  We present H1jet, a fast code that computes the total cross section and
differential distribution in the transverse momentum of a colour singlet. In
its current version, the program implements only leading-order $2\to 1$ and
$2\to 2$ processes, but could be extended to higher orders. We discuss the
processes implemented in H1jet, give detailed instructions on how to implement
new processes, and perform comparisons to existing codes. This tool, mainly
designed for theorists, can be fruitfully used to assess deviations of selected
new physics models from the Standard Model behaviour, as well as to quickly
obtain distributions of relevance for Standard Model phenomenology.
