What is a work breakdown structure
A work breakdown structure (WBS) divides the project scope into smaller and smaller deliverables, in tree form, down to work packages. It answers one question: what must be delivered for the project to be complete?
The WBS describes the what, not the when. It is the base for estimating hours and cost, assigning owners and building the Gantt chart. Without a WBS, the schedule is born with forgotten tasks.
WBS visual map
The map shows the 3-level tree of the example, with its work packages and the total of 440 hours.
The rules of a good WBS
- The 100% rule: the levels below an item add up to 100% of its work, no more and no less. Work outside the WBS is out of scope.
- Deliverables, not activities: name items with nouns ("Cell layout"), not verbs ("Do layout"). Verbs come later, in the schedule.
- No overlap: each package belongs to a single parent.
- Estimable work packages: small enough to have an owner, a duration and a cost. Many teams use packages of 8 to 80 hours as a rule of thumb; it is common sense, not a standard.
- Hierarchical numbering: 1, 1.1, 1.1.1, which makes rolling up cost and hours easy.
How to build a WBS in 5 steps
- Write the final deliverable at the top (level 1).
- Split it into 4 to 7 major deliverables (level 2), by phase or by product component.
- Go down to the work packages (level 3), stopping when you can estimate and assign.
- Check the 100% rule top-down and bottom-up.
- Write the WBS dictionary: for each package, scope, owner, acceptance criteria and estimate.
Worked example: implementing a manufacturing cell
Fictional example. Level 1 is the final deliverable; level 2 has 5 deliverables; level 3 has 10 work packages with hour estimates.
| Code | Deliverable / work package | Hours | % of total |
|---|---|---|---|
| 1.1 | Project management | 40 | 9.1% |
| 1.1.1 | Project plan and schedule | 16 | |
| 1.1.2 | Progress reports and meetings | 24 | |
| 1.2 | Engineering | 72 | 16.4% |
| 1.2.1 | Cell layout | 40 | |
| 1.2.2 | Machine technical specification | 32 | |
| 1.3 | Infrastructure | 168 | 38.2% |
| 1.3.1 | Floor and foundations | 120 | |
| 1.3.2 | Electrical and compressed-air network | 48 | |
| 1.4 | Equipment | 104 | 23.6% |
| 1.4.1 | Machine receiving and inspection | 24 | |
| 1.4.2 | Installation and leveling | 80 | |
| 1.5 | People and start-up | 56 | 12.7% |
| 1.5.1 | Operator training | 36 | |
| 1.5.2 | Testing and acceptance | 20 | |
| 1 | Manufacturing cell implemented | 440 | 100% |
Check: 16 + 24 = 40; 40 + 32 = 72; 120 + 48 = 168; 24 + 80 = 104; 36 + 20 = 56. Total: 40 + 72 + 168 + 104 + 56 = 440 hours, or 100%. Infrastructure carries 38.2% of the work, so that is where the estimate deserves the most attention.
Next, each package becomes tasks with durations and dependencies. Use PERT/CPM to compute the duration, FMEA to deal with what could go wrong in each deliverable, and 5W2H to link actions to owners.
Where a WBS helps on the shop floor
- Equipment installation and layout changes: each civil job, purchase and training becomes a package.
- Product launch: the WBS brings together engineering, APQP/PPAP, tooling and logistics.
- Maintenance shutdowns: the maintenance plan details deliverables per machine.
Common mistakes
- Mixing deliverables with activities ("Buy machine" next to "Floor and foundations").
- Forgetting project management, which also consumes hours.
- Breaking down too much: 2-hour packages create paperwork with no gain.
- Breaking down too little: packages of months cannot be tracked.
- Uneven levels for no reason, with one branch detailed and another vague.
- Skipping the dictionary: two teams read the same package in different ways.
Frequently asked questions
What is a WBS?
The hierarchical decomposition of project scope into deliverables and work packages. WBS stands for work breakdown structure.
What is the difference between a WBS and a schedule?
The WBS defines what will be delivered; the schedule defines when and in what order. The WBS comes first.
What is the 100% rule?
Child items must add up to all the work of the parent. In the example, the 5 groups add up to the 440 hours.
How many levels should a WBS have?
As many as needed to estimate and assign an owner. For small manufacturing projects, 3 levels are usually enough.
What is a work package?
The lowest level of the WBS: a deliverable with an owner, an hour and cost estimate and acceptance criteria.
What is the WBS dictionary for?
To describe each package (scope, owner, acceptance, estimate) and avoid different interpretations.
Sources
- PMI. Practice Standard for Work Breakdown Structures. Project Management Institute.
- PMI. A Guide to the Project Management Body of Knowledge (PMBOK Guide). Project Management Institute.
- KERZNER, H. Project Management: A Systems Approach to Planning, Scheduling, and Controlling. Wiley.
Keep studying: trusted sources and links
- PMI: Standards and publications (PMBOK Guide)
- ABEPRO: Associação Brasileira de Engenharia de Produção (in Portuguese)
- Wikipedia: Project management
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About the author
Lean Manufacturing & Behavioral Management Specialist
Over 20 years in the automotive and metalworking industries (GM and Dana), Lean Manufacturing practitioner since 2006. SENAI instructor and mentor in Brazil’s Brasil Mais Produtivo program, delivering consulting, training and audits for 50+ companies, combining quality, productivity and people development.
