What is the RBXM file format?
An .rbxm file is a binary model file used by Roblox Studio, the development environment for the Roblox platform. It stores a tree of Roblox instances - 3D parts, scripts, models, and GUI objects - without a root DataModel, making it a portable asset that developers can insert into any game.
The file begins with the 14-byte magic header <roblox! followed by control bytes and a 4-byte version field (0x00000000 for Version 0). The body is divided into typed chunks: INST (class definitions), PROP (property values), PRNT (parent-child links), META (metadata), SSTR (shared strings), and END (terminator). Each chunk is independently compressed with LZ4 (default) or Zstandard, or stored uncompressed.
Property data uses type-tagged encoding for types such as Vector3, CFrame, and Color3. Integer arrays use a byte-interleaved layout that improves run-length compression efficiency.
.rbxm has a companion text format - .rbxmx - which stores the same instance tree as human-readable XML. The binary .rbxm is smaller and faster to load; .rbxmx is preferred for version-controlled assets in pipeline tools like Rojo.
Roblox Corporation has not published an official format specification; the community-maintained rbx-dom project at dom.rojo.space documents the binary layout.
Security & safety
RISK: MEDIUM.rbxm files can contain Lua/Luau scripts which execute inside Roblox Studio. Malicious scripts in community-shared models can exploit Roblox Studio or in-game mechanics. Roblox has sandboxing, but running untrusted models in Studio carries risk. Download models only from Roblox's official Creator Marketplace or trusted developers.
Format details
in a nutshell- Roblox XML Model File (.rbxmx) - The XML counterpart to .rbxm - same data model, human-readable XML format; extension is .rbxmx not .rbxm.
Programs that open RBXM files
Windows1 app
ROBLOX Studio
Free File → Open from File → select .rbxm; or drag onto an open Studio place to insert the model. 
macOS1 app
Technical details
deep spec| Magic bytes | `3C 72 6F 62 6C 6F 78 21` (`<roblox!`) at offset 0; full 14-byte header |
| File version | Version 0 (bytes 9-12 = `0x00000000`); tracked in the file header |
| Byte order | Little-endian for most fields; byte-interleaved big-endian for integer arrays |
| Chunk compression | LZ4 (default) or Zstandard (`28 B5 2F FD`); uncompressed chunks also possible |
| Chunk types | `META`, `SSTR`, `INST`, `PROP`, `PRNT`, `END` |
| Instance model | Stores a selection of Roblox instances without a root `DataModel` |
| Property encoding | Type-tagged values: `Vector3`, `CFrame`, `Color3`, `BrickColor`, `UDim2`, etc. |
| Integer encoding | Byte-interleaving of array elements for run-length compression efficiency |
| XML counterpart | `.rbxmx` stores the identical instance tree as human-readable XML |
| Primary tool | Roblox Studio (Windows and macOS) |
| Format specification | Community-documented by rbx-dom at `dom.rojo.space/binary.html`; no official Roblox spec |
| Typical file size | 5 KB - 50 MB depending on model complexity and embedded assets |
| Integrity | No file-level checksum; integrity relies on chunk framing |
| Released | circa 2012 (binary format); XML .rbxmx predates it |
| Latest version | Binary Model Format Version 0 (as documented by rbx-dom community; format evolves with Roblox Studio) |
| Specification | dom.rojo.space |
RBXM conversions
Community Q&A
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