Person Month Calculator
Estimate effort, schedule, or team size using person-month planning.
What Is a Person-Month?
A person-month is a unit of effort. It represents the amount of work one person can complete in one month. Teams use this unit to estimate scope, budget, staffing, and timelines for projects in software development, construction, research, consulting, and operations.
For example, if one person typically contributes 160 hours per month, then:
- 1 person-month = 160 hours
- 6 person-months = 960 hours
- 12 person-months = roughly 1 person-year
Core Formulas
1) Effort in Person-Months
Person-months = Total hours ÷ Hours per person per month
2) Duration in Months
Duration = Person-months ÷ Number of people
3) Required Team Size
People needed = Person-months ÷ Duration in months
How to Use the Calculator
- Mode 1: Enter total hours to convert into person-months.
- Mode 2: Enter total hours and team size to estimate project duration.
- Mode 3: Enter total hours and desired duration to estimate team size.
- Set your monthly working hours (default is 160) to match your organization’s schedule.
Practical Examples
Example A: Software Feature Delivery
If backlog analysis suggests 2,400 hours and each person contributes 160 hours/month:
2,400 ÷ 160 = 15 person-months. With a team of 5, duration is about 3 months.
Example B: Business Process Migration
A migration program requires 1,120 hours. With 4 analysts:
1,120 ÷ 160 = 7 person-months. 7 ÷ 4 = 1.75 months (about 7–8 weeks).
Example C: Staffing for a Deadline
You have 3,200 hours of work and need completion in 4 months:
3,200 ÷ 160 = 20 person-months. 20 ÷ 4 = 5 people required on average.
Common Estimation Mistakes
- Ignoring coordination overhead: Larger teams can increase meeting and handoff time.
- Assuming 100% productive hours: Real schedules include admin work, leave, and interruptions.
- Not accounting for skill mix: One senior specialist may replace several junior contributors in some tasks.
- Treating all work as parallelizable: Some phases are sequential and cannot be sped up by adding people.
Better Planning Tips
- Use a realistic monthly hour value (for many teams, 120–160 effective hours is more accurate than 160+).
- Add contingency for risk-heavy or uncertain work packages.
- Break large estimates into milestones and validate after each phase.
- Track actuals weekly and update your model early instead of waiting until deadlines slip.
FAQ
Is a person-month the same as a calendar month?
Not exactly. A person-month is effort-based. Calendar duration depends on team size, dependencies, and non-project constraints.
Can I simply add more people to shorten any project?
Not always. Some work can be parallelized, but complex tasks often require onboarding, communication, and coordination that reduce gains.
What value should I use for hours per month?
Use your company standard. If unsure, start with 160 and adjust based on holidays, leave, meetings, and observed productivity.