The Scopus database works well to calculate H Indexes for small and moderate sized journals. It cannot calculate on the fly for more than about 2000 articles. If the journal you are interested in has more articles than that you'll need to go to another source. Below are the steps to calculate a journal H Index.
From the main search page:
Alternatively you can calculate an H5 Index, which is the H Index for the past 5 full years. So long as this is done for every journal title being considered it should be equally as illuminating as the full H Index.