manmonths

Man-month is a unit of work representing the amount of work performed by a single person in one calendar month under typical full-time conditions. In project management and cost estimation, total effort is commonly expressed in man-months, with staffing and schedules inferred by multiplying the number of people by the expected duration in months.

Calculation: The number of man-months equals the total number of person-hours divided by the standard hours worked in a month. In many industries this is taken as about 160 hours per person-month (40 hours per week for four weeks), but actual values vary by country, holidays, and company policy. It is a measure of effort, not a guaranteed calendar time.

Origins and usage: The term gained prominence in mid-20th century management and software engineering. It is central to estimates of required manpower, but its assumption of linear scalability has been questioned, notably by Brooks's Law, which observes that adding manpower to a late project can cause further delays.

Limitations and considerations: Man-months assume constant productivity and a linear relationship between staff and output, which are rarely realistic. They overlook ramp-up time, learning curves, coordination overhead, task dependencies, and breaks. The phrase has drawn criticism for gendered language, leading many to prefer person-months or similar neutral terms.

Applications and related terms: Used in software development, construction, and research to plan staffing, budgeting, and timelines. Related units include person-months, person-hours, and the broader concept of effort versus elapsed calendar time.