MS Project Calculating Duration Incorrectly

Iron Contributor

When I set a fixed duration task to start on 5/25 and finish on 6/7, this is a duration of 10 days. MS Project for some reason however, is displaying a duration of 12 days. This is incorrect. I don't' understand why its doing this. If I manually change the duration field to 10 days, then project changes the finish date to 6/6, which is still not correct because working days from 5/25 to 6/6 would be 8 days. I have May 30th scheduled as a non-working day for memorial day. See screenshot.  I'm not sure if it has anything to do with the options I have set for the project which is 6 hours per day, 30 hours per week to account for the reality that its not realistic for someone to be able to dedicate a full 8 hour day to project work.

 Screenshot 2022-05-22 113018.jpg

4 Replies
best response confirmed by JBLT-77 (Iron Contributor)
John Bacon,
I'm guessing you did not change the working calendar to reflect the 6 hour work day. By default, each work day is 8 hours (8:00 AM to 12:00 PM and 1:00 PM to 5:00 PM). The number of default days from 5/25/22 through 6/7/22, including the non-working memorial day, is 9. So 9 of the default 8 hour working days equals 72 hours. However, you have re-defined a "day" as 6 hours so 72 divided by 6 equals, wait for it, 12 "days".

The fact that resources may only be 75% efficient is not relevant for Project's scheduling calculation. That's something you will have to take into account when you estimate task durations and assign resources.

Hope this helps.
Yes. I checked and I did leave the project calendar work times from 8-12 and 1-5. I thought by changing working hours in project schedule options to 6, project would do its calculations accordingly assuming a resource assigned at 100% is still only 6 hours per day. So what you are suggesting is that I change the working hours per day back to 8, hours per week back to 40. Leave the project calendar working time as 8-12 and 1-5, but add resources with Max Units of 75% to account for administrative non-project related tasks performed during the day, and this should fix my calculation issue?

Or, if I just change the project calendar working times to 8-12 and 1-3.  Will that also resolve my duration, start date, finish date discrepancy?

Well my question for you is, what are your company working hours? Are they normal 8 hour working days or are they 6 hour working days? If they are 8 hour working days, then don't mess with the working times, unless you need to change them to say, 7:00 AM to 11:00 AM and 11:30 AM to 3:30 PM, or similar.

If a resource is a full time employee then their Max Units (Resource Sheet) should be 100%. The amount of time they are expected to spend on any given task is determined by the assignment units. So if a resource has other tasks, allocate their assignment across all those tasks. Project will warn you if a resource is overallocated (i.e. assignment level over a defined period exceeds their Max Units value). That defined level can be set via, Resource > Level group > Leveling options.

When it comes to resource efficiency the best way to handle that is to allow extra work hours for a task. Let's say a task should normally take 8 hours to complete but because human resources are normally not 100% efficient, you might enter a work content of 10 hours (80%). However, you'll likely find that trying to work a project at that fine a detail is more effort than its worth (i.e. resource efficiency is a very small, likely insignificant, part of the issues the project manager faces during execution of a plan).