  Compared to primordial perturbations on large scales, roughly larger than $1$
megaparsec, those on smaller scales are not severely constrained. We revisit
the issue of probing small-scale primordial perturbations using gravitational
waves (GWs), based on the fact that, when large-amplitude primordial
perturbations on small scales exist, GWs with relatively large amplitudes are
induced at second order in scalar perturbations, and these induced GWs can be
probed by both existing and planned gravitational-wave projects. We use
accurate methods to calculate these induced GWs and take into account
sensitivities of different experiments to induced GWs carefully, to report
existing and expected limits on the small-scale primordial spectrum.
