What is a Sankey diagram? A comprehensive guide to understanding energy and data flows

If you have ever wondered how the energy powering a factory, a city, or a digital platform moves from source to end use, a Sankey diagram is the answer. This distinctive type of flow diagram uses the width of arrows to encode the magnitude of flows, delivering an immediate visual balance between inputs and outputs. In simple terms, a Sankey diagram shows “where things go” and “how much goes there” at a glance. In this article we explore what is a Sankey diagram, how it works, when to use it, and how to create and interpret one that truly informs decision making.

What is a Sankey diagram? A precise definition

What is a Sankey diagram? Put plainly, it is a graphic that represents the movement of quantities through a system. The key characteristic is that the width of each flow (the arrows) is proportional to the amount of material, energy, money, or information moving along that path. Sankey diagrams are especially powerful for illustrating energy balances, but they are equally useful for other flows such as cost breakdowns, material streams, and data routing. In essence, the diagram answers the question: where do the resources come from, where do they go, and how much is used at each stage?

Origins and terminology

The term Sankey diagram is named after Captain Matthew Henry Phineas Sankey, who popularised this style of diagram in the late 19th century. His original illustration described the energy flows within a steam engine, demonstrating how the efficiency of the system could be improved by reducing waste. Since then, the method has evolved with modern software, but the guiding principle remains the same: a visual ledger of inputs, processes, and outputs where the widths of the lines encode quantity.

How a Sankey diagram works

At its heart, a Sankey diagram is a flow map. Nodes represent stages, processes, or repositories, and links (arcs) carry the quantities between them. The defining rule is conservation: the total width of the incoming flows equals the total width of the outgoing flows at each node (subject to rounding). This produces an immediate visual check of balance and loss. The more точно the data reflects the true balances, the more actionable the diagram becomes.

Core elements

• Flows as lines – arrows or ribbons showing the direction of movement.
• Widths proportional to quantities – the thicker a flow, the larger the amount it represents.
• Nodes – points where flows originate, split, or terminate (such as sources, processes, or sinks).
• Directionality – arrows typically flow from left to right, or from top to bottom, indicating progression.
• Balance – an implicit check that incoming flows equal outgoing flows, accounting for losses or storage.

Why the diagram is so readable

The visual language of a Sankey diagram makes it easy to spot where major resources are going, where bottlenecks exist, and where inefficiencies lie. Because width encodes magnitude, even small changes in a flow can be immediately visible when compared across the diagram. This combination of intuitive design and quantitative clarity is what makes the question “What is a Sankey diagram?” particularly pertinent for sustainability reporting, manufacturing optimisation, and data-driven storytelling.

When to use a Sankey diagram

Not every dataset is suitable for a Sankey diagram, but when you need to communicate the distribution of a resource through a system, the format shines. Consider some typical use cases and how they benefit from a Sankey diagram.

• Energy and fuel flows – visualising how electricity, gas, and heat move through a building or industrial process, including losses and conversions.
• Material and supply chains – illustrating the path of raw materials through manufacturing stages, waste streams, and recycling loops.
• Cost and financial flows – mapping how funds originate, move through departments, and are allocated to outputs or liabilities.
• Data and information flows – depicting how data travels across systems, platforms, or teams, including transformations and storage.
• Water and environmental tracking – representing river basins, water withdrawals, and usage across sectors.

Wherever you need to show a complex set of flows with a clear balance, a Sankey diagram can be the most effective storytelling tool. If a diagram simply shows a single chain, a standard flow diagram may suffice; if you need to compare multiple routes, a Sankey diagram offers greater nuance and comparability.

Designing an effective Sankey diagram

Design matters as much as data. A well-crafted Sankey diagram communicates quickly and avoids common pitfalls that can confuse or mislead the viewer. Below are practical guidelines to keep in mind when answering the question “What is a Sankey diagram?” in a way that is both informative and visually compelling.

Data preparation and structure

Start with a structured dataset that reflects a clear balance of inputs, processes, and outputs. Each flow should have:

• A source node and a target node
• A quantified value or magnitude
• Unit consistency across all flows

Verify conservation at every node: the sum of inflows should equal the sum of outflows, minus any defined losses or storage. If you have multiple levels of processes, consider whether you need a multi-step Sankey diagram or a hierarchical (tree-like) layout.

