Where 440 Hz came from
The standard dates to 1939, when delegates from France, Germany, the Netherlands, Italy and England met at Broadcasting House in London and agreed on A440. The ISO took it up as Recommendation R 16 in 1955 and formalized it as ISO 16 in 1975.
A440 is widely used as concert pitch in the United Kingdom and the United States. In continental Europe, A4 often lands anywhere from 440 to 444 Hz, and Leonard Bernstein often tuned the New York Philharmonic to A442.
Every other note follows from A4
Most modern keyboards use equal temperament, where each semitone is the same frequency ratio and 12 of them make an octave. Set A4 and every other note is fixed. Counted in semitones from A4:
- Middle C (C4), 9 semitones down: 261.63 Hz
- A3, 12 down: 220 Hz
- A4: 440 Hz
- E5, 7 up: 659.26 Hz
- A5, 12 up: 880 Hz
When A4 isn't 440
Some groups tune higher or lower. An A4 of 442 Hz shifts every note by 7.9 cents, and middle C moves to 262.81 Hz. Some musicians choose A4 = 432 Hz instead, and the gap between 432 Hz and 440 Hz is 31.8 cents. The 440 Hz vs 432 Hz page covers what that does and doesn't change.
Small gaps like these are easy to hear as beats. Two instruments tuned to 440 Hz and 442 Hz beat at 2 Hz when they play A4 together.
Tuning by ear
To tune an instrument, play the reference tone and the instrument together and listen for beats. A slow wobble means you're a few Hz off, and it slows to nothing as the pitches meet. A reference tone doesn't have to be fancy. The Web Audio API's oscillator even defaults to 440 Hz, so a tone generator in the browser starts on A4 unless you change it.