The Whistle Register

Lowest Live Note: G#2
Highest Live Note: G#7

Lowest Note: D#3 (Oh, By The Way) (Studio)
Highest Note: F#7 (Ruby Tuesday) (Live)

Lowest Live Note: Eb2 (on T.V. Show)
Highest Live Note: Eb8 (Rumored B9) (on T.V. Show)

Lowest Studio Note: C3 (Kingdom Come)
Highest Studio Note: C7 (Spring Summer Feeling)

People have stated hearing her hit the same notes live in concert but there isn't any footage or recording yet to confirm it.

Erykah Badu's Vocal Range
Lowest Studio Note: D3
Highest Studio Note: B6
Highest Live Note: C7

Lowest Notes: B2
Highest Note: E6
Longest Note: 38 Seconds Sustained / 42 Seconds Held

Miki Howard's Vocal Range
Lowest Note: B2
Highest Note: E6