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Sankey diagrams that look like real engineering drawings

FastSankey is a free online Sankey diagram maker for engineers. Draw energy and material flows on a proper drawing sheet – with title block, legend, drawing frame and print-ready PDF export in A4 to A1.

Start drawing – it's free

No sign-up · Runs in your browser · Your data stays on your device

Screenshot of the FastSankey editor showing an energy flow Sankey diagram on an A3 sheet with title block

Built for technical documentation

Most Sankey tools produce charts. FastSankey produces drawings – the kind you can put into a report, hang next to a P&ID or hand to a client.

📐 Real drawing sheets

Work on A4–A1 landscape sheets with a light grid, snap-to-grid nodes, a drawing frame and a classic title block with your own logo, project name, drawing number, author and date.

🖨 Print-ready PDF

One click exports a PDF at the exact sheet size with a 4 mm drawing frame – plus SVG for vector workflows and high-resolution PNG.

🧭 Orthogonal routing

Flows run with clean 90° arcs like piping on a plan. Vertical runs can be dragged sideways, value labels are movable with leader lines.

🔀 Flow merging

Arrows of the same flow merge into one thick band – both when leaving a source and when entering a target – with widths that add up correctly.

♻ Circular flows

Return flows (A → B → A) are detected automatically and routed around the diagram as loop arrows.

⚖ Node balances

Every node shows the sum of incoming and outgoing flows – imbalances are flagged instantly with a red Δ.

How to create a Sankey diagram with FastSankey

The sidebar follows the workflow from top to bottom – four steps from empty sheet to finished drawing.

  1. Define your flows

    Open Define flows… and create one entry per flow type – for example Natural gas, Heat, Electricity or Losses – each with its unit (MWh, t/a, €…) and colour. The legend panel on the sheet updates automatically.

  2. Place your nodes

    Activate the Node tool and click on the sheet, or drag “New node” from the sidebar onto the drawing. Nodes snap to the grid. Click a node with the Select tool to rename or recolour it.

  3. Insert arrows

    Pick the flow for new arrows, activate the Arrow tool, then click the source node and the target node. Enter the value – the arrow width scales with it. Two sliders control the overall thickness and a minimum width so thin flows stay visible.

  4. Arrange, document, export

    With Select you can move nodes, drag the vertical run of an arrow sideways, move value labels (with leader line) and add text comments. Fill in the Title block… – project, drawing number, author, your company and logo – and export as PDF, SVG or PNG. Save stores the whole project as a JSON file you can reopen anytime.

Tip: the Load example button at the bottom of the sidebar loads a complete CHP energy-flow diagram you can pick apart to learn from.

Frequently asked questions

Is FastSankey free?
Yes – free to use, no sign-up, no account.
Where is my data stored?
Nowhere but on your device. Editing runs entirely in your browser and projects are saved as local JSON files. Only the PDF export sends the rendered image to our server for conversion; it is not stored.
Which export formats are supported?
SVG (vector), PNG up to 4× resolution, and print-ready PDF at exact sheet sizes (A4–A1 landscape) with a drawing frame.
Can I use my own logo?
Yes – upload it in the title block dialog. It is embedded in the drawing and saved with your project file.
What is a Sankey diagram?
A Sankey diagram visualises flows – energy, material, money – between nodes, where the width of each arrow is proportional to the flow quantity. It is the standard way to show energy balances of plants and buildings.

Ready to draw your energy balance?

Open the editor and load the example – you'll have your first diagram in minutes.

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