In TV and movies, all computer operations take an exact, known amount of time to perform, and all progress bars move smoothly at a constant rate. This will always be 100% accurate. Generally, in Real Life, it's more complicated. Just try copying a large folder in Windows and see for yourself. In real life, some jobs are predictable and some are not, and this is always highly dependent on actual implementation details that are beyond this article. See also Beeping Computers and Magic Countdown. Examples of Exact Progress Bar include: