Five gallons of paint weighs approximately 56.5 pounds based on a 1-gallon weight of 11.3 pounds. This weight is for an interior latex paint based on the statistics in the MSDS (material data safety sheet) report found on the website of the paint company. Weights will vary.
The MSDS report for a product gives you all of the safety specifications including weight. Not all paint and do-it-yourself store websites provide the MSDS for the products that they sell. Professional paint sellers who deal with contractor paint offer access to MSDS on the pages of their websites with each individual product.