Duration vs Work in Microsoft Project: What’s the Difference?

What Duration means

Duration is the amount of working time between the start and finish of a task, measured on the calendar. A task with a duration of 5 days will span 5 working days, taking weekends and holidays into account through the project calendar. Duration answers the question: how long until this task is done?

What Work means

Work is the total effort required to complete the task, measured in person-hours (or person-days). It is the sum of the time all assigned resources spend on the task. Work answers a different question: how much effort does this take? A task can have a short duration but a lot of work if several people are on it, or a long duration with little work if one person touches it occasionally.

The relationship, with an example

Duration, Work, and the number of resources are linked. The simplest way to see it:

• A task needs 40 hours of effort. That is the Work.
• Assign one full-time person (8 hours a day). Duration becomes 5 days.
• Assign two full-time people instead. The same 40 hours of Work is now spread across two people, so Duration drops to about 2.5 days.

So the Work stayed the same (40 hours), but the Duration changed because the number of people changed. This is the heart of the confusion: if you think of a task only as “5 days,” you lose track of whether that means 40 hours of effort or 80, and your resource plan quietly breaks.

Why task types matter here

Microsoft Project decides what to recalculate using the task type, and this is where people get surprised. There are three types:

• Fixed Units (the default): the resource assignment stays fixed; changing Work or Duration adjusts the other.
• Fixed Duration: the Duration stays fixed; adding resources reduces the work each person does rather than shortening the task.
• Fixed Work: the total Work stays fixed; adding resources shortens the Duration.

There is also effort-driven scheduling, a setting that, when on, keeps total Work constant as you add or remove people, so more people means a shorter Duration. If you have ever added a resource and watched a task get unexpectedly longer or shorter, the task type is why.

Which one should you enter?

Enter the one you actually know, and let Project calculate the rest.

• If you know how long the task will take on the calendar, enter the Duration.
• If you know the effort it requires, enter the Work, assign your resources, and let Project work out the Duration.

The mistake to avoid is entering a number without deciding which it is. Be consistent across the schedule, and make sure your task types match how you want changes to behave. This is one of the common scheduling mistakes that makes a plan unreliable, and getting it right is part of building a schedule that is actually realistic.

See how healthy your schedule really is

Duration and work are about building tasks correctly. Whether the whole schedule holds together is a bigger question, and that is what GanttScore answers. Drop your Microsoft Project file in and it scores your schedule against the DCMA 14-point standard plus three checks of our own, seventeen in total, in about ten seconds. The free score shows what passes and fails; the full report gives you the fixes.