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About C♯1
C♯1 / D♭1 has a frequency of 34.65 Hz in standard tuning, where A4 is fixed at 440 Hz. It is MIDI note 25 and key number 5 on an 88-key piano.
In equal temperament every semitone multiplies the frequency by the twelfth root of two (≈1.05946), so C♯1 is 44 semitones below A4: f = 440 × 2^((25 − 69) / 12) = 34.65 Hz. The number in the name is the octave in scientific pitch notation, which starts each octave on C.
C♯1 and D♭1 are enharmonic: on a piano or in equal temperament they are the same key and the same frequency, but they are spelled differently depending on the key signature.
C♯1 frequency in different tunings
| Tuning reference | C♯1 frequency |
|---|---|
| A4 = 440 Hz (ISO 16 standard) | 34.65 Hz |
| A4 = 442 Hz (many European orchestras) | 34.81 Hz |
| A4 = 443 Hz (Berlin, Vienna) | 34.88 Hz |
| A4 = 435 Hz (French diapason normal, 1859) | 34.25 Hz |
| A4 = 432 Hz (alternative tuning) | 34.02 Hz |
| A4 = 415 Hz (Baroque pitch) | 32.68 Hz |
| C4 = 256 Hz (scientific pitch) | 33.9 Hz |
Intervals from C♯1
| Interval | Note | Frequency | Ratio (just) |
|---|---|---|---|
| Minor second | D1 | 36.71 Hz | 16:15 |
| Major second | D♯1 | 38.89 Hz | 9:8 |
| Minor third | E1 | 41.2 Hz | 6:5 |
| Major third | F1 | 43.65 Hz | 5:4 |
| Perfect fourth | F♯1 | 46.25 Hz | 4:3 |
| Tritone | G1 | 49 Hz | 45:32 |
| Perfect fifth | G♯1 | 51.91 Hz | 3:2 |
| Minor sixth | A1 | 55 Hz | 8:5 |
| Major sixth | A♯1 | 58.27 Hz | 5:3 |
| Minor seventh | B1 | 61.74 Hz | 9:5 |
| Major seventh | C2 | 65.41 Hz | 15:8 |
| Octave | C♯2 | 69.3 Hz | 2:1 |
Instruments and voices that can play C♯1
Frequently asked questions
What is the frequency of C♯1?
C♯1 is 34.65 Hz when A4 = 440 Hz (34.02 Hz if A4 = 432 Hz).
What MIDI number is C♯1?
C♯1 is MIDI note number 25.
Is C♯1 on a piano?
Yes – it is key 5 of 88.