How to Convert Person Months to Calendar Months

A classic problem that every software project manager has faced at some time or the other:

Given that there is a project of 100 person months. In how many months can the project be realistically implemented?

Can the project be done in 10 months with 10 people? Can it be done in 5 months with 20 people? What if there were 100 developers….

COCOMO offers an empirical based formula to compute the optimal schedule. I haven’t had much success in using that. To be fair to COCOMO, it does have a number of parameters that can be tweaked. I did try but COCOMO seems to be on the far higher side even when I set the parameters to an unrealistic extreme. Instead, the following formula worked much better for my past projects that have ranged from 100 person months to 800 person months.

Number of Calendar months = 2.5 x (person months).33

So, for a 100 person months project, the calendar months = 2.5 x (100).33 = 11 person months

Based on your past experience, you can tweak this result, for me, it is adds 2 extra months. The optimal calendar months for a 100 person months project is 9 months after adjusting for the 2 extra months.

I cannot recollect from where I landed on this formula. Perhaps it was Barry Boehm’s Software Engineering Economics (1979) or more likely from Fred Brooks classic The Mythical Manmonth.

Update: I just looked up The Mythical Man Month. Fred Brooks quotes this formula from Barry Boehm’s Software Engineering Economics (page 274, Indian Edition, 2006).

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