A440 is the reference frequency that almost all Western music is tuned to: concert A, labeled A4, vibrates exactly 440 times per second. Every other note's pitch is derived from that single value, and it's the international standard the tuners you use every day are calibrated to.
In short: a hertz is one vibration per second; A440 (A4) is the fixed point from which every other frequency is calculated, octave by octave and half step by half step.
What hertz are
A hertz (Hz) measures how many times something repeats in a second. For a guitar string, it's how many times it oscillates back and forth over the course of a second: the faster it vibrates, the higher the note; the slower it vibrates, the lower it is.
The open 6th string, E2, oscillates 82.4 times a second. The 1st string, E4, vibrates at 329.6 Hz — four times faster — and that's exactly why you hear it two octaves higher. Your ear translates this vibration speed into what you perceive as pitch: low frequencies are low sounds, high frequencies are high sounds.
A440: the reference standard
Every note has a precise frequency, but pinning them all down requires a shared starting point. That point is A4 = 440 Hz, the A above middle C on the piano. It's historically the note given by the tuning fork, the metal fork that, when struck, produces a stable A used for tuning before electronics came along.
The value of 440 Hz was adopted as the international standard over the course of the 20th century, so that instruments, orchestras, and manufacturers worldwide could tune to the same reference. Today it's the default setting on virtually every tuner and app. If you want to understand how an instrument uses this frequency to tell you whether you're in tune, see how a guitar tuner works; if you're starting from a classic tuning fork instead, you'll find the method in how to tune a guitar with a tuning fork.
The octave doubles, the half step has a fixed ratio
The relationship between frequency and note follows two simple rules. The first concerns the octave: going up an octave doubles the frequency, going down halves it. Starting from the reference A:
- A2 = 110 Hz
- A3 = 220 Hz
- A4 = 440 Hz
- A5 = 880 Hz
They're all A's, each one double the previous one. The second rule concerns the half step, the smallest step in Western music. Between two consecutive half steps, the frequency ratio is always the same: the twelfth root of 2, about 1.0595. Multiplying this ratio twelve times gets you exactly to double — the octave. This scheme, where every half step is equal, is called equal temperament: you'll find the full comparison in equal temperament vs. just intonation.
Standard tuning string frequencies
Applying these rules to A440 gives you the frequencies of the guitar's six strings in standard EADGBE tuning:
| String | Note | Frequency |
|---|---|---|
| 6th (lowest) | E2 | 82.4 Hz |
| 5th | A2 | 110.0 Hz |
| 4th | D3 | 146.8 Hz |
| 3rd | G3 | 196.0 Hz |
| 2nd | B3 | 246.9 Hz |
| 1st (highest) | E4 | 329.6 Hz |
These are the values every tuner compares against the string you're playing. For the note-by-note detail, you can start from standard guitar tuning EADGBE. If you want a quick check, EasyGuitarTuner is a chromatic tuner that runs in your browser: you play the string and bring it to the target note shown, with nothing to install.
432, 442, and 443 Hz: alternatives to 440
440 isn't the only value ever used. The most cited alternative is 432 Hz: some musicians prefer tuning a bit lower for reasons of taste or as a nod to tuning forks of the past. It's a legitimate choice, but it brings no proven technical or physical benefit — it simply shifts the reference point, and with it every frequency downstream.
On the opposite end, several orchestras, especially in Europe, tune to 442 or 443 Hz for a slightly brighter, more projected sound. Here too, only the starting point changes. For guitar, and for the vast majority of tuners, though, the default reference remains A440: it's the standard that lets you play with others without unpleasant surprises.