Deciding the proportion of online to face-to-face components of a blended learning course is a complex problem that entails many decisions. These decisions are affected by different criteria related to the nature of the course, the educational institution, the teacher, and the students. The impact of these influential criteria on the design process is not always obvious. The aim of this study is to assist academics to design successful blended courses by investigating the impact of criteria that need consideration when deciding the proportion of online components of a blended course. A modified Delphi survey was used in this study. Eighteen experts with experience in instructional design and online teaching participated in the study. Findings indicate that technological aspects, that is, Availability of technology and Students' access to technology, have a strong negative impact on the proportion of online to face-to-face components. On the other hand, students' flexibility and convenience as well as the availability of a high level of institutional support should motivate academics to incorporate a high proportion of online components in their courses. The results also indicate that blended learning courses should have medium to high proportions of online components.