System Dynamics Simulator

Don't just model it.
Watch it flow.

Build stock-and-flow simulations with real water flowing through tanks and pipes, right in your browser. Ask AI about your system's behavior, then export the whole thing as a shareable MP4 — no desktop software, no license fees.

  • ✓ Real-time fluid animation, not static charts
  • ✓ Export your simulation as MP4
  • ✓ Ask AI about your model, using your own API key
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No credit card required for the free tier.

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Why Nirai

Built different, not just styled different

Every part of Nirai is built to remove the friction that makes system dynamics tools feel like spreadsheets with extra steps.

✦

AI that edits your model, not just chats

Ask what's driving a bottleneck or how to model seasonal demand. The AI proposes actual stocks and flows with the right properties, rendered straight onto your canvas — not just a text explanation. Bring your own key: Gemini, OpenAI, or OpenRouter, stored locally, never on our servers.

ƒ(x)

A formula engine built for real systems

Native support for sin/cos to model periodic demand, lookup tables for nonlinear relationships, and random functions for stochastic behavior — all in the same expression syntax you use for basic arithmetic.

⚡

A simulation engine, not a spreadsheet

The core solver is Rust compiled to WebAssembly, running entirely in your browser. No round-trips to a server for every parameter tweak — sliders update the simulation.

⌗

No dragging nodes around

Right-click an existing stock or hit "Add system" and Nirai lays out the new nodes automatically using graph-based layouting. Build models by describing structure, not by fighting a canvas.

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See the bullwhip effect, don't just read it

Amplifying oscillations, delays, and feedback loops show up as visible surges and lags in the water itself — the kind of systemic behavior that's easy to miss in a line chart and hard to unsee once you've watched it happen.

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Export the simulation, not just a screenshot

Turn any model into a shareable MP4 — the full animated flow, ready to drop into a lecture slide, a client deck, or a video explainer. Free tier runs up to 1,000 time steps; Pro is unlimited.

FAQ

Learn more about Nirai

System dynamics is a modeling approach for understanding how stocks (things that accumulate, like inventory or population) and flows (rates of change) interact over time through feedback loops. Nirai is built for analysts, educators, and engineers who want to build these models visually and see the resulting behavior animated in real time, rather than reading it off a static chart.
Nirai runs entirely in your browser with no install, animates simulation results as actual flowing water through tanks and pipes rather than line charts alone, and lets you export that animation as an MP4. It also includes an AI assistant that can propose model changes directly onto your canvas, using your own API key.
Not currently. The simulation engine and canvas rendering are built for desktop-class compute and screen space, so Nirai requires a laptop or desktop browser. If you visit on a phone or tablet, you'll see a prompt to switch devices.
Your API key is stored locally in your browser and is never sent to or stored on Nirai's servers. Nirai supports Gemini, OpenAI, and OpenRouter — you bring your own key and are billed directly by your chosen provider for AI usage.
You can ask it questions about your system's behavior, and it can propose specific changes — adding, modifying, or removing stocks and flows with the relevant properties, rather than just returning a text explanation.
Nirai uses fasteval2 for parsing converter formulas, with native support for common functions like sin and cos for modeling periodic demand, plus built-in lookup tables for nonlinear relationships and random functions for stochastic behavior.
The free tier supports simulations up to 1,000 time steps. Upgrading to Pro removes that limit entirely, along with unlocking watermark-free MP4 export and unlimited models.
Yes — any model can be exported as an MP4 of the animated simulation. Free tier exports include a watermark; Pro exports do not.
No. Nirai uses automatic graph layouting, so new stocks and flows position themselves sensibly when you right-click an existing node or use the "Add system" button — you don't need to drag anything into place.