What is a WBS (Work Breakdown Structure)?

A WBS is a hierarchical breakdown of everything a project has to deliver, split level by level into smaller and more manageable pieces.

The short definition

A Work Breakdown Structure (WBS) takes the total scope of a project and divides it into deliverables. Each deliverable can be divided again into smaller ones, until every piece is small enough to be estimated, assigned and tracked. In Spanish it is known as EDT (Estructura de Desglose del Trabajo).

The WBS describes what has to be delivered, not when or who does it. Schedules, costs and resources are built on top of it.

The 100% rule

The elements at each level must add up to 100% of the scope of their parent, no more and no less. If something is not in the WBS, it is not part of the project. If something is in the WBS twice, the scope is being counted twice.

How to build a WBS

1. Write the project's final result as the top element (level 0).
2. Split it into its main deliverables. They can be organised by phase, by product component or by area of the organisation.
3. Break each deliverable down further until the pieces can be estimated and assigned.
4. Describe each element: what it includes, what is excluded and how you will know it is done.
5. Review that nothing is missing and nothing overlaps (the 100% rule).

Numbering: the WBS code

Every element gets a code that shows its position in the hierarchy: 1, 1.1, 1.1.1 and so on. The code is the identifier used in schedules, budgets and reports, so it has to stay consistent when elements are added, moved or removed.

The WBS dictionary

The WBS dictionary is the document that describes each element of the WBS: its code, name, description, owner, acceptance criteria, quality requirements, risks and milestones. The diagram shows the structure; the dictionary explains what each box really means.

What a WBS is not

• It is not a schedule: it has no dates or durations.
• It is not an organisation chart: it lists deliverables, not people.
• It is not a task list in execution order: the order of the boxes is not the order of the work.