Scope 3 Sankey Diagrams — When They Help and When They Mislead
Supply chain emission flows are tempting to draw as sankeys, but category boundaries and data gaps make many sankey figures misleading. Here is our decision framework.
Sankey diagrams have become the default visual shorthand for Scope 3 supply chain emissions. They look authoritative — flowing bands of colour implying precise quantification of material movement through tiers. But in practice, many Scope 3 sankeys mislead more than they clarify.
The data honesty test
Before commissioning a sankey, answer honestly: do you have activity data at each node, or are you allocating total Category 1 emissions across suppliers using spend-based proxies? If the latter, a sankey implies tier-level precision you do not possess. A ranked horizontal bar chart with explicit "spend-based allocation" footnotes communicates more honestly.
Sankeys earn their place when you have primary activity data at three or more tiers — for example, raw material extraction, component manufacturing, and final assembly — with distinct emission factors applied at each stage.
Node count limits
Beyond six nodes per side, sankey readability collapses on A4 print and standard projection. We cap primary sankeys at five source nodes and five destination nodes, aggregating remaining categories into an "other" band with a supplementary table listing constituents above the materiality threshold.
Category boundary labelling
Scope 3 categories have defined boundaries that do not always match intuitive supply chain stages. Category 4 (upstream transportation) emissions should not appear as a flow band between supplier tiers unless you have genuinely separated logistics emissions from purchased-goods emissions. Mixing categories within a sankey flow creates audit findings.
Alternative when sankeys fail
When data does not support sankey construction, we recommend a two-panel layout: treemap showing category weight on the left, and a simplified three-tier supplier flow schematic (without quantitative band widths) on the right. The schematic communicates supply chain structure; the treemap carries the quantitative weight. Separating structure from quantity prevents false precision.
Revision expectations
Scope 3 sankeys require more revision rounds than standard bar charts because supplier data shifts late in reporting cycles. Budget two revision rounds minimum, and plan for data freeze dates at least 12 business days before your publication deadline.
If you are uncertain whether your data supports a sankey, send your category outline when submitting a brief. We will recommend the appropriate visual form before quoting production scope.