  We study the influence of an external magnetic field on hadron yields and
fluctuations in a hadron resonance gas by performing calculations within an
updated version of the open-source Thermal-FIST package. The presence of the
magnetic field has a sizable influence on certain hadron yield ratios. Most
notably, it leads to enhanced $ p/pi$ and suppressed $n/p$ ratios, which may
serve as a magnetometer in heavy-ion collisions. By attributing the centrality
dependence of the $p/\pi$ ratio in Pb-Pb collisions at 5.02 TeV measured by the
ALICE Collaboration entirely to the magnetic field, we estimate its maximal
strength at freeze-out to be $eB \simeq 0.12$~GeV$^2 \simeq 6.3 m_\pi^2$ in
peripheral collisions. The magnetic field also enhances various conserved
charge susceptibilities, which is qualitatively consistent with recent lattice
QCD data and is driven in the HRG model by the increase of hadron densities.
However, the variances of hadrons do not show any enhancement when normalized
by the means, therefore, measurements of second-order fluctuations in heavy-ion
collisions do not appear to offer additional information about the magnetic
field relative to mean multiplicities.
