Why Your Progress Bar Gets Stuck At 99%
Progress bars count steps, not seconds, and the last one percent hides the work they never counted.
The script
Your progress bar isn't measuring time... it's guessing.
It counts steps, not seconds.
Copy a thousand files, and each one moves the bar the same amount... whether it's a tiny note or a huge movie.
So it races through the small ones... then crawls on the big one.
And the last one percent is the worst.
You see, that's where the computer checks everything, saves it to disk, and cleans up... work the bar never counted.
Some bars even lie on purpose. Researchers found people think a bar that speeds up at the end is faster.
So next time it freezes, remember...
The facts, checked
- Most progress bars report completed work units (files, bytes or install steps) divided by the total, not elapsed time; per-file overhead makes many small files and one large file move the bar at very different speeds.
- Final steps such as verifying, flushing write caches to disk, registering components and deleting temporary files are often not weighted into the bar, so it stalls near the end.
- Harrison, Amento, Kuznetsov & Bell, "Rethinking the Progress Bar" (UIST 2007), and Harrison, Yeo & Hudson, "Faster Progress Bars" (CHI 2010): bars that accelerate toward the end were perceived as faster.
