Introduction to Stock/Flow Diagrams (S/Fs) - Learning System Dynamics Modeling

Oct 26, 2026

Introduction to stock/flow diagrams (S/Fs) explains systems thinking through stocks (accumulations) and flows (rates of change). It uses everyday substitutions—such as water in a bathtub, CO2 in the atmosphere, and credit card debt—to show inflows and outflows, boundary limits, and feedback loops. The document then describes system dynamics computer modeling, including “what if” testing and a step-by-step workflow from identifying problems and variables to building diagrams, equations, running simulations, documenting results, and telling the model’s story.

LanguageEnglish

CategoryResearch & Report

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Pages2

Reading Time5 min

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Introduction to Stock/Flow Diagrams (S/Fs)

Stocks, accumulations, and flows

Modeling boundaries and feedback

Examples: bathtub, CO2, and credit card debt

System Dynamics Computer Models

Steps in the modeling process

Introduction to Stock/Flow Diagrams (S/Fs) A principle of systems thinking involves stocks, accumulations of things, and flows, measurement of the rates at which the stocks change. The clarity of stock/flow diagrams aids in appreciation for rates of change, feedback, and other elements of systems. “Water in a bathtub” is an example of a stock. The flow from the faucet is an inflow that fills the tub, while the drain is an outflow to empty it. Water in a bathtub flowing in flowing out The clouds on the ends of the flows mark the model boundaries. It is important to know where water comes from to fill the tub and where it travels when it drains, but this diagram does not model the larger system. This is a simple bathtub diagram. Substitution of other names for the elements in our diagram can give insight into other systems, e.g.,“CO2 in the atmosphere”: CO2 in the atmosphere emissions removal Most people agree this causes a big problem, climate change. If “CO2 emissions” level off close to 2020 levels, the inflow will still exceed the outflow. And when inflow > outflow, the stock increases. That’s true for anything we can represent as a stock and flow. Another substitution shows insight about the use of credit cards: Credit card debt Monthly charges Paying off If spending is more than what is being paid off, the stock of debt rises. Debt costs money in the form of interest, and adding another element helps present a more complete picture. “Monthly charges” are based on a rate represented by a circle called a converter. The “Interest rate” is multiplied by the existing debt, adding to the stock of“Credit card debt.” This feedback connection is shown by the arrows called connectors. Students can easily learn how to input numbers and equations in order to model this situation. Credit card debt Interest Paying off rate [See more about](See more about stock/flow diagrams at www.clexchange.org/gettingstarted/stockflow.asp)[ ](See more about stock/flow diagrams at www.clexchange.org/gettingstarted/stockflow.asp)[stock/flow diagrams at www.clexchange.org/gettingstarted/stockflow.asp](See more about stock/flow diagrams at www.clexchange.org/gettingstarted/stockflow.asp) [Permission granted for photocopying and/or electronic distribution for non-commercial educational purposes with attribution.](Permission granted for photocopying and/or electronic distribution for non-commercial educational purposes with attribution.) ©2022 Alan Ticotsky and Creative Learning Exchange, [clexchange.org](clexchange.org) 223 NOW WHAT? A CALL TO ACTION Introduction to System Dynamics Computer Models By creating system dynamics computer models, students of all ages can compress both space and time, thereby discovering long-term results of actions. Because modelers interact through computers and generate results in graphical, tabular, and animated formats, they maintain a high NOTE: This process can feel overwhelming to a novice, so one option is to find simple models that others have created that relate to the issue of concern. Then using the other steps in the modeling process, test and/or modify those models as a way to explore the issue. For example, if concerned about famine in a particular country, look for a basic population/resource model. Steps in the modeling process: level of attention, engagement, and, therefore, learning. Even the simplest modeling of a system can reveal insights typically missed without it. It’s possible to model any dynamic, interdependent system. The system dynamics modeling approach involves the development of computer simulation models that portray processes of accumulation and feedback and allow for the testing of “What if” questions. Systematically testing models can help with identification of effective strategies for overcoming a system’s natural tendency to return to a status quo or equilibrium. (Think about how difficult it is to change an ingrained habi

FAQs

What is a stock/flow diagram used to explain?

Stock/flow diagrams explain how accumulations (stocks) change over time based on rates (flows), including feedback and other system elements.

How do inflows and outflows affect a stock?

When inflow exceeds outflow, the stock increases; when outflow exceeds inflow, the stock decreases.

What are the main steps for building and testing a system dynamics model?

The process includes identifying a specific problem, brainstorming key variables, sketching behavior over time graphs, creating an endogenous stock/flow diagram, connecting loops and feedback, writing equations, running “what if” tests with relevant data, documenting the model, and telling the story to communicate learning.

Oct 26, 2026

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