Person-Months Calculator
Use this tool to estimate total effort in person-months, equivalent person-hours, and required duration for a target workload.
What is a person-month?
A person-month is a unit of effort, not calendar time. One person-month means one person working for one month at full allocation. If someone is only allocated 50% of their time, they contribute 0.5 person-month in that month.
This metric helps teams plan project workload, staffing, and budget with a shared language across engineering, operations, and management.
Core formula
Person-months = Team Size × Duration (months) × Allocation
- If allocation is 100%, use 1.0 in the formula.
- If allocation is 60%, use 0.6.
- To convert to person-hours, multiply person-months by your monthly hours assumption.
How to use this calculator
Step 1: Enter team size
Enter the number of people contributing to the project. Decimals are allowed if your staffing is fractional.
Step 2: Enter duration
Provide the planned calendar length in months.
Step 3: Set allocation %
If every person is fully dedicated, use 100%. For part-time assignments, reduce this value. For example, a 4-person team at 75% allocation over 8 months yields 24 person-months.
Step 4: Choose hours per person-month
Many organizations use 160 hours/month, but you can align this with your internal capacity model.
Step 5: Optional target effort
Enter a target person-month value if you want to estimate the duration required with the current staffing profile.
Example scenarios
Example 1: Full-time team
A 6-person team working for 4 months at 100% allocation:
- Person-months = 6 × 4 × 1.0 = 24 PM
- At 160 hours/PM, effort = 3,840 hours
Example 2: Part-time allocation
A 10-person cross-functional team contributes at 40% for 9 months:
- Person-months = 10 × 9 × 0.4 = 36 PM
- Even with many people, low allocation can extend delivery timelines.
Common planning mistakes
- Confusing effort with elapsed time: 12 person-months does not always mean one person for one year in practical delivery.
- Ignoring ramp-up and coordination: Bigger teams can increase communication overhead.
- Using unrealistic monthly hours: Holidays, meetings, and context switching reduce actual productive hours.
- Skipping allocation adjustments: Shared team members rarely contribute 100% continuously.
Quick interpretation guide
- Higher person-months = more total effort required.
- Same person-months with fewer people = longer schedule.
- Same person-months with more people = potentially shorter schedule, but only up to coordination limits.
Frequently asked questions
Is person-months the same as billable months?
Not always. Person-months represent effort. Billing terms may include rates, utilization targets, and non-project time.
Can I use this for software or research projects?
Yes. It is useful for software development, data science, consulting, academic projects, and operations planning.
What allocation should I use for realistic forecasting?
Most teams forecast between 60% and 85% effective allocation unless staff are fully dedicated.