Layout and aesthetics

Key layout decisions include how many levels of detail to display, the direction of flow (left-to-right or top-to-bottom), and spacing between layers. A few practical tips:

• Keep the diagram uncluttered; avoid too many narrow, overlapping flows.
• Use a consistent colour scheme, ideally one colour per node or per category of flow, to help viewers follow paths.
• Label major flows with units and magnitudes where space allows; consider including a legend for larger diagrams.
• Include a short caption or title that explains what the diagram represents and the time period covered.

Clarity over completeness

Limit the display to the most important flows if the dataset is large. A focused diagram often communicates more effectively than a complete but dense map of every minor detail. When you must show many elements, consider a staged approach: a high-level Sankey diagram with links to more detailed views.

Colour and accessibility considerations

Choose colours with perceptual uniformity in mind; avoid red-green combinations for colour-blind readers. Provide alternative text (alt text) for screen readers and ensure labels are readable at the chosen scale. A well balanced colour palette improves comprehension rather than merely aesthetics.

Key features and terms you should know

Understanding what is Sankey diagram opens up a toolkit of features and terms that are commonly used in practice. Here are some core concepts to familiarise yourself with.

• Itemised flows – individual streams that carry a quantity from one node to another.
• Conservation of flow – the principle that total inflows equal total outflows at each node, accounting for losses and storage.
• Balancing – adjusting data to reflect real-world constraints, ensuring the diagram remains an honest representation.
• Branching – the splitting of a flow into multiple onward paths, often shown as forks in the diagram.
• Leakage or losses – flows that disappear from the main pathway, such as heat loss, emissions, or waste.

Practical examples: what is a Sankey diagram in action?

To answer the question in a concrete way, here are a few illustrative scenarios where a Sankey diagram delivers real value.

Energy efficiency in a manufacturing plant

A plant might use a Sankey diagram to map all incoming energy types (electricity, natural gas, steam), the energy converted within processes, and the energy discarded as heat or lost through inefficiencies. The diagram quickly reveals which process steps dominate energy consumption and where improvements could yield the greatest savings.

Household electricity and heat use

At the city or district level, a Sankey diagram can display how electricity is generated, transmitted, and distributed to homes, with branch flows showing utilisation for lighting, appliances, heating, and charging. It highlights losses in transmission and distribution and can accompany targets for reducing peak demand.

Corporate cost allocations

In a business context, a Sankey diagram often traces the flow of funds from revenues through departments, projects, and overheads, culminating in profits or losses. This helps finance teams explain where money goes and supports budgeting and strategic planning.

Tools and software for creating Sankey diagrams

There are many tools available, from simple online generators to sophisticated data visualisation platforms. The choice depends on your data volume, the level of interactivity you require, and how you plan to share the diagram with stakeholders.

• Online and desktop builders – simple interfaces good for quick, single-purpose diagrams. Examples include Sankey generators and template-based editors.
• Spreadsheet add-ins – some Excel or Google Sheets add-ins can generate Sankey diagrams from structured data, suitable for quick iterations.
• specialised visualisation tools – RAWGraphs, Flourish, and similar platforms offer more nuanced controls, multi-level diagrams, and presentation-ready exports.
• enterprise BI tools – Tableau, Power BI, and similar suites can create interactive Sankey diagrams through extensions or custom visualisations.

When evaluating tools, consider how well the software handles data updates, scales to larger datasets, supports multi-level or multi-direction diagrams, and allows easy sharing with colleagues.

A step-by-step guide to building a Sankey diagram

If you are asking “What is a Sankey diagram?” and want to create one yourself, here is a practical workflow you can follow. This guide is designed for readers who prefer a clear, reproducible method.

1. – Decide what you want to illustrate and the key decision questions you hope to answer.
2. – Gather all sources, ensure units are consistent, and verify conservation at each stage.
3. – Pick a platform that suits your data size and the level of interaction you need.
4. – List all stages, sources, processes, and sinks that will appear as nodes.
5. – Record every flow with a source, target, and magnitude. Ensure flows connect logically.
6. – Assign colours, set minimum flow thresholds to reduce clutter, and add labels.
7. – Check that the diagram reflects a balanced system; make adjustments for accuracy.
8. – Include captions, axes labels, and a legend; gather feedback from stakeholders.
9. – Produce publication-ready graphics or interactive versions for dashboards.

By following these steps you’ll move from raw data to a compelling narrative about how resources move through a system. The question “What is Sankey diagram?” becomes a practical solution you can deploy in real-world reporting.

