If you have ever had the pleasure of working with Microsoft Project you will likely be quite happy with it until you start changing the RESOURCE UNITS percentages (in the TASK INFORMATION pane).  If you make changes you will find that it often miscalculates (or appears to miscalculate) the number of HOURS in WORK.  After reading many many blogs and help files, I was about to give up on this when I found a reproducable process to have it calculate the number of days in DURATION correctly:

  1. Make sure the task is set to AUTO SCHEDULE
  2. Double click on the task and click the ADVANCED tab.  Set the TASK TYPE to FIXED WORK (see below for definitions) and CALDENDAR to STANDARD (or whatever you want)
  3. Type in the name of the TASK (do NOT type in the WORK or DURATION fields)
  4. Go the RESOURCE NAMES field and type the names and percentages using the following syntax:
    • name[PercentPersentSign],name[PercentPersentSign],name[PercentPersentSign] and so on
    • Ted[10%],John[40%]    This would mean that there will be a total of 4Hr per day worked on the project (8hr day * [40%+10%])

Click on these screen shots for details:


The three task types used in Project are:

  • fixed-unit task: A task in which the assigned units [or resources] is a fixed value and any changes to the amount of work or the task’s duration do not affect the task’s units. This is calculated as follows: Duration x Units = Work
  • fixed-work task: A task in which the amount of work is a fixed value and any changes to the task’s duration or the number of assigned units [or resources] do not affect the task’s work. This is calculated as follows: Duration x Units = Work
  • fixed-duration task: A task in which the duration is a fixed value and any changes to the work or the assigned units [that is, resources] don’t affect the task’s duration. This is calculated as follows: Duration x Units = Work.

Project uses fixed units by default.

Each of the task types affects scheduling when you edit one of the three elements as follows.

In aIf you revise unitsIf you revise durationIf you revise work
Fixed units taskDuration is recalculated.Work is recalculated.Duration is recalculated.
Fixed work taskDuration is recalculated.Units are recalculated.Duration is recalculated.
Fixed duration taskWork is recalculated.Work is recalculated.Units are recalculated.


You may find the following links useful if you need more details and


Ray · January 8, 2023 at 2:26 am

Your articles are extremely helpful to me. MS Project calculations are a hard to understand.

Muhammad Azzizan · August 26, 2015 at 11:57 pm


kiran · December 30, 2013 at 2:07 pm

nothing is working with my problem.

i’m just calculating days (duration) from (start) 01.11.2013 to (finish) 30.12.2013. simply it should count 60 days in duration column but its calculating 42 days :/

i have tried changes in MS Project options but problem still exists 🙁

any solution please???

    Kevin · March 17, 2014 at 10:05 am


    Duration does not count non-working days (weekends, holidays).

rsc · February 7, 2013 at 12:29 am

I have a MS Project problem: say you have 3 rooms and 5 clerks, they will work for 3 weeks, one clerk in each room daily , 5 days a week. How do you get that schedule done (which clerk works when) ?

DCapps · October 13, 2011 at 10:09 am

Had an activity that was not started with 2 days in the duration field and yet showed 5 days in the between the scheduled start and finish date. It made no sense as there were no resources involved, the predecessors were not driving out the finish date and the calendar did not have any non-working periods within the start and finish dates, apart from a sunday. Everything looked right and I experimented with all kinds of options. Finally, I started making random changes to one field after another, calcing after each one. That is when I hit pay-dirt. I changed the % complete from 0 to 1 and hit enter and the program finally calculated the activity correctly. I then changed the % complete back to zero and all was fine.

Has anyone experienced anything like that before?

    Henning · July 10, 2012 at 2:52 pm

    OMG! Thank you for adding your 2 cents on this. It totally helped me out. I had the exact same issue and it was highly annoying.

    Thanks again!

    Sloane · January 30, 2013 at 4:10 pm

    I thought I’d be here all night trying to figure this out. Thank you so much. Completely saved me a night’s sleep!

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