  We perform de Haas-van Alphen measurements and quasiparticle self-consistent
\textit{GW} (QS\textit{GW}) calculations on FeS. The calculated Fermi surface
(FS) consists of two hole and two electron cylinders. We observe all the eight
predicted FS cross sections experimentally. With momentum-independent
band-energy adjustments of less than 0.1 eV, the maximum deviation between the
calculated and observed cross sections is less than 0.2\% of the Brillouin zone
area for $B \parallel c$. The carrier density is $\sim$0.5 carriers/Fe. The
mass enhancements are nearly uniform across the FS cylinders and moderate,
$\sim$2. The absence of a third hole cylinder with $d_{xy}$ character is
favorable for the formation of a nodal superconducting gap.

Superconductivity;Strongly Correlated Electrons