Fractional Durations Explained


You may have noticed in your schedule that the total float appears to be calculating incorrectly. Case and point, the image below shows the finish date is equal to the constraint date, but the total float is one day (it should be zero).

When I first saw this, I used to think it was a bug in P6. I used to complain, "P6 is the worst! There's so many bugs in it!" But like most things in P6, there was something happening beneath the surface that I didn’t understand.

Let's run through a few of the reasons why this may be happening in your schedule. 


First thing to understand is that P6 calculates by the minute and not just by the day. When we change our display to show P6 calculating by the minute, we can see that although our finish date is the same as our constraint date, our finish time is finishing five hours earlier than our constraint time.

Which leads us to our next reason: since P6 calculates by the minute, the durations can calculate by increments less than one day. When we change our settings to show decimal points for durations, we'll see our total float change to 0.5.

If you're wondering how to show decimals on durations, go to "Edit > User Preferences" and change the duration format to show 2 decimals as shown below.

Also, if you're wondering how to show the times on dates, again, go to "Edit > User Preferences" and click on the "Dates" tab and show the time and minutes as highlighted below.

So now that we can see that P6 is calculating correctly, how can we fix this issue? 


Solution 1 - Check fractional remaining durations:

The first thing we can do is check for remaining durations that aren't whole numbers. The problem in this case is that the driving activity for my project has 3.50 days remaining which causes the activity to finish in the middle of the work day instead of at the end of the work day. This has a downstream effect on the rest of the activities as they will then start and finish in the middle of the day instead of at the beginning and end of the day as they should.

To fix this, simply change the remaining duration to a whole number.

Solution 2 - Check the times on the constraint dates:

The other thing that can happen is that the time on your constraint date does not match your calendar. You can see below that my constraint date finishes at 12 AM… 

But the workday in my calendar finishes at 5 PM.

In order to fix this issue, make sure the time in the constraint matches the time in your calendar. So if the workday in your calendar finishes at 4 PM, your constraint date should end at 4 PM.