personmonths

Person-month is a unit of effort used in project management and related fields to express the amount of work that a single person can complete in one month of full-time work. It represents effort rather than a guaranteed calendar duration. A common approximation is about 160 hours (8 hours per day for 20 workdays), though actual hours vary by country, organization, and work schedule.

The term is used for planning, budgeting, staffing, and cost estimation. For example, a task estimated at 6 person-months could be assigned to one person for six months, or to three people for two months, assuming full-time engagement and similar productivity. It is important to note that person-months are an estimate of effort and do not guarantee an exact finish date.

Limitations include the assumption of linear scalability and constant productivity, which rarely hold in practice. Real projects incur overhead, learning curves, holidays, vacations, and differences in skill. Adding more people can increase coordination costs and lead to diminishing returns or delays (a reference point often cited as Brooks’s law). Because of these factors, person-month estimates are most useful for rough planning in early project stages rather than precise scheduling.

See also: man-month, person-hours.