  We investigate the prospects of measuring the cosmic magnification effect by
cross-correlating neutral hydrogen intensity mapping (HI IM) maps with
background optical galaxies. We forecast the signal-to-noise ratio for HI IM
data from SKA1-MID and HIRAX, combined with LSST photometric galaxy samples. We
find that, thanks to their different resolutions, SKA1-MID and HIRAX are highly
complementary in such an analysis. We predict that SKA1-MID can achieve a
detection with a signal-to-noise ratio of $\sim 15$ on a multipole range of
$\ell \lesssim 200$, while HIRAX can reach a signal-to-noise ratio of $\sim 50$
on $200 < \ell < 2000$. We conclude that measurements of the cosmic
magnification signal will be possible on a wide redshift range with foreground
HI intensity maps up to $z \lesssim 2$, while optimal results are obtained when
$0.6 \lesssim z \lesssim 1.3$. Finally, we perform a signal to noise analysis
that shows how these measurements can constrain the HI parameters across a wide
redshift range.
