What Is a Sankey Diagram? Definition, Parts and an Example
A Sankey diagram shows how a quantity splits and combines as it moves between stages, with ribbons as wide as the amounts. Learn what the parts mean, with an example.
A Sankey diagram is a flow diagram in which the width of every band is proportional to the quantity it carries. It answers one question well: where does it all go?
Take a month's pay and the things it is spent on. A list of numbers tells you the amounts; a Sankey diagram shows them as bands leaving a single block, each as wide as its share, so the biggest claims on the money are obvious without reading a figure.
The example above is exactly that, with invented numbers. It is also editable: change an amount and the band resizes.
The parts of a Sankey diagram
Every Sankey diagram is built from the same three things.
- Nodes: the blocks, such as a source, a stage or a destination. A node's height is the total that passes through it.
- Flows (also called links or ribbons): the bands between nodes. Each flow's width is its amount.
- Columns or stages: nodes are arranged left to right, so the diagram reads as a journey from where something starts to where it ends.
When a Sankey diagram is the right chart
Use one when you have a quantity that is split, combined or passed on, and the question is about proportions: money through a budget, energy through a system, people through a funnel, material through a process.
It is the wrong chart when there is no conserved quantity, when you need exact values more than shapes, or when the diagram would need dozens of nodes. A table or a bar chart is clearer for exact comparison.
Where the name comes from
The diagram is named after Matthew Henry Phineas Riall Sankey, an Irish engineer who used it in 1898 to show the energy efficiency of a steam engine. Flow maps with widths proportional to quantity are older still, such as Charles Minard's 1869 map of Napoleon's Russian campaign.
Key points
- Band width is the amount: the rent band is twice as wide as the food band because 1,200 is twice 600.
- What goes into a node comes out of it, so the node's height is its total.
- Read left to right, from where the quantity starts to where it ends up.
- The figures in the example are invented.
The example’s data
Invented monthly figures in euros, chosen so that the five spending bands add up to the pay.
| From | To | Amount (€) |
|---|---|---|
| Monthly pay | Rent | 1200 |
| Monthly pay | Food | 600 |
| Monthly pay | Savings | 500 |
| Monthly pay | Transport | 300 |
| Monthly pay | Other | 400 |
The same data as CSV
source,target,value
Monthly pay,Rent,1200
Monthly pay,Food,600
Monthly pay,Savings,500
Monthly pay,Transport,300
Monthly pay,Other,400How it works
Decide what is flowing
Pick one quantity, such as money, energy, people or material, and one unit for the whole diagram.
List the stages and the nodes
Name where it starts, where it can go, and any steps in between.
Add the amounts
For each flow, enter how much moves from one node to the next. Make what leaves each node equal what entered it.
Frequently asked questions
What is a Sankey diagram used for?
To show how a quantity is divided and moved between stages: budgets, energy, materials, customer journeys, website traffic and similar flows.
Is a Sankey diagram a flowchart?
No. A flowchart shows the sequence of steps and decisions in a process. A Sankey diagram shows how much of something moves along each route.
Make your own Sankey diagram
Open this example as your own chart, then change the amounts and names.