Final answer:
The commonly used hash to calculate the ETag for files less than 5GB in Amazon S3 is MD5. It is automatically generated during the file upload process if the upload is in a single operation.
Step-by-step explanation:
The hash that is commonly used to calculate the ETag for files less than 5GB in Amazon S3 is MD5. When you upload a file to Amazon S3, it automatically generates an ETag for the object. For objects that are uploaded in a single operation (files less than 5GB), the ETag value is the MD5 hash of the object's content. If the file is uploaded in parts (multipart upload), the ETag is not an MD5 digest, but instead generated by a different process that can include the MD5 hashes of the individual parts and the number of parts.