Microsoft Project vs Primavera P6: Which Schedules What

A dispute over a finish date rarely turns on the Gantt chart. It turns on which constraint pinned an activity, which calendar counted its days, and whether float went negative somewhere nobody looked. Those are exactly the places where Primavera P6 and Microsoft Project differ.

The short answer. Primavera vs Microsoft Project comes down to control versus reach. P6 is the scheduler for large, audited capital schedules, with layered calendars, two constraints per activity and configurable float. Microsoft Project is the general-purpose scheduler most delivery teams already know. If you receive both file types from different contractors, compare them on six axes, not on the bars.

Reviewers already audit schedules on these axes. The US Defense Contract Management Agency's 14-point schedule assessment, which scheduling software vendors such as Deltek document in their Acumen metrics, says no more than 5% of incomplete tasks should carry a hard constraint and no more than 5% of logic links should carry lag. Both limits police constraints and logic, the first axis below where the tools part ways.

Primavera vs Microsoft Project: how do they compare on six axes?

The two schedulers compare most usefully on how they handle constraints, calendars, float, resource levelling, file formats and who normally uses them. The table gives the short form; the sections after it explain what each row means in a dispute.

How does each tool honour constraints?

Microsoft Project gives each task one constraint from eight types, and a "must" constraint overrides the logic around it; all eight, ranked by risk shows which ones to distrust. P6 lets an activity carry a primary and a secondary constraint, so a start-no-earlier-than date and a finish-no-later-than date can sit on the same activity. A constraint is the usual reason an activity shows negative float: the date you imposed contradicts the logic feeding it.

How do calendars and float differ?

Calendars decide how many days an activity occupies, so the same duration lands on different dates under a five-day and a seven-day calendar. P6 assigns calendars per activity and per resource at global and project level; Microsoft Project does the same through its base, task and resource calendars. Float then follows the calendar. Total float versus free float is the distinction that matters when two files disagree, and negative float is the number to find first.

The diagram below shows the order to read an unfamiliar file in.

What does each tool do about resource levelling?

Both tools level overallocation by delaying work, and both can push the finish date out when they do. P6 uses activity priority to decide who waits; Microsoft Project uses priority and a levelling order. The mechanics are the same as in resource levelling explained: levelling trades dates for capacity, so a levelled file and an unlevelled one are not comparable until you know which you hold.

What does each file format carry?

A Microsoft Project schedule travels as a binary .mpp or as MSPDI XML; a Primavera schedule travels as a tab-delimited .xer file or as P6 XML. Moving between them is a conversion, not a rename, and calendars are the field most likely to change on the way.

Which should you use, and what are the trade-offs?

The choice is usually made for you by the contract. When it is not, pick the tool your reviewers can read. For the person holding both file types, Onplana Schedule opens .mpp, Microsoft Project XML and .xer on Windows and Mac, and draws the Gantt, critical path and float the same way for each. Any file whose contents are Microsoft Project XML is read on your machine; everything else, including a binary .mpp and a P6 .xer, is converted on Onplana's servers and the uploaded copy is deleted within 24 hours. Before you commit to either side's numbers, run the free Schedule Health Check on the file you have.

Check the schedule before you commit to it Run the free Schedule Health Check on a file you have: it reads the plan and flags dangling tasks and broken logic, up to 20 MB and 20,000 tasks. No signup required. → Open the Schedule Health Check

Microsoft Project™ is a trademark of Microsoft Corporation. Oracle Primavera P6™ is a trademark of Oracle. Onplana is not affiliated with Microsoft or Oracle.