Supply Flow Diagram

Draw the physical and information flows on one page: who orders what, from whom, with which lead time.

Time1 h
FormatTeam
StageMap

Supply Flow Diagram: what it is and why it works

A supply flow diagram shows on one page how goods and information move between customers, your sites, warehouses and suppliers. Physical flows are drawn as solid arrows, information flows such as orders, forecasts and call-offs as dashed arrows, and every arrow carries its lead time and lot size. The diagram is built outward from the customer order, so each movement is linked to the signal that caused it. Handoffs, the points where responsibility changes between people, departments or companies, are circled because that is where delays and errors collect.

The diagram works because most supply chain decisions rest on an assumed flow that nobody has drawn. Planners assume a transfer takes two days, buyers assume the supplier sees demand weekly, and the reality often differs. Putting both flows on the same page exposes duplicated orders, batching of information, and signals that pass through too many hands. It is simpler and faster than a full value stream map and covers the multi-site network rather than one product family's shop-floor routing. Use it before a VSM to agree on scope, alongside SCOR to check which process owns each arrow, and before a bottleneck walk to decide where to look.

What you need

A defined scope: one product line or customer segment and the sites it passes through
Knowledge of how orders are placed and transmitted at each link (EDI, email, portal, phone)
Typical lead times and lot sizes for each shipment and each order transmission
People who actually handle the orders, not only their managers

What you get

A one-page diagram of physical and information flows with lead times and lot sizes
A marked list of handoffs with the delay each one adds
A shared understanding of the true order-to-delivery path
One handoff selected for a real-time information redesign

When to use it

When decisions rely on assumptions about flows nobody has drawn.

How to do it, step by step

Start from the customer order and draw every physical movement of goods.
Overlay the information flows: who places orders, who receives them, with what delay.
Annotate each arrow with lead time and lot size.
Circle every handoff — most delays live between boxes, not inside them.
Pick one handoff and design its information flow in real time.

Worked example: Reagent supply to a group of water treatment plants

Illustrative scenario — figures are realistic but not from a real company.

A utility operated four water treatment plants that consumed coagulant and sodium hypochlorite from two chemical suppliers through a central warehouse. Plants ran short about once a month although the warehouse usually held stock.

The team drew tanker deliveries from suppliers to the warehouse and from the warehouse to each plant, noting a two-day transfer lead time and minimum deliveries of 4,000 gallons.
Overlaying information flows showed that plant operators read tank levels daily but emailed the warehouse only on Mondays, and the warehouse consolidated requests into a Thursday purchase order.
Annotating each arrow revealed that a low-tank signal could wait up to nine days before a supplier saw it, although physical movement took only four.
They circled three handoffs: operator to warehouse clerk, clerk to buyer, and buyer to supplier order desk.

Result. The team redesigned the first handoff by sharing tank-level readings from the plant control system with the warehouse every morning, with a reorder point per tank. Shortage alarms dropped to zero over the following quarter and emergency deliveries, which carried a surcharge, stopped. The diagram showed that the problem was never the trucks but the calendar of emails.

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Common pitfalls and how to avoid them

Drawing only material movements and leaving out the ordering signals.Overlay every information flow with its frequency and delay; the slowest signal often sets the real lead time.
Using planned lead times from the ERP instead of observed ones.Check a sample of recent orders and shipments to record typical and worst-case times on each arrow.
Adding so much detail that the diagram no longer fits on a page.Keep one page per product line; move exceptions and variants to notes rather than extra boxes.
Redesigning every handoff at once.Pick the handoff that adds the most delay and fix its information flow first, then revisit the diagram.

Frequently asked questions

What is the difference between a supply flow diagram and a value stream map?

A supply flow diagram covers the network: suppliers, plants, warehouses and customers, with shipment and order lead times. A value stream map usually zooms into one product family inside a plant or site and adds process data, inventory between steps and a value-added timeline. Teams often draw the flow diagram first to see the whole chain, then map the value stream where the diagram shows the biggest delay.

What symbols should you use in a supply chain flow diagram?

Keep the set small: boxes or icons for sites, solid arrows for physical movements, dashed arrows for information, and circles for handoffs. Write lead time and lot size on each arrow. Standard value stream icons can be reused if the team already knows them, but a clear legend matters more than following any particular symbol library.

Why do most delays happen at handoffs?

At a handoff, work changes owner, and each owner has its own queue, schedule and priorities. Information is often batched, for example read daily but sent weekly, and requests may be re-entered by hand. None of this shows up in the processing time of either party, so it is easy to overlook. Mapping handoffs explicitly makes this hidden waiting time visible and measurable.

Origin

Flowcharting tradition — industrial engineering, mid-20th century.