As the crowdsourcing strategy becomes better known, the managerial decisions necessary to establish it as a viable business process are becoming increasingly important. However, a divide and conquer approach, currently dominant in the field, leads to scattered decision support for the crowdsourcing processes. We propose an ontology-based decision tool that supports the whole business process crowdsourcing. The advantage of the ontology approach is that it collects and consolidates knowledge from the existing literature to provide a solid knowledge base for the tool construction. Operationalising the ontology, the tool helps make the decision to crowdsource or not, and choose appropriate design alternatives for the crowdsourcing process. We evaluated the tool through a controlled experiment with 190 participants. The obtained results show that the tool is useful by significantly increasing: (1) the performance in making the decision to crowdsource or not, and (2) the design of crowdsourcing processes.