  Fastlim is a tool to calculate conservative limits on extensions of the
Standard Model from direct LHC searches without performing any Monte Carlo
event generation. The program reconstructs the visible cross sections from
pre-calculated efficiency tables and cross section tables for simplified event
topologies. As a proof of concept of the approach, we have implemented searches
relevant for supersymmetric models with R-parity conservation. Fastlim takes
the spectrum and coupling information of a given model point and provides, for
each signal region of the implemented analyses, the visible cross sections
normalised to the corresponding upper limit, reported by the experiments, as
well as the exclusion $p$-value. To demonstrate the utility of the program we
study the sensitivity of the recent ATLAS missing energy searches to the
parameter space of natural SUSY models. The program structure allows the
straight-forward inclusion of external efficiency tables and can be generalised
to R-parity violating scenarios and non-SUSY models. This paper serves as a
self-contained user guide, and indicates the conventions and approximations
used.
