Double perovskite halides are a class of materials with diverse chemistries that are amenable to solution-based synthesis routes, and display a range of properties for a variety of potential applications. Starting from a consideration of the octahedral and tolerance factors of ∼2000 candidate double perovskite compounds, we compute structural, electronic, and transport properties of ∼1000 using first-principles calculations based on density-functional-theory methods. The computational results have been assembled in a database that is accessible through the Materials Project online. As one potential application, double perovskites are candidates in the search for lead-free halide photovoltaic absorbers. We present the application of our database to aid the discovery of new double perovskite halide photovoltaic materials, by combining the results with optical absorption and phonon stability calculations. From three distinct classes of chemistries, 11 compounds were identified as promising solar absorbers and the complex chemical trends for band gap within each of these are analyzed, to provide guidelines for the use of substitutional alloying as a means of further tuning the electronic structure. Other possible applications of the database are also discussed.