Risk analysis on Android is aimed at providing metrics to users for evaluating the trustworthiness of the apps they are going to install. Most of current proposals calculate a risk value according to the permissions required by the app through probabilistic functions that often provide unreliable risk values. To overcome such limitations, this paper presents RiskInDroid, a tool for risk analysis of Android apps based on machine learning techniques. Extensive empirical assessments carried out on more than 112 K apps and 6 K malware samples indicate that RiskInDroid outperforms probabilistic methods in terms of precision and reliability.