For project managers assigning tasks and resources to a schedule, you will see a simple calculation made on every task which may be confusing at first. It's not confusing at all once you examine this simple formula.
Task Work Hours ÷ (Task Duration x Resource available hours per day)
In the above example a simple 8 hour/day resource with 20 hours of work assigned to a five day duration is easy to calculate.
20 ÷ (5 x 8) = .5
The .5 displays as 50%
As with any simple three-value formula, you can manually input any two values and the third must calculate. The default setting is to allow the project manager to enter duration and work hours, allowing the system to calculate the percentage. You can change that setting any time you'd like and choose whichever two values you'd like to enter manually.
See Advanced Task Settings for further information about changing task calculations to fit the way you think and work.