Basic Concept

A "man-hour" (sometimes rendered "man-hour") is the amount of work performed by an average worker in one hour. It is used in estimates of the total amount of uninterrupted labor required to perform a task. For example, researching and writing a term paper might require twenty man-hours. Preparing a family banquet from scratch might require ten man-hours.

Man-hours do not take account of the breaks that human beings generally require from work--e.g. for rest, eating and bodily functions. They only count pure labor. Managers take the man-hour count and add break time to estimate the amount of actual time a task will require. Thus, while one college course's term paper might consume twenty man-hours, it almost certainly will not get done in twenty hours. Its progress will be interrupted by work for other courses, meals, sleep and the manifold distractions of college life. (The exception to this rule is "cramming"---a.k.a. "pulling all-nighters"--which occurs when a student makes the common adolescent mistake of confusing man-hours with the amount of 'actual' time required to complete a project. Thus, the student works for twenty straight hours, the day before the paper is due, to complete a term paper that could have been done much more comfortably by distributing those twenty man-hours over a month of actual time.)

Real-World Applications

The great advantage of the man-hour concept is that it can be used to estimate the impact of staff changes on the amount of time required for a task. This is done by dividing the number of man-hours by the number of workers available. For example, a banquet consuming ten man-hours in its preparation would require one (presumably harassed) person to work ten straight hours. However, if the chef-for-the-day could draft a helper (like an sympathetic grandmother), the total task would consume only five hours of actual time. The two chefs combined still expend ten hours of labor; but by dividing the work between them, they get the work done in half the time. A term paper requiring twenty man-hours could be done in five actual hours if four friends collaborated on it. (See Cheating.) Cheating can be to create an unfair advantage, usually in ones own interest, and often at the expense of others. ...

In a more serious vein, let's say that a corporation's senior management requests a weekly report on its factory's operations. Assume that all administrative personnel work forty hours per week. The company's operations staff estimates (based on their experience with such things) that producing any report on the operations will take eighty man-hours. This means that, if management is to get its weekly report, the corporation must hire two employees. If the corporation can only afford to hire one employee, then management will have to content itself with a bi-weekly report.

Man-Years

The similar "man-year" concept is used on very large projects. It is the amount of work performed by an average worker during one year. Obviously, the number of hours worked by an individual during a year varies greatly according to cultural norms and economics, but a man-year seems to hover around 2000 man-hours. See, e.g. Solicitation Number 05-0002-02: Support Services for the Office of Naval Research for the Legislative Affairs Office (United States Navy Office of Naval Research: Arlington, Virginia, USA, 2004 (http://www.onr.navy.mil/02/matoc/05_09/solicitations/docs/05-0002-02.pdf) (man-year defined as 2000 hours); Report 5, International Federation Of Professional And Technical Engineers Local 32: San Diego, California, 2000 (http://www.ifptelocal32.com/CA/Report%205.htm) (man-year defined as 2087 man-hours, counting 311 "Non-available/Nonproductive" man-hours).