We propose a new metric called ‘alt-index’, which is analagous to the h-index, but uses altmetrics data to measure both the volume and social media activity of scientific literature. The dataset includes over 1.1 million papers and their associated altmetrics score. A correlation analysis of the h-index and alt-index is conducted at three different levels: field level, source (journal) level, and author level. Our results show a strong positive correlation between the two indexes across all levels. We conclude that the alt-index can be adopted to measure the social activity of publications in situations where there is agreement amongst various social media channels.