What the Standard Says
ISO 16:1975, "Acoustics: Standard tuning frequency (Standard musical pitch)", specifies 440 Hz for the note A in the treble stave. It also says instruments used for tuning should produce that note to within 0.5 Hz. ISO last reviewed and confirmed the standard in 2022, so it remains current.
The US National Institute of Standards and Technology (NIST) broadcasts the same note. Its radio stations play a 440 Hz tone once an hour, in minute 2 on WWV and minute 1 on WWVH, except in the first hour of each UTC day.
How 440 Hz Became Standard
Before 440 Hz, pitch was defined by a physical object. NIST's history describes the French "Diapason Normal", a tuning fork kept at the Paris Conservatory and stated to sound 435 Hz at 15 °C. A congress in Vienna in 1891 adopted it as "International Pitch".
NIST gives the steps that led to 440 Hz:
- The US music industry agreed on A = 440 Hz as a compromise among the pitches different groups used.
- 1936: the American Standards Association adopted it as an industrial standard.
- 1939: a conference in London, with delegates from France, Germany, Great Britain, Holland, and Italy, adopted 440 Hz unanimously.
- 1955: it was accepted as an ISO Recommendation at Stockholm. ISO 16 followed in 1975.
The old and new pitches are closer than the numbers suggest. NIST notes that an organ pipe tuned to A = 435 Hz at 15 °C plays almost exactly A = 440 Hz at 20 °C (68 °F): the pitch of an air column rises as the air warms.
What Changes at 432 Hz
Every note moves by the same ratio, 432 ÷ 440, which is 31.8 cents lower. A semitone is 100 cents, so the change is about a third of the step between two neighboring notes. Here is a guitar in standard tuning:
- Low E (E2): 82.41 Hz at A = 440, 80.91 Hz at A = 432
- A (A2): 110.00 Hz, 108.00 Hz
- D (D3): 146.83 Hz, 144.16 Hz
- G (G3): 196.00 Hz, 192.43 Hz
- B (B3): 246.94 Hz, 242.45 Hz
- High E (E4): 329.63 Hz, 323.63 Hz
Middle C moves from 261.63 Hz to 256.87 Hz. Chords and intervals don't change, only where the whole set sits.
Play with other people at the pitch they use. A band or a recording at 440 Hz sounds about a third of a semitone sharp against an instrument tuned to 432 Hz.
How to Tune to 432 Hz or 440 Hz
Open the chromatic tuner, tap Settings, and choose a reference pitch (A4) of 440 Hz or 432 Hz. Every note name, cents reading, and reference tone moves with it, and the choice is saved on your device. The same setting works on the guitar tuner and the other instrument tuners.
The setting also offers 442 Hz, 7.9 cents above standard, and 415 Hz, which is close to a semitone below 440 Hz (415.3 Hz).
A tuner that detects pitch measures frequency against its reference, so set the reference before you tune. To check a tuner by ear, NIST's hourly 440 Hz tone is a free reference.