Introduction to Functional Sizing

The functional size of a piece of software represents the size of the functional user requirements. Functional user requirements represent what user functions will be included in the software. Functional requirements are the business processes performed by, or supported by, the software, (eg record and store ambient temperature), and include what functions the software must do. Functional requirements are the "floor plan" for software. Just as square meters or feet are important measures for building construction estimation, so functional size is an important input to software estimation.

For a new development project, the functional size is the size of all of the delivered or installed functional user requirements. For an enhancement project, the functional size is the sum of the total of the size of all functional user requirements that are either new, renovated(changed) or removed from the software.

Functional size can be sized in functional size units using a measurement method such as IFPUG, NESMA, COSMIC, FiSMA or Mark ll).

The functional size of the software is not the total size of a software project.

Non-functional requirements (eg portability, security, maintainability) are not included in the functional size but can be allowed for utilising a Value Adjustment Factor, or should be the subject of an adjustment to the effort estimate.

Technical (build) requirements which address how the softwarewill be developed, or built, and include tools, methods, resource skill levels etc., are not reflected in the functional size and should be the subject of an adjustment to the effort estimate.