Box3D Deterministic Physics for Unreal Engine 5.8 is now on Fab

Deterministic rigid-body simulation for Unreal Engine 5.8, running independently of Chaos — now available on Fab.

Box3D, the plugin that brings Erin Catto's Box3D physics engine into Unreal Engine 5.8 as a first-class simulation runtime, is now on Fab. Version 1.0.0 ships with full C++ source, Blueprint access to the everyday API surface, and twelve ready-to-play demo maps.

Get it on Fab: Box3D Deterministic Physics

A fuller overview, with demo renders and the complete feature list, lives on the Box3D plugin page.

What it is

Box3D is a self-contained physics runtime for Unreal. A world subsystem steps the simulation at a fixed 60 Hz with four substeps, entirely separate from Chaos. Body and shape components drive your existing actors, so you can simulate one system deterministically while Chaos continues to handle the rest of your game — or replace Chaos per actor, wherever it matters.

The engine underneath is Box3D by Erin Catto, the 3D sibling of Box2D. It is a data-oriented C17 solver built for large piles of bodies, with continuous collision detection, island-based sleep, and cross-platform determinism.

Why determinism

The fixed timestep runs on a single solver worker by default, so the same scene steps the same way run after run. That property is covered by automated tests in the plugin, and it is what makes the rest possible: simulations can be captured to .b3rec recordings and replayed in-engine, and simulated actors can be baked into a Level Sequence through Take Recorder.

If you have ever shipped a physics puzzle, a replay system, or a networked game that needed the same inputs to produce the same outcome, this is the part that matters.

What's in the box

• Rigid bodies — sphere, box-hull and capsule primitives, plus triangle-mesh and convex collision mirrored directly from a Static Mesh's own authored collision.

• Joints — revolute, distance, spherical and weld, with break events, motors and live setters.

• Sensors with begin and end overlap events, and conveyor surfaces.

• Queries — overlap, sweep and line trace against the Box3D world.

• Contact hit events and solver statistics exposed at runtime.

• An instanced-mesh path for scenes running into the thousands of bodies.

• Simulate-In-Editor support, so you can watch a scene settle without entering Play.

• Take Recorder capture of simulated actors into a Level Sequence.

• Offline collision baking via commandlet for static geometry.

Bodies, shapes, joints, sensors, queries, contact and sensor events and solver settings are all exposed to Blueprint, so a Blueprint-only project can build with the plugin. A few editor-side tools — the bake commandlet and the demo map generator — remain C++ only.

Twelve demo maps, not a screenshot reel

The plugin ships its demos as playable content, not marketing footage. Every one is engine-content-only, carries a live stats overlay, and exposes its parameters for tuning.

Bridge builds a suspension deck out of revolute joints and lets you drop weight on it. Ball and Chain is the joint chain stress test. Jenga and Large Pyramid exercise stacking stability, the case where solvers usually show their seams.

Dominoes is the determinism showcase: run it, record it, replay it, get the same cascade. Inclined Plane isolates restitution and friction against a tunable ramp. Ragdoll Pile throws articulated bodies into a heap. Conveyor Circuit demonstrates conveyor surfaces moving cargo around a loop. Car Racing and Smoke round out the set, and Monte Cristo is a narrative machine built entirely from Box3D parts.

Honest limitations

Worth knowing before you buy:

• Triangle-mesh collision is Static-body only. That is a Box3D engine constraint, not a wrapper gap — a moving object needs convex or primitive shapes.

• No height-field shapes yet.

• No .b3rec to Sequencer import. Take Recorder captures live actor transforms instead.

• Chaos and Box3D do not interact. A Box3D body is static as far as Chaos is concerned, and vice versa.

• Windows (Win64) only in 1.0.0.

Requirements and licensing

Unreal Engine 5.8, Win64. C++ source included.

Box3D is free for personal use and $99 for the Professional licence, handled through Fab's standard licensing tiers.

Try it, then buy it

A free compiled sample project (UE 5.8, Win64) is linked from the Fab listing, so you can play the demos before you commit to a licence.

Download Box3D Deterministic Physics on Fab: https://www.fab.com/listings/2c829d74-8be3-4b7f-85b6-c51784570fbc

Attribution

Includes Box3D by Erin Catto, MIT licensed (github.com/erincatto/box3d), built from the publisher's fork at github.com/kerinzeebart/box3d.