A4 at a glance
All values assume twelve-tone equal temperament at the international standard pitch of A4 = 440 Hz. In that system every semitone has the same size - a frequency ratio of 2^(1/12) - so A4 is exactly the tuning reference itself. Under alternative references the note shifts with the reference: at A4 = 432 Hz, A4 becomes 432.00 Hz.
A in every octave
| Note | Frequency | MIDI | Piano key |
|---|---|---|---|
| A0 | 27.50 Hz | 21 | 1 |
| A1 | 55.00 Hz | 33 | 13 |
| A2 | 110.00 Hz | 45 | 25 |
| A3 | 220.00 Hz | 57 | 37 |
| A4 ← | 440.00 Hz | 69 | 49 |
| A5 | 880.00 Hz | 81 | 61 |
| A6 | 1760.0 Hz | 93 | 73 |
| A7 | 3520.0 Hz | 105 | 85 |
Each octave doubles the frequency - the simplest relationship in music, and the reason notes an octave apart sound like “the same note, higher”.
Neighboring notes
One semitone down is G♯4 at 415.30 Hz; one semitone up is A♯4 at 466.16 Hz. Need a frequency between notes, or a different waveform? The tone generator plays anything from 20 Hz to 20 kHz; the note ↔ frequency converter does the math both directions, and the chromatic tuner listens to your instrument and shows the offset in cents.