This widget allows the theme to show convert the theme default measurement unit to another value selected from the widget.

Default theme measurement unit is set in Theme Options > General > General Settings.

The widget is added from Admin > Appearance >  Widgets > Select Sidebar > Add Widget and optionally add Widget Title

Note: If you use a cache plugin you must exclude this cookie from being cached for the widget to work. Cache plugins are caching everything including the Measurement cookies and the Measurement switch will show your chosen Measurement only after the cache is deleted from the plugin.
Help article about how to use a cache plugin with measurement widget dropdown