A♯4 / B♭4
Reviewed for accuracy · Maintained by Justin Evans
A♯4 highlighted — use the view toggle to see all 88 keys.
Piano key
50 of 88
Octave
4
Frequency
466.16 Hz
MIDI number
70
Key color
Black key
Pitch class
A# · Bb · Cbb
What is A♯4?
A♯4 is the A♯ in octave 4 — the 50th key of the 88 on a full-size piano, counted from the bottom. In standard tuning it sounds at 466.16 Hz. It sits 10 half-steps above middle C (C4).
To find it, locate any group of two black keys: the white key just to their left is a C. Count up from middle C (C4) within the same octave to reach octave 4, then step to A♯.
Every octave doubles the frequency: the A♯ one octave up sounds at 932.32 Hz and the one below at 233.08 Hz. The same piano key can be written as A#, Bb, Cbb depending on the musical context — same sound, different spelling.
Nearby notes
Every A♯ on the keyboard: