How to use
Choose note to frequency, enter a scientific pitch name such as C4 or Bb3, and set the A4 reference. For frequency to note, enter hertz instead; the calculator returns the nearest equal-tempered note and its cents difference.
A positive cents value means the entered frequency is sharper than the nearest note; a negative value means flatter. A difference of 100 cents equals one equal-tempered semitone.
Equal-temperament formula
Frequency = A4 reference × 2^((MIDI − 69) ÷ 12). MIDI 69 is A4, MIDI 60 is middle C (C4), and each octave changes frequency by a factor of two.
For frequency input, the formula is reversed and rounded to the nearest MIDI number. The cents difference is calculated before display rounding.
Examples and spelling
A4 with a 440 Hz reference
A4 is exactly 440 Hz and MIDI note 69.
C4 with a 440 Hz reference
Middle C is approximately 261.63 Hz and MIDI note 60.
Bb4 and A#4
In 12-tone equal temperament they have the same frequency, although harmonic spelling and musical function differ.
Limits and conventions
- Only 12-tone equal temperament is modeled; historical temperaments and just intonation differ.
- The accepted range is MIDI 0–127, from C-1 through G9.
- The nearest-note result does not judge tuning quality or instrument intonation.
- No microphone, audio upload, pitch tracking, or calibration measurement is performed.
Frequently asked questions
Can I use a reference other than 440 Hz?
Yes. Set A4 from 300 to 500 Hz; every calculated note changes by the same tuning ratio.
Why are A# and Bb both shown?
They are enharmonic spellings of the same equal-tempered pitch, but the correct written name depends on musical context.