The objective of this paper is to describe the use of a probabilistic approach to Web effort estimation by means of a Bayesian Network. A Bayesian Network is a model that embodies existing knowledge of a complex domain in a way that supports reasoning with uncertainty. Given that the causal system relative to Web effort estimation has an inherently uncertain nature the use of Bayesian model seemed a reasonable choice. We used a cross-company data set of 150 industrial Web projects volunteered from Web companies worldwide, which are part of the Tukutuku database. Results showed that the effort estimates obtained using a Bayesian Network were sound and significantly superior to the prediction based on two benchmark models, using the mean and median effort respectively.