We show that whenever the q-dimensional Minkowski content of a subset A of a Euclidean space exists and is finite and positive, then the "S-content" defined analogously as the Minkowski content, but with volume replaced by surface area, exists as well and equals the Minkowski content. As a corollary, we obtain the almost sure asymptotic behaviour of the surface area of the Wiener sausage in dimension greater or equal to 3.