What is 12 20mm on aps c in full frame?

12-20mm on APS-C gives the view of 18-30mm on full frame.

The same view on every common format:

• Medium format (44 x 33): 23-38mm
• Full frame: 18-30mm
• APS-C (yours): 12-20mm
• APS-C (Canon): 11-19mm
• Micro Four Thirds: 9-15mm
• 1-inch: 6.7-11mm
• 1/1.3-inch: 5-8.3mm
• 1/1.7-inch: 3.9-6.5mm
• 1/2.3-inch: 3.2-5.4mm

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.