Microsoft Project considers many factors when setting a task’s start and finish dates (such as contraints you’ve set on the dates, dependencies on other tasks, and the task’s duration). Task duration is crucial. Your project’s finish date is determined by the latest finish dates of its tasks. Those finish dates are affected by how long the tasks take to complete. The duration of each task is determined by resource availability and, most important, by the formula Duration = Work ÷ Resources. If you assign resources, Microsoft Project uses this formula as the basis for all its scheduling.
By default, Microsoft Project calculates duration for you. In most cases, you’ll want to use Microsoft Project’s default settings. However, if you need to, you can choose which piece of the scheduling equation Microsoft Project calculates by setting the task type: fixed duration, fixed units, or fixed work. By default, units are fixed, causing Microsoft Project to calculate duration.
A task type is a method Microsoft Project uses to calculate the work, duration, and resource units for a task. You can choose the method you want Microsoft
Project to use on a task-by-task basis. The three task types are fixed units, fixed work, and fixed duration.