Courant

Courant is a professional-quality System Dynamics environment available completely free of charge. Build models that reveal how complex systems behave over time — and why they so often surprise us — with a tool that rivals commercial offerings at no cost.

If you've ever struggled with unintended consequences, policy resistance, or the gap between short-term fixes and long-term outcomes, System Dynamics gives you a way to see the whole system at once. Courant gives you the tools to do it.

Methodology and Interaction

Sketch your system's causal structure: Start where every SD practitioner starts: with a causal loop diagram. Courant automatically detects your feedback loops and classifies them as reinforcing or balancing — so you can see the engines of growth and the leverage points where intervention matters most.

Build and simulate stock-and-flow models: Write equations with autocomplete that suggests variable names and built-in functions as you type. Press Ctrl+R to simulate and see results immediately in an interactive chart. Break large models into reusable modules with input/output ports. Add subscripts to expand elements across dimensions (regions, age groups, product lines) without duplicating your diagram.

Import and export freely: Courant reads Vensim .mdl and XMILE files directly, preserving equations and diagram layout. Export back to these formats or save in Courant's own JSON format. Your work is never locked in.

Quantifying Uncertainty

System Dynamics is most valuable when you use it to ask "what if?" Parameter sweeps reveal sensitivity and thresholds at a glance. Monte Carlo analysis assigns probability distributions to your uncertain assumptions and runs thousands of trials. The result is an envelope of possible futures — not a single prediction, but a realistic picture of the range of outcomes your system can produce.

Optimization finds the best answer automatically. Define what "better" means — minimize cost, hit a target, fit observed data — and let the optimizer search for the parameter values that achieve it. This is how you calibrate models against reality.

Comparison of Tools

A Programmable Engine

The same engine that powers the visual editor is available as a Java library. Define models in code, run headless simulations, sweep parameters programmatically, or embed System Dynamics in a larger application. Models built in code and models built in the editor use the same format — save from one, open in the other.

Open Source

Courant's source code and its library of example simulation models are freely available as open source. You can study how it works, contribute improvements, or adapt it for your own purposes. See the licensing documentation for details. The repository is hosted on GitHub at github.com/Courant-Systems/Courant.