This article looks carefully at the making of the tax gap as a calculative practice at the Swedish Tax Agency. It points to the challenges with assessing tax gap numbers and reveals examples of its careless usage by various stakeholders in Swedish society in order to impact the discourse on tax compliance. Based on an ethnographic study of documents, it is an example of a number that performs a dual function: on the one hand mobilizing people’s morals and subsequent commitments and on the other hand measuring such commitments. On a more general level, the tax gap is an example of how a number that describes a complicated reality is calculated with many caveats attached to it, yet used with certainty in society. I thus challenge the metaphor of the ubiquitous ‘black box’. The Swedish tax gap number is a calculated device and can be unpacked as a specific number. But if we think about what a tax gap ought to contain, we have to think outside the box and beyond: what was not included in the published tax gap number and why there was no room for it.