Test Coverage Target Calculator

A team with a coverage gate in CI keeps running into the same three sums. The first is straightforward: I am at 75%, the target is 80%, how many lines do I need to cover? The second turns up mid pull request and is far less obvious. And the third almost nobody does — it is the one that prevents the surprise of breaking the gate without having touched a single test.

The sum that misleads is the pull request one. Covering 80% of the new lines does not keep the total at 80%: if the base sits above that, there is slack and you can cover less; if it sits below, covering even 100% of the new lines does not recover it. Enter how many lines the PR adds and the page says how many of them need tests — and warns when the gate is unreachable from the PR alone, which is the case where the problem is the base rather than what you are writing.

The third sum answers how much untested code still fits before coverage falls below the gate. With 900 of 1000 lines covered and a gate at 80%, another 125 lines fit with no tests at all — after that, any line brings it down. All the arithmetic runs on integers: 80% of 1000 is exactly 800, and an implementation that rounds up would ask for 801, sending you to cover one line more than necessary.