What is 120 300mm on canon aps c in full frame?

120-300mm on APS-C (Canon) gives the view of 192-480mm on full frame.

The same view on every common format:

• Medium format (44 x 33): 243-608mm
• Full frame: 192-480mm
• APS-C: 128-320mm
• APS-C (Canon) (yours): 120-300mm
• Micro Four Thirds: 96-240mm
• 1-inch: 71-178mm
• 1/1.3-inch: 53-133mm
• 1/1.7-inch: 42-104mm
• 1/2.3-inch: 34-86mm

Crop factors are by the sensor diagonal, rounded as they're usually quoted; a format with a different shape (4:3 against 3:2) frames a little differently across the width. More in the focal length converter.

Computed from the sensor sizes, not written by a model.