also: にんび
noun
man-day; person-day
Unit of work representing one person working for one day, used in project management and cost estimation.
このプロジェクトには延べ100人日の作業が必要です。
This project requires a total of 100 man-days of work.
見積もりは5人日で計算されています。
The estimate is calculated at 5 person-days.
人月 is 'man-month', a larger unit of work effort, while 人日 is 'man-day'.
Compound of 人 (person) and 日 (day), directly meaning 'person-day'.