C4 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 C4 is exactly 440 × 2^(-9/12) Hz. Under alternative references the note shifts with the reference: at A4 = 432 Hz, C4 becomes 256.87 Hz.
C in every octave
| Note | Frequency | MIDI | Piano key |
|---|---|---|---|
| C1 | 32.70 Hz | 24 | 4 |
| C2 | 65.41 Hz | 36 | 16 |
| C3 | 130.81 Hz | 48 | 28 |
| C4 ← | 261.63 Hz | 60 | 40 |
| C5 | 523.25 Hz | 72 | 52 |
| C6 | 1046.5 Hz | 84 | 64 |
| C7 | 2093.0 Hz | 96 | 76 |
| C8 | 4186.0 Hz | 108 | 88 |
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 B3 at 246.94 Hz; one semitone up is C♯4 at 277.18 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.