The idea
A stock is an accumulation measured at a moment; a flow changes that accumulation over time. Keeping the distinction explicit prevents confusion between having a large backlog and receiving work rapidly. The units themselves help reveal whether an explanation is coherent.
Worked example
A repair library begins Monday with 40 unprocessed items. During the week it receives 25 and completes 30. Ignoring losses and rework, it ends with 35. The stock is items awaiting processing; the flows are items received and completed per week. A manager who celebrates a lower arrival rate without checking completion could miss a growing backlog. The simple accounting identity forces the team to connect its operational story to quantities.
Use it carefully
A stock can have several inflows and outflows, and records may omit transfers or errors. Choose consistent time intervals and units. A completion count that includes reworked items may overstate the reduction in unfinished work. Before building a complicated model, reconcile the opening balance, changes, and closing balance. Accounting consistency is necessary for explanation, but it does not identify the causes of each flow.
FROM UNDERSTANDING TO ACTION
Put it to work.
Draw a stock-and-flow diagram for unread email. Include arrivals, completed messages, and deletions without action.
RETRIEVE. REASON. REMEMBER.
Knowledge check.
Choose one answer for each question, then check your reasoning.
Sources & further reading.
Links support the core concepts or identify the book. Examples, exercises, and critical analysis are original Institute material. Source checks are not an independent expert review of every claim.
- MIT System Dynamics: mapping stocks and flows ↗
MIT System Dynamics: mapping stocks and flows; supports the core concept. Scenarios, calculations, and practice questions are original. Source reviewed 2026-10-19.