A0
The lowest note on the piano
Reviewed for accuracy · Maintained by Justin Evans
A0 highlighted — use the view toggle to see all 88 keys.
Piano key
1 of 88
Octave
0
Frequency
27.5 Hz
MIDI number
21
Key color
White key
Pitch class
A · G## · Bbb
What is A0?
A0 is the A in octave 0 — the 1st key of the 88 on a full-size piano, counted from the bottom. In standard tuning it sounds at 27.5 Hz. It sits 39 half-steps below middle C (C4).
To find it, locate any group of two black keys: the white key just to their left is a C. Count down from middle C (C4) 4 octaves to reach octave 0, then step to A.
Every octave doubles the frequency: the A one octave up sounds at 55 Hz and the one below at 13.75 Hz. The same piano key can be written as A, G##, Bbb depending on the musical context — same sound, different spelling.
Nearby notes
A0A♯0 →
Every A on the keyboard: