From FIXwiki

FIX Field 9

Message length, in bytes, forward to the CheckSum field. ALWAYS SECOND FIELD IN MESSAGE. (Always unencrypted)

Notes

Error in FIX Specification:

The above definition of this field (taken from the FIX repository) is a bit vague. I suggest using the following more precise definition taken from elsewhere in the specification:

The message length is ... verified by counting the number of characters in the message following the BodyLength field up to, and including, the delimiter immediately preceeding the CheckSum tag ("10=")