At the unit which shows man-hour and the work load such as system development, at "number of people × time (month)" it is displayed. Size of project is expressed when and, hybridizing the cost month amount (man month unit cost), per 1 it is utilized in cost computation of system development/estimate.

The foundation in man-hour calculating is "the work load which the human (engineer) who has standard skill does in unit time (1 month)", but at the time of software development because the general standard which defines that does not exist, calling the same "1 man month", really the possible work quantity differs depending upon the development bender. In addition, if about it has made 1 man month definition "of month" and not to be fixed, with the work of 22 days or more there is, there is also about job of week 4 day is designated as 1 man month. Furthermore also unit cost of 1 man month being disjointed, there is opening 10 times or more depending upon the bender. When the fee which differs every level of the staff inside the same company (development bender) is set, it is.

Because in man month indication in regard to calculation, "3 human ×8 mosquito months" also also "6 human ×4 mosquito months" become the same "24 man months", if number of people are increased, when payment date can be made quick, there are times when it is misunderstood, but job of software development relative importance of communication between the necessary personnel is large, staff addition in order to decrease the efficiency of intention understanding between the member, always is not connected to payment date shortening. As for error of the thing which applies the method of thinking the man month to management of progress, old deflection derrick and Jr (Frederick P. Brooks and Jr. ) Doctor book "The Mythical man-month: With essays in software engineering "(1975) it points out.

In addition, when system development trust contract is done in the man month base, as for the bender which it receives an order appointing the excellent engineer, rather than being able to finish work quickly, superior making use of the engineer who does not excel several months, in order the method which could point to work to make a profit, for a long time there is also the criticism that to be high the buying ? it is done the software whose quality is bad.

But, from the fact that it is easy to understand as a scale, even presently extremely it is utilized generally in cost estimate of system development.