Sankey diagram maker

What is a Sankey diagram?

A Sankey diagram shows where something flows. Every band is as wide as the amount it carries, so you see at a glance where most of it goes. Energy is the classic use: what goes into a power plant, a factory or a home, what comes out as something useful, and what gets lost on the way.

Captain Matthew Sankey drew one in 1898 to show where the energy in a steam engine went.

How do you read one?

In my course I ask three questions about every Sankey:

1. Which band is widest? That's where most of the energy goes, and usually where the biggest savings are.
2. What's useful, and what's lost? In the course colours, useful flows are coral and losses are grey. A wide grey band is worth a second look.
3. Who counted it, and how? A Sankey is only as good as its numbers. Check where they come from and what was left out.

How does this maker work?

Pick an example or start with empty paper. Drag a flow out of the right edge of a box and drop it on another box, or on empty paper to make a new one. Then type how much flows. That can be a number, or a percentage of what goes into the box, like 29% for an efficiency. Change the input later and the percentage moves with it.

While you draw, a bookkeeper checks every box. A green tick means what goes in also comes out. A red label like "12.5 left" means something is missing. Click the box and you choose: draw the difference as a grey loss, or fix the numbers yourself. When you draw a new flow, the tool suggests what's left. It never adds a loss unless you ask it to.

Can it draw loops?

Yes. Heat recovery, or condensate that goes back to the boiler, is energy that comes round again. Drag a flow back to an earlier box and the tool draws it as a loop, over the top or underneath, whichever way is shorter. The bookkeeper counts what comes back as input to that box.

Questions you might have

Can I use the images in my report?

Yes. Export a PNG or JPG, or an SVG that stays sharp at any size. A small "engineeronanapkin.com" stays in the corner.

Is my data sent anywhere?

No. Your diagrams are kept in this browser. A share link carries the whole diagram inside the link itself, so nothing is stored on a server. The page loads fonts and drawing libraries from the web, but your numbers stay here.

Which unit should I use?

Whatever your numbers are in. The unit is only a label, and the tool doesn't convert anything. Use an energy unit such as kWh or PJ and you can switch to napkin units: litres of water boiled, or Dutch homes for a year (2,500 kWh each).

Can I get the numbers out again?

Yes. Export to Excel and you get every flow with its source, destination, amount and kind.

Does it work on my phone?

You can open and view a shared diagram on a phone. Drawing works best with a mouse or a trackpad.

Spot a mistake, or missing something? Email [email protected] and I'll have a look.