A4
440Hz at concert pitch
Press to hear a pure sine tone at this pitch.
MIDI note
69
Piano key
49
Octave
4
Frequency
440 Hz
Period
2.2727 ms
Wavelength
0.7795 m
Pitch class
A
Notation
SPN
MIDI is the note number synths and software pass around (0–127). Pitch class is the note without its octave — all the As share one.
At other reference pitches
| Reference (A4) | A4 frequency | Where you meet it |
|---|---|---|
| 432 Hz | 432 Hz | the alternative-tuning circle |
| 440 Hzstandard | 440 Hz | the ISO 16 standard — concert pitch |
| 442 Hz | 442 Hz | many European orchestras tune a touch sharp |
| 443 Hz | 443 Hz | some sharper continental orchestras |
Every row is A4 — only the A4 you tune to changes. A frequency scales straight with the reference: A4 at 432 Hz is the 440 Hz value times 432⁄440.
About this note
This is concert pitch — the A that orchestras tune to and the anchor for the whole formula. 440 Hz was agreed at an international conference in 1939 and standardized internationally, then codified as ISO 16 in 1955. "Concert pitch" just means the reference frequency an ensemble tunes to; set A4 and every other note follows.
Move around
Every A
Chords with A in them
A appears in each of these — see the whole chord grid.