How to Build a Data Flow Diagram (DFD) | The Logic of Levelling and the Four Symbols

A DFD turns up in a system document and you cannot tell how it differs from a flowchart. Or you sit down to draw one and cannot decide how far to break things down. A DFD has only four symbols, but it comes with a distinctive idea called levelling. This article covers what the symbols mean and the rules that govern levelling.

What a DFD Is: A Diagram of How Data Moves

A DFD (data flow diagram) represents how data travels through a system. It shows where data comes in, what transforms it, where it is stored, and where it goes out.

The decisive difference from a flowchart is that a DFD has no concept of time. Where a flowchart expresses the sequence of "what happens next," a DFD expresses only the relationship of "where the data goes." Branches and loops are not drawn.

What it is used for

The main use is sharing a picture of the whole system among the people involved. It is particularly effective when replacing an existing system, or when connecting several systems together.

Questions like "where does this information come from?" and "why are we holding the same data in two places?" become visible once you draw a DFD. It is also an opportunity to notice duplicated data, or outputs nobody uses.

The Four Symbols

A DFD has only four building blocks. The exact shapes vary between conventions, but the meanings are consistent.

Process (circle or rounded rectangle)

Something that transforms data. Write it ending in a verb: "register the order," "calculate the invoice amount."

One rule matters here: a process always has both an input and an output. A process with input but no output, or output but no input, is a drawing error.

Data store (two parallel lines)

Where data is held. A database table, a file, a paper ledger. Drawn as a shape enclosed by lines above and below, or as a rectangle open on the right.

A data store does nothing by itself. Processes write to it and processes read from it. You cannot draw an arrow directly from one data store to another.

External entity (rectangle)

Someone or something outside the system that exchanges data with it. Customers, suppliers, other departments, external services.

Settling these is in fact the most important judgment in drawing a DFD, because how much you treat as your own system determines the scope of the diagram.

Data flow (arrow)

The direction data moves. Always label the arrow with what is flowing: "order data," "customer information," "stock count" — a noun.

An unlabelled arrow carries no meaning in a DFD. If you cannot write what is flowing, the arrow probably should not be there.