Project Planning using Phase Ratios

When planning a project, the ISBSG phase ratios provide a guide to the percentage effort required for each project phase. 5 min read. 

Data Set Analysis

The Release 10 Repository contains 94 projects, which covers six phases,

  1. Plan
  2. Specification
  3. Design
  4. Build
  5. Test
  6. Implement

This data set is split ~50/50 across new developments and enhancements.

Phase Ratios

Figures shown are the mean (average) figure for the data set.

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Rules of Thumb

In general, assume the following,

9% for planning, 9% for specification, 14% for design and 6% for implementation.

Build + test percentages show greater variations, particularly for enhancement testing.

Using Phase Ratios in Project Planning

Phase ratios are used during project planning to help define the likely phase effort,

  1. Establish your project’s functional size.
  2. Using ISBSG work regression tables, estimate the likely total project effort.
  3. Using the phase ratios, split the likely total project effort into likely effort by phase.

Example Phase Ratio in Project Planning

A new development project is sized as being 540 function points and will be developed for a mid-range platform environment. The regression tables indicate a likely total project effort of 4,600 hours. Then using the ratios above results in the following,

  • Planning: (4,600/100) *9               =    414 hrs
  • Specification: (4,600/100) *11     =    506 hrs
  • Design: (4,600/100) *15                 =    690 hrs
  • Build: (4,600/100) *43                    = 1,978 hrs
  • Test: (4,600/100) *16                      =    736 hrs
  • Implementation: (4,600/100) *6 =    276 hrs

Using Phase Ratio’s during Project Execution

As each phase is completed, the actual effort taken for that phase will be known, which is then used to reverse engineer the likely effort needed for the remainder of the project and the individual phases.

For example, instead of 414 hours for planning, 450 hours were consumed, which results in a revised likely total project effort of,

  • Total project effort: (450/9)*100    = 4,500 hrs

Consequently, the revised totals for each of the remaining phases becomes,

  • Specification: (5,000/100) *11      =   550 hrs
  • Design: (5,000/100) *15                 =    750 hrs
  • Build: (5,000/100) *43                    = 2,150 hrs
  • Test: (5,000/100) *16                      =    800 hrs
  • Implementation: (5,000/100) *6 =   300 hrs


If you are planning or quoting a new software project else want to benchmark your project, then the phase ratios provided are useful guidelines. Please note the example shown is for development projects only, and in practice figures should be tailored to suit your particular project environment.

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