He has an ultimate 4 Octave vocal range. Source is Michael Jackson's official facebook page.
Freddie Mercury and Michael Jackson were one of the only singers to have a 4 Octave vocal range in their time.
As you may know , high notes are a piece of cake for Michael. His normal singing pitch is already higher than 90% of the male singers today. When he hits a high note , like when he hit a G5 live in I'll Be There performance in 2001 , it's unmatchable. In "Elizebath I Love You" , he held a note for like 18 Seconds. His song "All In Your Name" , the who song is of high notes when Michael sings expect the words "Only god knows". He had a rare and great vocal range. Hope that helps.
