If you've used Algodoo, you already know why it has such a loyal following: it's a genuinely good 2D physics sandbox with a long history, a huge community, and some features (optics especially) that go deeper than most alternatives. If you're reading this, there's a good chance you searched for something like "algodoo alternative," "algodoo online," or "algodoo no download," which usually means one specific thing: you want to skip installing anything and just start building.

That's the honest, direct comparison this post is trying to be. Physion and Algodoo solve a lot of the same problem, drawing shapes, connecting them with joints and springs, watching physics happen, but they make different tradeoffs. We'll cover both fairly: what each one does well, where Algodoo is still ahead, and where Physion's browser-first design changes the equation.

The Short Version

Neither column is "wrong." If you want dedicated optics tooling or you're building on an iPad with your finger, Algodoo's native app is a real advantage. If you want to open a link, start dragging shapes, and send that exact scene to a student thirty seconds later with zero download on either end, that's what Physion is built for.

What Algodoo Is Today

Algodoo has a longer history than most of its category. It began life as Phun, a 2D physics sandbox Emil Ernerfeldt built for his master's thesis at Umeå University in 2008, and which went viral after a demo video spread online. Algoryx Simulation AB picked it up commercially in 2009 and renamed it Algodoo, and in 2013 the desktop version became free to download for Windows, Mac, and iPad. Along the way it grew a full optics engine (lenses, prisms, refraction, laser tools), incompressible fluid simulation, and Algobox, a long-running community library where users share their scenes.

For roughly a decade after 2013, Algodoo saw no major updates, which led a lot of people to assume the project had been abandoned. That's no longer accurate: Algoryx resumed active development, shipping a 2.2.0 beta in 2024 followed by stable releases 2.2.3 (December 2024) and 2.2.4 (July 2025), the current version at the time of writing. So it's fair to describe Algodoo as maintained again after a long quiet stretch, not abandoned, just slower-moving than most actively developed software.

What hasn't changed: Algodoo is still a download. The desktop app installs on Windows or Mac; the iPad version is a $7.99 App Store purchase. There is no official browser or web build. A few third-party sites advertise "Algodoo online," but at the time of writing the most visible one lists Algodoo as an unreleased item on a user-voted wishlist, not something you can actually open and use, so treat "run Algodoo in your browser" claims with some skepticism until you've confirmed one actually works.

What Physion Is

Physion is a browser-based 2D physics simulator: you draw shapes, connect them with joints and springs, and press play, entirely inside a tab, with nothing to install. It's free to open and start building, no download, no sign-up required. There's a metered free tier (1 saved scene per month, ad-supported), plus two paid plans, Basic ($6/mo or $60/yr) and Pro ($10/mo or $100/yr), that raise the monthly scene limit and remove ads; current plan details are always available in the in-app Pricing dialog. Every tier gets the same full toolset, which includes:

• Shapes: circles, rectangles, capsules, polygons, regular polygons, text, polylines, spirals, and gears
• Joints and mechanisms: revolute, weld, distance, and wheel joints (built for vehicles and suspensions), plus a dedicated Spring Tool and Rope Tool
• Liquids and soft bodies: the Particle Group Tool turns any shape into a group of LiquidFun particles, configurable as fluid, elastic, or viscous
• Particle emitters: continuous particle streams via the Particle Emitter Tool
• JavaScript scripting: Node Scripts let you attach real JavaScript classes to any object for custom behavior, collision handling, and game logic
• Boolean operations, mirroring, and snapping for precise construction
• Instant scene sharing: make a scene public and share it with a single link, no account needed for the person opening it

No sign-up is required to open Physion and start experimenting; you only need an account (Google or Facebook sign-in) if you want to save your work or make a scene public. On mobile, Physion currently works in view-only mode: you can open and watch a shared scene on your phone, but building one needs a desktop-sized browser window.

Where Algodoo Is Still Ahead

To be direct about it, because a comparison that only flatters one side isn't useful to anyone deciding between them:

• Optics. Algodoo's lens, prism, and refraction system, plus laser tools that route light through fiber-like paths, is a dedicated feature area with no real equivalent in Physion. Physion has a laser raycasting tool that bounces off surfaces and can trigger script callbacks on hit, which covers reflection-based scenes well, but there's no lens or refraction system behind it. If your use case is specifically geometric optics, Algodoo is the more complete tool today.
• Native touch editing. The Algodoo iPad app is a real, paid, first-class touch experience: you can build a scene with your finger. Physion's design tools are built for a mouse and keyboard; on a touchscreen, Physion is currently for viewing rather than building.
• A longer track record. Algodoo (and Phun before it) has been around since 2008, with a correspondingly large back catalog of tutorials, classroom materials, and community scenes built up over more than fifteen years. That kind of accumulated third-party content takes time to match.

Where Physion Pulls Ahead

• Genuinely zero install, on every tier. Not a "portable" build, not a browser wrapper around a desktop app, not a separate paid mobile purchase: free tier or paid Basic/Pro plan, Physion is the same web app on a school Chromebook, a locked-down office laptop, or a personal machine with no admin rights to install software. Algodoo's only paid option, the iPad app, is a one-time $7.99 purchase that still requires installing an app from the App Store.
• Scripting in a language people already know. Node Scripts are plain JavaScript, the same language taught in most intro programming courses and used across the web. Algodoo's Thyme is a capable scripting language, but it's proprietary to Algodoo, so the skill doesn't transfer anywhere else.
• Share by link, not by file. Sharing an Algodoo scene generally means exchanging a save file. Sharing a Physion scene means sending a URL that opens directly in the recipient's browser.

See It for Yourself

The fastest way to judge any of this is to actually open a scene. A few real, public Physion scenes that show off the range covered above:

• Centrifugal Force: a simple rigid-body demo of circular motion
• Musical Pendulums: revolute joints plus Node Script audio, several pendulums of different lengths each playing a note
• Water Maze Solver: the liquid/particle-group system finding its way through a maze
• Jellies: the same particle-group tool configured for soft, elastic bodies instead of fluid
• Chebyshev's Linkage: a classic mechanism built entirely from rigid bodies and joints
• Unicycle Ragdoll Race: wheel joints, ragdolls, and Node Scripts driving a PID controller to keep each unicycle balanced
• Multiply or Release: the community-built marble race and territory game that turned into its own genre; see the full write-up for the story behind it

Every one of those opens directly in your browser from the link above. That's the pitch in practice, not just in theory.

Which Should You Use?

If your project genuinely needs geometric optics, lens design, or you want to build scenes by hand on an iPad, Algodoo is the more complete choice for that specific job today, and it's earned its long-running community. If what you want is to open a tab, start dragging shapes, script behavior in JavaScript, and send a working link to a classroom or a friend without either of you installing anything, that's exactly the gap Physion was built to fill.

Plenty of people reasonably use both. There's no download standing between you and finding out which one you reach for more: open Physion and start building.