I am helping a colleague "debug" an MS-Project project plan today, where there seem to be unexplained over-allocations. They have chosen quite a complex method for restricting allocations and I don't quite understand what I am seeing.

The units that are allocated to a task, for any particular resource, seem to be governed by settings in three places:

  • The Resource Availability in the resource calendar
  • The Resource Max Units in the resource sheet
  • The max units allocation on the task

I don't understand the relationship between these settings and so I cannot determine what this colleague is actually forcing Project to do.

If Monday is a fully working day, with a default of 8 hours for Resource A. If I then set their availability via the calendar to 50% I assume that they can only spend 4 hours on the project on that day.

If I then set their Max Units on the Resource Sheet to 50%, does that mean they will only spend 2 hours per day (i.e. 50% of their availability, 50% of 4 hours)?

If I then allocate them to a task at 50% does that provide them with 2 hours availability on the task (i.e. 50% matches their max units from the Resource Sheet and so is allowed) or does it provide 1 hour, being 50% of the availability I have inherited from the Resource Sheet/Calendar combination?

In the past I always have 100% resource availability within time blocks they are on the project, and restrict their availability to tasks within the Resource Allocation. I have no experience of this, apparently 3-layer, description of a Resource's task-hours allocation.

It is driving me nuts! What is the relationship between these settings?

2 Answers 2


When you edit a resource's availablility - you are changing his/her max units over time. It will match the value in the max units field for the applicable time frame. For example: From April 1 to April 24, the resource's max units are 100%. After April 24, their max should only be 50%. If I Look at the resource sheet anytime between April 1 and April 24 - I will see 100%. But, if I open the project on April 25 and look at the resource sheet, I will see 50%.

I don't agree with your statement "The max units on the Resource Sheet matches the availability as defined by the calendar". I can change a resource's calendar to only allow work from 08:00 am to 12:00 pm - 4 hours per day. If I set the resource's max units to 50%, I am saying the resource can produce 30 minutes of effort for every one hour of working time set in his/her calendar. So 50% of 4 hours will be 2 hours per day.

By default, Project will assign a resource at his/her max units as long as: 1) max units are not above 100%. In that case the resource will still be assigned at 100%. 2) You have contoured the resource's max units over time using Resource Availability in the Resource Information dialog and the assignment spans over a change. In that case, the resource will be assign at his/her max units at the start of the task.

In your scenario, let's say your task suddenly increases in duration. Instead of neatly fitting into week one, it starts in week one and extends into week two. The resource will be overallocated because of a 100% assignment for the entire task - but his/her availability drops.

I would not recommend getting so fine grained as changing a resources max units every week. You (and your client) will spend their entire time managing a Project file - not managing the project.

  • Thanks @JulieS, I was hoping to hear from you! In respect of my statement you disagree with- I tested this by setting a resource's availably to 50% in their calendar (note, not the work hours, which I agree can clearly be anything, but the Availability, I then saw that this was reflected in the Max Units on their Resource Sheet entry. I did subsequently prove to myself that it matches the calendar for that period- perhaps 50% this week and 100% next week, or whatever. Sadly my colleague has chosen to contour his resources' availability in this way, some weeks 50%, some 100% and some zero!
    – Marv Mills
    Apr 23, 2015 at 14:31
  • Ah, sorry. My mistake. I though when you said you had set their availability in the calendar, that you had changed working time. Why is your colleague getting so fine grained in his plan? For zero availability I'd really use non-working time. With contouring, he's going to have a time of it with overallocated resources.
    – JulieS
    Apr 23, 2015 at 15:23
  • For zero availability he is just failing to add an actual availability in Resource Availability on the Resource Calendar screen, rather than setting an actual availability as zero, I just couldn't be bothered to explain that :) And that appears to override the Working Days on the resource calendar. I don't know why he has chosen that method, but I am assisting him with techniques for simplifying his planning and scheduling :)
    – Marv Mills
    Apr 23, 2015 at 15:33
  • Yes, setting a resource to zero, or skipping a time period in the availability dialog, does render the working time in the resource's calendar moot. With zero availability, you are saying they are not available at all. The problem with gaps or zero availability is it does not move the task like setting non-working time in the resource's calendar. The zero availability just marks the resource as overallocated without giving you any real hint about what is going on.
    – JulieS
    Apr 24, 2015 at 14:43
  • In your example about editing a resource's availability: you say that if you look at the resource sheet anytime between April 1 and April 24 - you will see 100% for the resource's max units. And if you look at it on April 25, you will see 50%. That would seem logic to me too, but in my (Project Server 2013) environment, the max units is not updated automatically!!!! Only when you open the resource in MS Project, retrieve the resource's information (double-click on it) and save it again, the resource's max units value is updated. Perhaps a bug? Anyone else who encounters this problem? May 7, 2019 at 14:04

In order to try and reach an understanding I have performed various tests in model situations and I have discovered the situation is usually straightforward, but is counter-intuitive in one example.

The max units on the Resource Sheet matches the availability as defined by the calendar. If you change the calendar then the Resource Sheet changes and vice versa. If you say a resource is only 50% available then their max units on the resource sheet is also 50%. For an 8 hour day you could apply them at 50% and get 4 hours work. If you apply them at 100% then you are breaking the max units "rule" and you get an over-allocation.

So there is no "relationship" between these settings, they are all a reflection of the same underlying measure- How much of a project-day can that Resource be applied without over-allocating.

However there is a problem, and this is at the root of my colleague's problem with his Project. In the resource calendar you can set different blocks of time at different availability of units. Suppose in Week 1 a resource was available 100% and in Week 2 they had 50% availability. If you allocate them 100% to a task that is scheduled in Week 1 then no problem. If that task then moves to Week 2 as a result of scheduling slippage, the resource allocation will remain at 100% and therefore cause an over-allocation.

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