This entry is part of the Chart Index, the reference library for the Chart Design Field Guide.
The Sankey diagram visualises flows between nodes as ribbons whose widths are proportional to the quantities flowing through them. It is the chart for where does it come from, where does it go? — the conversion funnel that does not reduce to a funnel, the energy budget that does not reduce to a pie, the customer journey that splits and rejoins. The form's name comes from Captain Matthew Henry Phineas Riall Sankey, who used it in 1898 to show steam-engine energy losses.
Sankey diagrams are dense and information-rich, but they are also surprisingly easy to make unreadable. Too many flows, too many splits, ribbons of similar widths crossing in the middle — the elegance of the form collapses quickly. Done well, they are unmatched for showing multi-step flows; done poorly, they are decorative rivers.
What it is
A Sankey diagram maps a flow process to nodes (boxes or columns) and ribbons (curved bands) between them. Each node has an incoming total and an outgoing total; the ribbons' widths are proportional to the flow they carry. The total flow in must equal the total flow out (conservation) — though some Sankey variants relax this for visualisation purposes.
Five stages from landing page to first activated user. Each stage's column shows the incoming flow; the ribbons to the right show where they go (continue, drop, branch). The thick ribbon down the centre is the main path; the smaller ribbons branching off are the alternative routes and the drop-offs.
When to use it
Sankey diagrams are the right choice when:
- The data is multi-stage flow with splits, merges, or drop-offs.
- The reader's question is "where does the volume go through this process?"
- Each stage's total is conserved (or near-conserved) so the ribbons have a meaningful sum.
- You have few enough stages and flows for the chart to remain readable (typically 3–6 stages, 5–20 flows per stage).
- The audience is familiar enough with the form or will be guided.
When not to use it
- Single-stage flow. Use a bar chart or treemap.
- Many splits and merges. Above 8–10 stages or 30 flows per stage, the ribbons cross into an unreadable tangle.
- Quantity comparison rather than flow. If the reader needs to compare totals across categories, a bar chart is more precise.
- Hierarchical data. Use a treemap or sunburst; Sankey's flow semantics are not for tree structures.
Design principles
Order columns by stage
Sankey diagrams should read left-to-right (or top-to-bottom) along the process. Each column is one stage; ribbons flow from earlier to later. Resist the urge to rearrange stages for compactness; the temporal or process order is the chart's organising principle.
Sort nodes within each column
Within a column, order nodes by size (largest at top) or by domain meaning (e.g., user segment). Random order produces visual chaos. Consistent ordering helps the eye trace specific paths.
Use a quiet ribbon palette
Saturated multi-coloured ribbons look like a tropical river system. Use a single hue family, or colour by source (so all ribbons from the same starting node share a colour). Subdued colour lets the widths — the actual data — carry the story.
Direct-label the major flows
The widest ribbons should carry numeric labels — Activated: 8,400 (35%) — placed on or near the ribbon. Small ribbons can remain unlabelled or be shown only in tooltips.
Show drop-offs explicitly
If users drop off at each stage, show the drop-off as its own ribbon (typically curving downward into nothing, or terminating at a drop-off node). Hiding drop-offs by closing the system at the last visible stage is misleading.
Use small multiples for cohort comparison
To compare flows across cohorts (e.g., conversion funnels across two campaigns), use side-by-side Sankey diagrams rather than overlaying. Overlapping Sankeys are unreadable.
Resist the urge to encode time on ribbons
Some Sankey variants try to encode time as ribbon curvature or animation. The form's strength is its static, conserved-flow semantics. Animation belongs in tools, not in editorial communication.
Anatomy
A Sankey diagram's anatomy is columns of nodes connected by width-proportional ribbons. Drop-offs are explicit; node order within columns is meaningful; conservation is the perceptual contract.
Related types
- Funnel chart — for a single linear flow with no branches. Sankey's simplest case.
- Alluvial diagram — Sankey for shifting category memberships over time.
- Chord diagram — for all-to-all flows within one set.
- Network graph — for arbitrary relationships without a directional flow.
- Treemap — for static part-to-whole; no flow semantics.
Reading list
- Schmidt, M. (2008). The Sankey Diagram in Energy and Material Flow Management. The form's modern formalisation.
- Riehmann, P., Hanfler, M. & Froehlich, B. (2005). Interactive Sankey Diagrams. The interactive variants.
- Sankey, M. (1898). Introductory Note on the Thermal Efficiency of Steam-Engines. The original chart.