On June 1, 2011, Heroku switched from billing for “dynos” to billing for “dyno-hours”, which is more clear. This brings pricing inline with the new process model, wherein you can run one-off processes as well as manipulate the dynos for each of your app’s process types independently.
The vast majority of apps will see a 1% - 10% reduction in overall monthly dyno charges.
Previously, Heroku included “1 free dyno” with each app, which is equivalent to between 672 and 744 dyno-hours (depending on the length of the month). With this switch to dyno-hours, we are converting the included free dyno to a fixed 750 free dyno-hours. This makes more sense, is more consistent, gives all users additional free dyno-hours, and allows the free dyno-hours to be used for any kind of process (web, workers, one-off admin process, or any other Procfile process).