Wi-Fi offloading is one of the most effective approaches to relieve the cellular radio access from part of the burgeoning mobile demand. To date, Wi-Fi offloading has been mainly leveraged in limited contexts, such as home, office or campus environments. In this paper, we investigate the scaling properties of Wi-Fi offloading, by studying how it would perform on a much larger scope than those considered today. To that end, we consider a real-world citywide scenario, built on data about actual infrastructure deployments and mobile traffic demand, and observe which amount of traffic could be accommodated by the existing pervasive Wi-Fi access infrastructure, were it opened to mobile users. We find that more than 80% of the mobile traffic demand in a large urban area may be easily served by Wi-Fi access points, under a wide range of system settings.