One of the biggest mistakes in software project planning is assuming that adding more people will automatically speed up delivery. In reality, the relationship between effort (man-days) and delivery time isn’t linear. The Putnam-Norden-Rayleigh (PNR) curve helps us find the optimal balance between team size and project duration.
The Formula for Optimal Time and Team Size
A simplified formula from the PNR curve helps estimate the optimal project duration based on effort:
T = C * E^0.33
Where:
- T = optimal time (in months)
- E = total effort (in person-months, calculated as man-days / 20)
- C = productivity factor (typically 2.5 for software development)
Example: A 1000 Man-Day Project
Let’s assume we have a software project estimated at 1000 man-days. How long should it take, and with how many people?
First, convert estimation to person-months:
E=1000/20=50 person-months
T = C * E ^ 0.33
T = 2.5 * 50 ^ 0.33 =9.2 months
Now, we calculate the optimal team size: N=50/9.2 ≈5.4
So, for a 1000 man-day project, the optimal schedule is around 9-10 months with a team of 5-6 people.
If you overstaff a project, you hit diminishing returns due to:
- Communication overhead (more coordination required)
- Task fragmentation (breaking work into smaller, inefficient chunks)
- Ramp-up time (new team members need onboarding)
On the other side, too few people lead to excessive delays. The goal is to find the sweet spot between delivery speed and team efficiency.
Key Takeaways
✅ Project duration follows a nonlinear relationship with effort—you can’t simply divide effort by the number of developers.
✅ There is an optimal balance between team size and timeline, which avoids inefficiencies.
✅ For a 1000 man-day project, a team of ~5-6 people over 9 months is ideal—forcing it faster may backfire.
By respecting the PNR curve, software teams can achieve realistic schedules and maintain quality without unnecessary stress. 🚀
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