What is URDF?

URDF (Unified Robot Description Format) is an XML format that describes a robot as a tree of rigid links connected by joints — the standard robot model format for ROS, Gazebo, RViz and MoveIt.

Glossary · ~2 min read

A URDF file is the "what does this robot look like and how does it move" file. Everything downstream — motion planning, simulation, visualization, collision checking — reads the same URDF.

What's inside a URDF

• Links — the rigid bodies (base, upper arm, gripper finger, wheel...).
• Joints — how two links connect and move relative to each other: revolute, prismatic, continuous, fixed, floating or planar, each with an axis and, for revolute/prismatic, limits.
• Visual geometry — what's rendered (usually a mesh).
• Collision geometry — what's used for physics/contact checks (usually simplified).
• Inertial — mass, center of mass and the inertia tensor for each link. See what an inertia tensor is.

A minimal example

<robot name="simple_arm">
  <link name="base_link"/>
  <link name="arm_link"/>
  <joint name="shoulder" type="revolute">
    <parent link="base_link"/>
    <child link="arm_link"/>
    <axis xyz="0 0 1"/>
    <limit lower="-1.57" upper="1.57" effort="10" velocity="1"/>
  </joint>
</robot>

Where URDF is used

ROS and ROS 2 (motion planning with MoveIt, visualization in RViz), Gazebo and other simulators, and increasingly reinforcement-learning pipelines that import URDF before converting to MJCF (MuJoCo) or USD (Isaac Sim).

Where a URDF comes from

Historically, by hand — measuring or exporting each link's geometry, placing joint frames, and writing the XML directly. Automated tools like Jointly generate a URDF straight from CAD (STEP, mesh, SolidWorks, Onshape), inferring the links, joints, axes, inertia and collision instead of hand-authoring them. See our STEP-to-URDF guide for how that works.