Journal impact is derived from various metrics that track how often a journal has been cited in recent years. This information is most helpful for conducting and publishing research, but may also be useful in preparations for reappointment, advancement, and grant funding requests. The Big Three for data are:
Each contains a different dataset of publications, and uses its own metrics to determine impact. It may be prudent to consult multiple resources for journal impact, depending on your information need: no single resource will contain all publications, and each uses its own metrics to determine impact.
For more information on journal impact and metrics, an excellent guide has been published by the Pao Yue-Kong Library (Hong Kong Polytechnic University): https://libguides.lb.polyu.edu.hk/journalimpact/intro
Eigenfactor offers free journal metrics for over 11,000 titles in a searchable database. Metrics are derived from Thomson-Reuters Journal Citation Reports (JCR) data (also available in Web of Science). Publications from 1997-2015 can be searched by journal name, ISSN, publisher, year, or ISI Category. The Eigenfactor is also adjusted by discipline, acknowledging that the average citations differ from one discipline to another.
Screenshot of Eigenfactor Journal Ranking
Google Scholar provides a list of the top 100 publications based on its calculation of h5-index and h5-median. It can be generated by category (subject area) as well. Click on the h-index for a publication to see which articles were cited the most and who cited them.
Scimago (SJR) offers free journal metrics for over 34,000 titles in a searchable database. Metrics are derived from Scopus data. Publications can be searched by journal name, ISSN, or publisher, and browsed by subject area and category, country, or year. Scimago provides an h-index, SJR metric, and data visualizations for included titles.
Scimago has also published a massive data visualization project called the Shape of Science, "whose aim is to reveal the structure of science."
Open access means that anyone, anywhere can access your work at no cost. Open access is NOT the same as self-publishing, and many open access journals are peer reviewed. To maximize your work's impact, or to meet a grant funding requirement, you may be interested in an open access publication. Many traditional publishers are also offering open access options for individual articles.