Cumulative Flow Diagram
A stacked area chart that visualizes workflow states over time, revealing bottlenecks, WIP trends, and flow efficiency in Kanban and Lean systems.
Also known as: CFD, Cumulative Flow Chart
Category: Techniques
Tags: lean, kanban, metrics, visualization, agile, project-management
Explanation
A Cumulative Flow Diagram (CFD) is a stacked area chart where the x-axis represents time and the y-axis represents the cumulative number of work items. Each colored band represents a workflow state (e.g., Backlog, In Progress, Done), and the bands stack on top of each other.
**How to Read a CFD**:
- **Vertical distance** between bands shows the **Work in Progress (WIP)** for that state at any point in time
- **Horizontal distance** between bands shows the **Lead Time** (how long items spend between two states)
- **Slope of the top line** indicates the **arrival rate** (how fast new work enters the system)
- **Slope of the bottom line** indicates the **throughput** (how fast work is completed)
- **Parallel bands** indicate a stable, predictable flow
**What CFD Patterns Reveal**:
| Pattern | Meaning |
|---------|----------|
| Bands widening | WIP is growing — work is entering faster than it's leaving |
| Bands narrowing | WIP is shrinking — the system is catching up |
| Flat bottom line | No items being completed — a blockage exists |
| Bulging in one band | A bottleneck at that workflow stage |
| Parallel, evenly spaced bands | Healthy, stable flow |
| Top line steeper than bottom | Demand exceeds capacity — unsustainable |
**CFD vs. Other Charts**:
- **Burndown charts** show remaining work in a sprint — CFDs show flow across the entire system continuously
- **Burnup charts** show completed work — CFDs additionally show WIP distribution across states
- **Kanban boards** show current state — CFDs show the historical trend, making patterns visible
**Using CFDs Effectively**:
1. **Detect bottlenecks early**: When a band widens, that stage is accumulating work faster than it processes it
2. **Validate WIP limits**: If WIP bands stay consistent, your limits are working; if they bulge, limits need tightening
3. **Predict delivery**: Use the current throughput slope to forecast when remaining work will be completed
4. **Spot systemic issues**: A gradually widening overall WIP suggests the team is starting more than it finishes
5. **Measure improvement**: After process changes, the CFD shows whether flow has actually improved
**Connection to Little's Law**:
CFDs make Little's Law (Lead Time = WIP ÷ Throughput) visually intuitive. You can literally see that reducing WIP (narrowing the bands) reduces lead time (the horizontal distance between bands), assuming throughput stays constant.
**Common Mistakes**:
- Not including all workflow states (hiding queues between stages)
- Using it without WIP limits (the CFD will just show growing chaos)
- Ignoring the arrival rate — even perfect flow breaks down if demand exceeds capacity
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