a one-month snapshot and fill in the blanks by person.
Now you have a list of what everyone is working on--and when--for the next month. And I'll bet there's more work from your Universe of Work list. List that work on the "unstaffed" list.
Listing the Unstaffed Work
Underneath the last person's name, write "Unstaffed work." Now, week by week, add in the unstaffed work row everything that's in the Universe of Work that is not assigned to anyone. You should have a picture that looks something like this:
This is a portfolio for a cross-functional team. If you manage or are part of a single-function team, the tasks in the boxes would reflect that.
Estimate the People Required to Complete the Unstaffed Work
With this picture, you can see who's working on what--and whether those people are multi-tasking--and how much work is not staffed. Now you can estimate how many people you would require to staff the unstaffed work.
Manage the Portfolio
I find that the hardest part of this is starting. Once I have a rolling wave, four-week portfolio, I find it relatively easy to maintain. As you complete one week, add the data for the next week at the end of the plan. If you're not sure how long a duration portfolio you need, start with a four-week plan. If you don't have enough insight into the future, consider adding another week or two onto the end. But I find that few people or organizations are able to maintain detailed tactical plans longer than a few weeks, so more detailed planning isn't useful.
Once you have a detailed rolling-wave plan, you'll be able to answer the question of how many more people you need and how you could restaff the projects to take advantage of more people.
But I find the project portfolio chart much more useful than just showing my management where we are understaffed. I use the chart when my manager assigns my group more work. I show my manager the chart and ask, "My staff are working as hard as they can. They can't do more. What work would you like me to move to the unassigned work?" Now we can have a conversation about how much work needs to be done, the criteria for completing the work, and the relative priority of the work.
Knowing how much work you group can accomplish--and when they can finish that work--is critical to your success as a manager. With a project portfolio, you can manage the work. You can make tradeoff decisions about which projects to staff and which ones to leave unstaffed. And you'll know how many more people you need for how long. Make sure you're managing the project portfolio, not allowing it to manage you.