  We report on quantitative calculations of the upper critical field $H_{c2}$
for clean type-II superconductors Nb, NbSe$_{2}$, and MgB$_{2}$ using Fermi
surfaces from {\em ab initio} electronic structure calculations. The results
for Nb and NbSe$_2$ excellently reproduce both temperature and directional
dependences of measured $H_{c2}$ curves without any adjustable parameters,
including marked upward curvature of NbSe$_{2}$ near $T_{c}$. As for MgB$_2$, a
good fit is obtained for a $\pi / \sigma$ gap ratio of $\sim 0.3$, which is
close to the value from a first-principles strong-coupling theory [H. J. Choi
\textit{et al}. Nature, \textbf{418} 758 (2002)]. Our results indicate
essential importance of Fermi surface anisotropy for describing $H_{c2}$.
