The C=O stretch appears as a very sharp and intense peak in an IR spectrum. Since, C=O is a double bond, it appears in the "double bond" region of the IR spectrum, which is typically in the 1500-2000 cm-1 range. More specifically, C=O generally falls in the range of 1650-1850 cm-1. The reason for the range is that there are many types of functional groups that contain a carbonyl (C=O), such as a ketone, aldehyde, amide, or ester. Each of these will have a slightly different value as each stretch will have a different energy due to various factors such as conjugation.