The amount of work a person, who may be a woman, will finish in a working day (about 8-10 hours).

Also, a unit of project management resource which is used to measure how much resource is required to perform a task. The man day does not adhere to the normal rules of arithmetic. To take an extreme example, if it takes 200 ManDay''''''s to design a piece of software, it does not mean that 200 people can design it in one day. Furthermore, without weightings, the unit does not take into account that some people achieve a lot in one day and some people achieve nothing.

Indeed, a 200 manday task might optimally be delivered by a team of three people over a three-month period, according to the sqrt(manmonths) rule of thumb.

See MythicalManMonth for further information on the pitfalls of resource management.