What is 18mm on aps c in full frame?

18mm on APS-C gives the view of 27mm on full frame.

The same view on every common format:

• Medium format (44 x 33): 34mm
• Full frame: 27mm
• APS-C (yours): 18mm
• APS-C (Canon): 17mm
• Micro Four Thirds: 14mm
• 1-inch: 10mm
• 1/1.3-inch: 7.5mm
• 1/1.7-inch: 5.9mm
• 1/2.3-inch: 4.8mm

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.