  In Euclidean space, it is well known that any integration by parts formula
for a set of finite perimeter $\Omega$ is expressed by the integration with
respect to a measure $P(\Omega,\cdot)$ which is equivalent to the
one-codimensional Hausdorff measure restricted to the reduced boundary of
$\Omega$. The same result has been proved in an abstract Wiener space,
typically an infinite dimensional space, where the surface measure considered
is the one-codimensional spherical Hausdorff-Gauss measure $\mathscr
S^{\infty-1}$ restricted to the measure-theoretic boundary of $\Omega$. In this
paper we consider an open convex set $\Omega$ and we provide an explicit
formula for the density of $P(\Omega,\cdot)$ with respect to $\mathscr
S^{\infty-1}$. In particular, the density can be written in terms of the
Minkowski functional $\p$ of $\Omega$ with respect to an inner point of
$\Omega$. As a consequence, we obtain an integration by parts formula for open
convex sets in Wiener spaces.
