In Euclidean space, it is well known that any integration by parts formula for a set of finite perimeter is expressed by the integration with respect to a measure P( , •) which is equivalent to the one-codimensional Hausdorff measure restricted to the reduced boundary of . 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 S ∞−1 restricted to the measuretheoretic boundary of . In this paper, we consider an open convex set and we provide an explicit formula for the density of P( , •) with respect to S ∞−1 . In particular, the density can be written in terms of the Minkowski functional p of with respect to an inner point of . As a consequence, we obtain an integration by parts formula for open convex sets in Wiener spaces.