Note Frequency Calculator
Every musical note in standard 12-tone equal temperament has a precise frequency in hertz. This calculator uses the formula f = 440 * 2^((n - 49) / 12), where n is the piano key number and 49 corresponds to A4 (440 Hz). Select a note name and octave to get its frequency instantly. Understanding note frequencies is essential for synthesizer tuning, audio filters, EQ notching, and instrument acoustics. Middle C (C4) is 261.63 Hz, the lowest A on a standard piano (A0) is 27.50 Hz, and the highest C (C8) is 4,186.01 Hz.
Note frequency formula
f = 440 * 2^((n - 49) / 12)
n = piano key number (A4 = 49, middle C = C4 = 40)
Key number = (octave + 1) * 12 + semitone_offset + 1
where C = 0, C# = 1 ... B = 11
The formula places A4 at exactly 440 Hz. Each octave doubles the frequency. Each semitone multiplies by the twelfth root of 2 (approximately 1.05946). Piano key 1 is A0 (27.50 Hz) and key 88 is C8 (4,186.01 Hz).
Reference frequencies for common notes
- A0 (key 1): 27.50 Hz - lowest note on a standard piano.
- C4 / middle C (key 40): 261.63 Hz - center of the keyboard.
- A4 (key 49): 440.00 Hz - concert pitch reference.
- A5 (key 61): 880.00 Hz - one octave above concert pitch.
- C8 (key 88): 4,186.01 Hz - highest note on a standard piano.
Note frequency: frequently asked questions
What is the formula for note frequency?
f = 440 * 2^((n - 49) / 12), where n is the piano key number (A4 = key 49 = 440 Hz). This is the standard 12-tone equal temperament tuning used worldwide.
Why is A4 = 440 Hz the reference?
A440 was standardized by the International Organization for Standardization (ISO 16:1975) as concert pitch. Most orchestras and electronic instruments tune A4 to 440 Hz. Some orchestras use A = 441 or 442 Hz but 440 Hz is the international standard.
What is the difference between equal temperament and just intonation?
Equal temperament divides the octave into 12 equal semitones (each a ratio of 2^(1/12) = 1.05946), making every key playable together. Just intonation uses simple integer ratios for pure intervals but cannot change key without retuning.
How many semitones are in an octave?
Exactly 12. Each semitone doubles the frequency by a factor of 2^(1/12), and 12 semitones therefore double the frequency exactly, giving the next octave.
What frequency is middle C?
Middle C (C4) is piano key 40, which gives f = 440 * 2^((40-49)/12) = 440 * 2^(-0.75) = 261.63 Hz.
Official sources
- MIDI Association: midi.org - MIDI note number and frequency specifications.
- Audio Engineering Society (AES): aes.org - equal temperament and tuning standards.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.