What Is Waterfall in Simple Terms?
Waterfall is a linear, sequential approach to project management where you complete one phase entirely before moving to the next—like water flowing down a series of steps. You gather all requirements first, then design, then build, then test, then deploy. There's no going back upstream. It's the 'measure twice, cut once' approach: extensive planning upfront to avoid changes later.
Waterfall answers the question: 'How do we deliver a predictable result when requirements are clear and stable?'
The Linear Flow: Why It's Called Waterfall
Waterfall gets its name from the visual representation of its process—like water cascading down a cliff, you cannot swim upstream. Once a phase is signed off with stakeholder approval, you move irreversibly to the next. Going back is expensive, often requiring formal change control processes and rework.
The Waterfall model was first described by Winston W. Royce in a 1970 paper on software development (though, ironically, he was critiquing it rather than advocating for it). It became the dominant methodology because it mirrored how physical products had been built for centuries—from architecture to manufacturing.
This model emerged from manufacturing and construction where physical deliverables make iteration costly. You can't 'un-pour' concrete, unbuild a bridge, or recall a million manufactured units cheaply. The assumption is that requirements can be fully defined upfront and will remain stable.
The 5 Phases of Waterfall (In Detail)
Each phase has specific deliverables that must be completed and approved before the next phase begins. This creates clear milestones and documentation at every step.
Requirements Gathering & Analysis
All requirements are gathered, analyzed, and documented in detail upfront. This typically results in a comprehensive Business Requirements Document (BRD) or Software Requirements Specification (SRS) that stakeholders sign off on. The goal is to capture every feature, constraint, and acceptance criterion before design begins. Changes after this phase trigger formal change control processes.
System Design
Based on the requirements, architects and designers create detailed technical specifications. For software, this includes system architecture, database schemas, interface designs, and technical documentation. For construction, it includes blueprints, engineering calculations, and material specifications. This phase produces the 'blueprint' that guides implementation.
Implementation (Build)
The actual construction or coding phase. Developers, engineers, or builders create the product according to the design specifications. In software, code is written module by module. In construction, physical work begins. The key: builders follow the plan exactly as specified, with minimal deviation.
Verification (Testing)
Testing and quality assurance validate that the product meets the original requirements. Testers compare actual functionality against the requirements document. Bugs are logged, fixed, and retested. This phase continues until the product meets predefined acceptance criteria. Testing happens after the build is complete—not during.
Deployment & Maintenance
The completed product is deployed or delivered to the customer. The project formally closes with documentation and handover. Maintenance begins—handling bug fixes, updates, and operational support. The product enters its operational lifecycle, often managed by a different team.
The "Ironclad" Promise: Predictability
Waterfall's greatest strength is predictability. Because everything is planned upfront, you can answer three critical questions before work begins: 'How much will this cost?' 'When will it be done?' and 'What exactly will be delivered?' This is why CFOs, procurement departments, and government agencies often prefer Waterfall—it enables fixed-price contracts, budget forecasting, and clear accountability.
⚠️ The catch: This promise holds only if scope doesn't change. The moment requirements shift mid-project, the cost and timeline certainty evaporates—often dramatically.
The Cost of Change Curve
One of the most important concepts in Waterfall is understanding how the cost of changes escalates as the project progresses. Fixing a bug or changing a requirement costs exponentially more in later phases:
| Phase | Relative Cost | Why |
|---|---|---|
| Requirements | 1x | Changing a document is cheap |
| Design | 5x | Redesigning impacts multiple systems |
| Implementation | 10x | Rewriting code or rebuilding is expensive |
| Verification | 20x | Bugs found require regression testing |
| Maintenance | 100x | Changes in production affect users |
In contrast, Agile keeps this cost relatively flat because changes are expected and incorporated continuously through short iterations.
Pros and Cons of Waterfall
Advantages
- Clear, predictable milestones and deliverables
- Easy to understand and manage for traditional organizations
- Well-documented requirements support compliance and audits
- Enables fixed-price contracts and accurate budgeting
- Works well when requirements are truly stable and well-understood
- Clear handoffs between specialized teams (designers, developers, testers)
Challenges
- •Late discovery of problems—testing happens at the end
- •Difficult and expensive to accommodate requirement changes
- •Users don't see working product until very late
- •Risk of delivering something users don't actually want
- •Documentation-heavy process can slow progress
- •Long time-to-market as nothing ships until everything is complete
When to Use Waterfall
Waterfall is the right choice when certain conditions are met:
Use Waterfall When:
- •Requirements are crystal clear and unlikely to change
- •The technology is mature and well-understood
- •Regulatory compliance requires extensive upfront documentation
- •You're working with fixed-price contracts requiring detailed specifications
- •Physical deliverables make iteration expensive (construction, manufacturing)
- •Stakeholders need firm budget and timeline commitments upfront
When Waterfall May Not Be Ideal
- •Requirements are unclear or expected to evolve
- •You're building innovative products where learning is essential
- •Time-to-market pressure requires early delivery
- •Users need to provide feedback during development
- •The project environment is dynamic and unpredictable
Real-World Example: Construction Projects
Building a bridge is the quintessential Waterfall project. Before construction begins, engineers spend months (sometimes years) on requirements: traffic analysis, load calculations, environmental impact assessments, and stakeholder approvals. Then detailed designs are created—every bolt, beam, and cable specified. Only after blueprints are approved does physical construction begin. You can't pour the foundation, decide midway that the bridge should be 20% longer, and iterate from there. The cost of change in physical construction makes Waterfall not just appropriate, but necessary.
Key Takeaways
- 1Waterfall is sequential and linear—each phase completes before the next begins
- 2It excels when requirements are stable, well-defined, and unlikely to change
- 3The cost of changes increases exponentially as the project progresses
- 4It provides predictability for budgeting, scheduling, and compliance
- 5Not a failure—just designed for different circumstances than Agile
- 6Often combined with Agile in hybrid approaches for complex projects
