Convert any musical note and octave to its frequency in Hz, using standard 12-tone equal temperament (A4 = 440 Hz).
Calculator verified • Last updated: August 2026
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Every note's frequency is a fixed ratio away from the tuning reference, A4. Moving up one semitone multiplies the frequency by the 12th root of 2 (≈1.05946); moving up a full octave doubles it. That single rule generates every note frequency on a piano from one reference point:
$$f = 440 \times 2^{n/12}$$
f: the frequency of the target note, in Hz.
440: the A4 reference pitch, in Hz (adjustable above — some orchestras use 442-444 Hz).
n: the number of semitones the target note is above (positive) or below (negative) A4.
For example, middle C (C4) is 9 semitones below A4, so $n = -9$: $440 \times 2^{-9/12} \approx 261.63$ Hz. One octave up, C5, is exactly double that: 523.25 Hz.
An octave is a 2:1 frequency ratio by definition — that's what makes two notes sound like "the same note, higher." Equal temperament splits that doubling into 12 equal semitone steps, which means each step must be the same multiplicative ratio, and 12 of them must multiply together to reach exactly 2. That ratio is $2^{1/12}$, the 12th root of 2 — an irrational number, about 1.059463. Older tuning systems (just intonation, meantone temperament) used simpler whole-number frequency ratios that sounded purer in some keys but badly out of tune in others; equal temperament trades a little purity in every key for consistent, playable tuning in all of them.
A guitar's low E string is tuned to E2. E2 is 29 semitones below A4 (count down: A4→A3 is 12, A3→A2 is another 12, A2→E2 is 5 more — 12+12+5=29). $f = 440 \times 2^{-29/12} \approx 82.41$ Hz — the standard reference frequency electronic tuners use for that string.
Unlike a physical constant, 440 Hz is a human agreement — it became the international standard (ISO 16) in 1955, after centuries of wildly inconsistent tuning references across regions and eras (some historical tunings were closer to 415 Hz, nearly a semitone flatter than today's standard). Many professional orchestras today tune slightly sharp of standard, commonly 442 or 443 Hz, for a marginally brighter ensemble sound — this calculator's A4 Reference Pitch field lets you recompute any note's frequency under a different reference.
Semitone — the smallest interval in standard Western tuning; the distance between any key and the very next one on a piano, black or white.
Octave — the interval between one note and another with exactly double (or half) its frequency, perceived as "the same note" at a different pitch.
MIDI Note Number — a standard integer numbering of every pitch (middle C = 60, A4 = 69), used by digital instruments and music software to represent notes.
Scientific Pitch Notation — the note-plus-octave-number naming system used here (C4, A4, etc.), where each octave is numbered starting from C.
440 Hz became the international standard pitch reference in 1955 (ISO 16), after decades of orchestras and instrument makers using inconsistent tuning references. It's a convention, not a physical law — some orchestras tune slightly sharp of it (442-444 Hz) for a brighter sound.
It's the tuning system where each octave is divided into 12 equal semitone steps, each a frequency ratio of the 12th root of 2 (about 1.0595) from the last. It's the standard tuning for nearly all modern Western instruments, replacing older systems that had unequal, key-dependent intervals.
No — that's A4, not middle C. Middle C is C4, which is about 261.63 Hz. A common mix-up: A4 (440 Hz) is the tuning reference, but middle C (C4) is a different, lower note roughly a minor 6th below it.