The unit system of the active document is stored on an ON_3dmUnitsAndTolerances class that is located on the CRhinoDoc object.

If you inside of a CRhinoCommand-derived object's RunCommand() member, you can get the current units system as follows:

 const CRhinoDocProperties& doc_props = context.m_doc.Properties();
 const ON_3dmUnitsAndTolerances& units = doc_props.ModelUnitsAndTolerances();
 ON::unit_system units_system = units.m_unit_system;

As a shortcut, you can do the following:

 ON::unit_system units_system = context.m_doc.UnitSystem();

If you outside of a CRhinoCommand-derived object's RunCommand() member, you can get the current units system as follows:

  CRhinoDoc* doc = RhinoApp().ActiveDoc();
  if( doc )
  {
    ON::unit_system units_system = doc->UnitSystem();
  }

This has changed for Rhino 4

With the addition of layouts in Rhino 4, there are now two unit systems associated with a document, model and page units. A new class has been added to OpenNURBS call ON_UnitSystem which makes it easier to work with custom units. The Rhino document class now contains two functions

• const ON_UnitSystem& ModelUnits() const;
• const ON_UnitSystem& PageUnits() const;

 C++
  ON_UnitSystem units = doc->ModelUnits();


 VB.NET
  Dim units as IOnUnitSystem = doc.ModelUnits()


 C#
  IOnUnitSystem units = doc.ModelUnits();