Learn in the textbookEstimating Development Effort
DefinitionFunction Point analysis (ファンクションポイント法) estimates development size by turning the number and complexity of a system's functions into points.
ExplanationYou count the functions users can see, such as inputs, outputs, files and inquiries, weight each by its complexity, and add them up; the resulting score is the FP count, which serves as the development size. Because it counts functions rather than how they are built, the result depends little on the programming language, and the count can be made as soon as a list of functions exists. The effort needed (the amount of work, in units such as person-months) is found by dividing this size by the productivity taken from past results (the size one person can handle per month).
AnalogyIt is like judging how much work a dinner will take not by the size of the pot, but by how many dishes you will make and how hard each one is.
Memory hook"Function" here means a feature, not a function in program code: you count the chunks of functionality users can see.