The minimum amount of tooth demineralization required for radiographic detection of dental caries is
Lesions confined to enamel may not be evident until approximately 30% to 40% demineralization has occurred. For this reason, the actual depth of penetration of a carious lesion is often deeper than seen in the image.
 
Oral radiology by White & Pharaoh 7th ed. Page no 288
20%
40%
60%
80%