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
  • ✓ Import existing Vensim & Stella models (XMILE/STMX)
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Features

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.

〰

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.

▶

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.

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How it works

From blank canvas to shareable simulation

No tutorial needed. Most users have a running model within minutes.

⌗
Step 1

Build your model

Add stocks and flows on a canvas that lays itself out. Define formulas using plain arithmetic, lookup tables, or trig functions for periodic behavior.

  • › Right-click any stock to extend the model
  • › Auto-layout — no dragging nodes
  • › Or ask the AI to suggest a structure
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Step 2

Watch it simulate

Hit run and watch water flow through your system in real time. Adjust sliders and see the simulation respond instantly — no page reload, no server round-trip.

  • › RK4 solver runs entirely in your browser
  • › Update the sliders and refresh the simulation
  • › Feedback loops and delays become visible
▶
Step 3

Export and share

Export the full animated simulation as an MP4 — ready to drop into a slide deck, lecture, or client presentation. Or share the model link directly.

  • › MP4 export with or without watermark
  • › Share a link to your live model
  • › Free tier: up to 1,000 time steps

Already using Vensim or Stella?

Import your existing XMILE or STMX file and skip step 1 entirely. Model structure transfers instantly — formulas using standard functions carry over automatically.

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No credit card required · 3 free models

How we compare

Nirai vs the alternatives

Every tool below does system dynamics. Here's where they differ.

Price

Nirai

Free / $99 once

Vensim

Free PLE / $395+/yr

Stella

$509/mo

InsightMaker

Free

Free tier

Nirai

Vensim

PLE (limited)

Stella

InsightMaker

No install required

Nirai

Vensim

Stella

InsightMaker

Offline use

Nirai

Vensim

Stella

InsightMaker

Fluid animation

Simulates as flowing water — not just charts

Nirai

Vensim

Stella

InsightMaker

MP4 export

Nirai

Vensim

Stella

InsightMaker

Auto-layout canvas

No manual node dragging required

Nirai

Vensim

Stella

InsightMaker

AI assistant

Nirai suggests formula and stock changes via instruct-mode; Stella suggests formulas only

Nirai

Vensim

Stella

InsightMaker

RK4 solver

Nirai

Vensim

Stella

InsightMaker

Stochastic / random functions

Nirai

Vensim

Stella

InsightMaker

Lookup tables

Nirai

Vensim

Stella

InsightMaker

XMILE / STMX import

Nirai imports structure; formulas with unsupported functions or subscripts may not transfer

Nirai

Vensim

Stella

InsightMaker

Unit checking

Nirai warns on mismatched units; Vensim and Stella enforce strictly

Nirai

Vensim

Stella

InsightMaker

Sensitivity analysis

Nirai

Vensim

Stella

InsightMaker

Monte Carlo simulation

Thousands of stochastic runs to explore parameter uncertainty

Nirai

Vensim

Stella

InsightMaker

Calibration against real data

Fit model parameters to observed time-series data

Nirai

Vensim

Stella

InsightMaker

Agent-based modeling

Nirai

Vensim

Stella

InsightMaker

Supported Partial Not supported

Pricing as of 2026. Stella Architect from $509/mo · Vensim PLE free (limited), Professional from $395/yr · InsightMaker free.

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Fit

Who is Nirai for?

Nirai is not trying to replace Vensim. It's built for a different job.

Nirai is a good fit if you…

  • Present or teach system behavior to othersFluid animation and MP4 export make feedback loops visible to people who have never seen a stock-and-flow diagram.
  • Want a browser-based tool with no installWorks on any device, in any classroom or client meeting, with nothing to install or license.
  • Are migrating from Vensim or StellaImport your existing XMILE or STMX files and continue working immediately — structure transfers instantly.
  • Want to build and share models fastAuto-layout canvas means no dragging nodes. Describe structure, hit run, share the link or export the video.
  • Are a student or independent analyst on a budgetFree tier covers most learning use cases. Pro is $99 once — not a recurring subscription.

Nirai is not the right tool if you…

  • Need Monte Carlo or sensitivity analysisNirai does not support multi-run experiments or parameter sweeps. Use Vensim or Stella for this.
  • Need to calibrate a model against real dataVensim has dedicated calibration tooling. Nirai does not.
  • Need agent-based modeling alongside system dynamicsInsightMaker or AnyLogic support hybrid agent + stock-and-flow models. Nirai is stock-and-flow only.
  • Need strict unit enforcement across a large team modelNirai shows unit mismatch warnings but does not block simulation. Vensim and Stella enforce this strictly.
  • Need to publish interactive web simulationsStella Architect has a dedicated interface builder for interactive learning environments. Nirai exports video, not interactive embeds.
Start free — no credit card

Not sure? The free tier gives you 3 models to find out.

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.
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.
Yes. Nirai supports importing XMILE and STMX files — the standard formats used by Vensim and Stella Architect. You can bring in your existing models and continue working in Nirai immediately, without rebuilding from scratch.
Yes. The free tier gives you 3 models and up to 1,000 simulation time steps with no credit card required. Pro is a one-time $99 lifetime license — not a subscription — and unlocks unlimited models, unlimited time steps, and watermark-free MP4 export.

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