What is 400 800mm on aps c in full frame?

400-800mm on APS-C gives the view of 600-1200mm on full frame.

The same view on every common format:

• Medium format (44 x 33): 759-1519mm
• Full frame: 600-1200mm
• APS-C (yours): 400-800mm
• APS-C (Canon): 375-750mm
• Micro Four Thirds: 300-600mm
• 1-inch: 222-444mm
• 1/1.3-inch: 167-333mm
• 1/1.7-inch: 130-261mm
• 1/2.3-inch: 107-214mm

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.