glTF Units for Spatial Dimensions and Distance

The GL Transmission Format (glTF), established by the Khronos Group, is an open-standard runtime asset format designed for the efficient delivery and loading of 3D content. To maintain consistency across various renderers, game engines, and platforms, the glTF specification establishes strict conventions for real-world measurements. This article explains how glTF defines spatial dimensions, the rationale behind these standard units, and how scale translates across 3D applications and augmented reality environments.

The Standard Metric Unit

According to the official glTF 2.0 specification, all spatial linear dimensions and distances are defined in meters. This means that a standard coordinate distance of 1.0 in a glTF asset represents exactly one meter in real-world physical space.

By adhering to the International System of Units (SI), glTF eliminates ambiguity when exchanging assets between different tools. In addition to meters for distance, glTF standardizes:

• Angles: Radians (for rotation in animations and camera definitions).
• Time: Seconds (for animation timelines).

Coordinate System Orientation

Spatial dimensions in glTF operate within a right-handed Cartesian coordinate system. In this layout:

• +X points to the right.
• +Y points up.
• +Z points forward (toward the viewer in scene coordinates, while standard camera definitions view along the negative Z-axis).

Distances measured along these axes directly correlate to metric displacement, providing a predictable orientation and scale for imported meshes.

Handling Non-Metric DCC Tools

Many 3D Digital Content Creation (DCC) software tools, such as Autodesk Maya, 3ds Max, or Blender, allow artists to work in centimeters, inches, or arbitrary generic units.

When exporting to glTF, exporters must apply appropriate scaling transformations so that the exported vertices conform to meters. For example, if a model is built in a workspace configured for centimeters, the exporter converts the values by a factor of 0.01. This conversion can happen either by baking the scale directly into the mesh geometry vertices or by applying a scale factor to the root node of the scene hierarchy.

Impact on AR and Physics Simulation

Standardized units are crucial for real-time applications such as Augmented Reality (AR) and physics simulations. Frameworks like WebXR, Apple ARKit, and Google ARCore depend on meter-based scaling to place digital twins and virtual products accurately in real-world environments. Because glTF defines one unit as one meter, an asset representing a 0.5-meter chair displays at its precise physical size automatically without manual recalibration.