The progress is recorded individually for each task so the progress of the subtasks does not affect the progress of the parent task at all.

The formula of calculation is "∑d*p / ∑d", where "d" is task duration in days, "p" is the task progress and "∑" stands for a sum of all loaded tasks.

Basically, go through all loaded tasks, add up duration in days multiplied by progress, and divide it by the sum of durations in days for all tasks. Keep in mind that only the loaded tasks are used for calculating the task list progress, and the completed tasks are not loaded by default.

So, unless you click Show Completed, the task list progress will be represented based on the loaded tasks instead of the whole task list.

ie: If you have a task list with two tasks with one task dependent on a single subtask
  • Task 1: (Review UI before going live) has a duration of 12days - 63% of the total task list percentage if completed,
  • Task 2: (Review CSS) has a duration of 7 days - 37% of the task list progress if completed,
  • The subtask of Review CSS has a duration of 5 days - Progress for the subtask will not count towards the task list progress on the Gantt chart
Task list on creation:

If you mark the progress on: 
  • "Review UI before going live" to 30% complete, this will account for 19% of the task list progress
  • "Review CSS" to 70% complete this will account for 26% of the task list progress
  • The total progress for the task list is 45% complete

If you complete "Define methods" and "Review CSS" this will mean that the task list is 56% complete based on
  • 37% for "Review CSS" been completed and
  • 19% for the part progress of "Review UI".
The task can be completed via the Gantt chart by clicking on the check mark on the left pane after you hover over the task.

When you access the Gantt chart at a later stage the completed task will not appear so the current progress of the active task(s) will be what is taken for the task list progress. 

You can display the completed tasks on the Gantt chart by clicking on the "show Completed" button

The part percentage of progress that represents the 100% progress of the completed task "Review CSS" will be added to the task list total. This brings you back to the following calculation
  • 37% for "Review CSS" been completed and
  • 19% for the part progress of "Review UI".