What Is 440 Hz, and Why Is Everything Tuned to It?

Pluck your open A string and the string swings back and forth 110 times every second. Each complete back-and-forth is one cycle, and “hertz” (Hz) simply counts cycles per second. The string shoves the air 110 times a second, the air shoves your eardrum 110 times a second, and your brain files that rhythm of pressure as a pitch: the note A. Frequency is not something the note has — frequency is what the note is.

The octave is a doubling

Here is the most elegant fact in all of acoustics: double any frequency and you get the same note, one octave higher. A is 110 Hz, and also 220, and also 440, and also 880. Your guitar demonstrates this physically — press the 12th fret and you are dividing the string exactly in half, which doubles its frequency, which produces the octave. The fret markers on your neck are a graph of this math. Frets get closer together as you go up because each semitone multiplies the frequency by the same ratio (about 1.0595, the twelfth root of 2), and equal ratios mean shrinking distances on a string.

This is also why pitch perception is logarithmic. The jump from 110 to 220 Hz sounds exactly as big as the jump from 440 to 880 — one octave — even though the second jump covers four times as many hertz. Ears hear ratios, not differences.

So why 440, specifically?

Because everyone finally got tired of arguing. For most of music history there was no standard pitch at all. “A” was whatever the local church organ said it was — surviving organs and tuning forks from the 1600s and 1700s put A anywhere from about 380 Hz to over 480 Hz, nearly a fifth apart. Handel’s personal tuning fork rang at 422. Pitch even inflated over the 19th century as orchestras discovered that tuning slightly higher sounds slightly more brilliant — an arms race that kept rising until singers, whose instruments cannot be re-strung, openly revolted.

France responded first, legislating A = 435 Hz in 1859 — the diapason normal, a literal law about a musical note. It competed with other standards for decades until an international conference in 1939 settled on 440 Hz, a number already common in American orchestras and conveniently used for radio broadcast tuning tones. The ISO blessed it as an official standard (ISO 16) in 1955. That is the whole story: 440 is not cosmically special. It is the frequency equivalent of driving on the right — an arbitrary choice whose entire value is that everyone made it together.

A440 is not a law of nature. It is a peace treaty — and like most peace treaties, its power comes from everyone agreeing to it, not from the number itself.

About the 432 Hz thing

You will eventually meet the claim that A = 432 Hz is “natural,” “ancient,” or “healing,” and that 440 was some kind of mistake or conspiracy. The honest version: Verdi did once advocate a lower pitch (to spare his singers, not the cosmos), and lower tunings do sound slightly darker and more relaxed — because everything is slightly lower, the same way a whole song sounds mellower a half step down. But there is no physics, biology, or history behind 432 being special. All musical meaning lives in the ratios between notes, and those ratios are identical at any reference pitch. Tune to 432 if you like the color. Just know you are choosing a shade, not unlocking a secret.

What your guitar and tuner are actually doing

Standard tuning in hertz: low E at 82.41, A at exactly 110, D at 146.83, G at 196.00, B at 246.94, high E at 329.63. Notice that only the A string lands on a round number — that is no accident. It is exactly two octaves below the A440 reference (440 → 220 → 110), which is why orchestras tune to A and why the fifth string carries the tuning’s name. Every other string’s awkward decimal comes from repeatedly multiplying by the twelfth root of 2.

When your tuner shows “+12 cents,” it is speaking in hundredths of a semitone: 100 cents per fret, 1200 per octave. Most people begin to notice pitch drift somewhere around 5–10 cents, which is why a tuner can call you sharp when you would swear you sound fine — and why the last few cents of tuning are done better by ear, listening for the slow “beating” wobble that two nearly-matched pitches make as their sound waves drift in and out of step. When the wobble stops, you are in tune. That beating is hertz made audible: two strings at 110 and 112 Hz will pulse against each other exactly twice per second.