How to read a Sankey diagram (in about a minute)
A Sankey diagram looks complicated and reads simple, which is the opposite of most charts. There is exactly one rule to internalise: the width of a ribbon is the size of the number it carries. Everything else follows from that.
Left to right is time, or logic, or both
Sankeys are drawn in columns. The leftmost column is where quantity enters the system — revenue, energy generated, visitors arriving. Each column to the right is a later stage. Nothing ever flows backwards, so if you find yourself tracing a ribbon right to left you are reading the chart in reverse.
Every split is a decision, every merge is a total
When one node fans out into several, something has been divided: revenue into cost and profit, electricity into consumption and losses, applicants into hires and rejections. When several ribbons converge into one node, you are looking at a subtotal. Reading a Sankey well means noticing which of the two is happening at each column.
Conservation is the honesty check
In a correct Sankey, what enters a node equals what leaves it. If a node has 100 coming in and only 80 going out, either 20 is missing or the chart is wrong. This is the fastest way to audit someone else's diagram: pick the biggest node and add up both sides.
Colour carries the second layer of meaning
Most well-made financial Sankeys use colour as a verdict rather than decoration: green for what you keep, red for what you lose, neutral grey for the trunk that is neither. Once you know the convention, you can read the outcome of a chart before reading a single label.
- Widest ribbon at the far left = where everything comes from.
- Widest ribbon at the far right = where everything ends up.
- The thinnest ribbon in a chart is usually the point of the chart.
- A node with many thin outputs is a category that should probably be grouped.
If you only remember one habit: find the widest ribbon and follow it to the end. In almost every Sankey worth publishing, that path is the story the author wanted you to see.
Frequently asked
What do the widths in a Sankey diagram mean?
Width is proportional to quantity. A ribbon twice as wide as another carries twice the value, which is why Sankeys communicate proportion faster than tables.
Why do some ribbons get thinner as they cross the chart?
Because something was subtracted at a split — a cost, a loss, a drop-off. The lost amount always leaves through another ribbon; if it does not, the diagram is not conserving its totals.