb dressed up as data. That is a deliberate constraint rather than an oversight: figures of that kind need a source, sourcing them properly is real research, and publishing them without one would be inventing data on a site whose entire proposition is that the arithmetic is checkable.
The constraint is enforced mechanically. A test scans every published page for unsourced claims about averages and typical values, and the site does not build if one appears. The reference chart is built the same way: every cell in it is derived from a formula printed above the table, so nothing in it was observed anywhere and nothing in it can go out of date.
How the tools are built
Each calculation is a plain JavaScript function that runs in your browser. There is no server, no API call, and no database. The functions are covered by an automated suite that runs on every change, including checks that invalid input returns nothing rather than a confident-looking zero — a tool that answers a question it was not given enough information to answer is worse than one that says so.
Where a result depends on an assumption, the assumption is an input you control. The lifetime value model, for instance, treats expected lifetime as the reciprocal of your churn rate, which is a simplification; the page says so rather than hiding it behind the result.
What it deliberately does not do
It will not ask you to create an account, will not email you, and does not keep the figures you enter. It is not financial advice, and it is not affiliated with any advertising platform. Where a tool disagrees with a platform's own reporting, the usual explanation is a different denominator or a different attribution window rather than an arithmetic error, and the published formula should make it possible to tell which.
How the site is funded
The tools are free and will stay free. Some outbound links to third-party marketing software may earn a commission in future, which would be disclosed on the affiliate disclosure page and marked wherever it applied. No result changes because of a commission — every formula is printed on the page that uses it, so a result bent toward an advertiser would be visibly wrong.
Reporting an error
If a tool's arithmetic is wrong, that is a bug worth reporting, and the formula printed on each page should make it straightforward to point at the exact step. Disagreements about definitions — what counts as an impression, which click rate a platform means — are worth reporting too, because the fix there is usually clearer labelling rather than different maths.