What is 18mm f2 on aps c in full frame?

18mm on APS-C gives the view of 27mm on full frame. For the same depth of field, f/2 there behaves like about f/3 on full frame. Exposure doesn't change: f/2 gives the same brightness on any sensor.

The same view on every common format:

• Medium format (44 x 33): 34mm at about f/3.8 for the same depth of field
• Full frame: 27mm at about f/3 for the same depth of field
• APS-C (yours): 18mm at about f/2 for the same depth of field
• APS-C (Canon): 17mm at about f/1.9 for the same depth of field
• Micro Four Thirds: 14mm at about f/1.5 for the same depth of field
• 1-inch: 10mm at about f/1.1 for the same depth of field
• 1/1.3-inch: 7.5mm at about f/0.8 for the same depth of field
• 1/1.7-inch: 5.9mm at about f/0.7 for the same depth of field
• 1/2.3-inch: 4.8mm at about f/0.5 for the same depth of field

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.