Interpreting a Sankey diagram: tips for readers

Once a Sankey diagram is in front of you, what should you look for? Here are straightforward tips to read a diagram quickly and accurately.

• Identify the main inputs and outputs at a glance by looking for the thickest flows.
• Trace a path from source to final sink to understand how a particular resource travels through the system.
• Check for conservation at each node; mismatches may indicate data gaps or losses that deserve closer inspection.
• Note where flows diverge; branching highlights areas where decisions or processes create multiple outcomes.
• Consider time: if the diagram represents a period, compare it with another diagram from a different period to observe trends.

Advanced variations and evolving formats

The classic Sankey diagram is just the beginning. Designers and analysts frequently adapt the basic idea to suit more complex problems.

Multi-layer and nested Sankey diagrams

In some scenarios, you may want to display several levels of detail. Multi-layer diagrams show a high-level overview with the option to drill into more detailed sub-flows. Nested variants organise flows within sub-systems, making it easier to isolate drivers of change in a large organisation.

Flow diagrams and dual-axis approaches

Some applications combine Sankey-like visuals with other chart types, such as bar charts or time-series lines, to provide contextual data alongside the flow information. These hybrid formats support deeper analysis without sacrificing readability.

Animated and interactive Sankey diagrams

For digital reporting and dashboards, animated diagrams can illustrate how flows change over time, while interactive versions let users filter, reorganise layers, or highlight specific pathways. This interactivity enhances engagement and comprehension, especially for complex datasets.

What is Sankey diagram in the context of sustainability and reporting?

In environmental accounting and sustainability reporting, the Sankey diagram has become an essential communication tool. It helps organisations demonstrate progress toward energy efficiency, emissions reductions, and circular economy goals. By making the flow of resources explicit, stakeholders can track improvements, identify high-leverage interventions, and communicate performance to regulators, investors, and the public.

Common pitfalls and how to avoid them

As with any data visualisation, there are traps that can mislead or confuse. Here are common pitfalls and straightforward strategies to mitigate them.

• Overclutter – too many tiny flows can blur the key messages. Solution: filter out minor ones or use layering to reveal details progressively.
• Inconsistent units – mixing kilograms with tonnes or kilowatt-hours with joules undermines readability. Solution: standardise units before plotting.
• Ambiguous labels – vague or crowded labels hinder interpretation. Solution: label major flows clearly and provide a succinct legend.
• Unclear directionality – arrows with confusing orientation reduce comprehension. Solution: keep direction consistent and use layout conventions (left-to-right or top-to-bottom).

Frequently asked questions about What is a Sankey diagram?

What is a Sankey diagram used for?

A Sankey diagram is used to visualise flows of energy, materials, costs, or information, with flow widths proportional to quantities. It helps diagnose inefficiencies, optimise processes, and communicate complex systems succinctly.

Can I make a Sankey diagram in Excel?

Yes, you can create a basic Sankey diagram in Excel using data visualisation techniques or using add-ins. More advanced features, such as multi-level layouts or interactive capabilities, may require dedicated software or online tools.

How accurate does the data need to be?

The value of a Sankey diagram rests on the quality of the underlying data. Accurate data supports meaningful balance checks and actionable insights. It is common to document assumptions, normalise time periods, and disclose any data limitations.

What is the best practice for presenting Sankey diagrams?

Good practice includes keeping the diagram legible, limiting the number of flows, presenting a clear legend, and providing context through a short textual description. Where possible, accompany the diagram with a narrative that explains the key takeaways and recommended actions.

A concise conclusion: what is Sankey diagram and why it matters

What is Sankey diagram? It is a powerful visual tool that communicates how resources move, where losses occur, and where opportunities for improvement exist. Whether you are analysing energy use in a factory, tracing financial allocations, or visualising data traffic through a network, a well-crafted Sankey diagram delivers clarity, balance, and insight. By focusing on flow, proportion, and direction, it turns complex datasets into intuitive visuals that persuade, inform, and drive better decisions.

Further reading and next steps

If you want to explore more about what is a Sankey diagram and how to apply it in your organisation, consider these practical next steps:

• Start with a small, well-defined problem and build a Sankey diagram to illustrate the core flows.
• Experiment with two or three alternative visualisations to see which communicates most clearly to your audience.
• Share drafts with stakeholders and incorporate feedback on both accuracy and readability.
• Document data sources, units, and timeframes to ensure the diagram remains credible and reusable.