A Note Frequency — A0 to A7
A is the reference pitch of modern music. A4 — the A above middle C — is defined as 440 Hz, the international tuning standard (ISO 16). Orchestras tune to the oboe’s A, and every other note is derived from it.
Every A on an 88-key piano
Frequencies use equal temperament with A4 = 440 Hz. Range shown: A0 to A7 — 8 of the 88 keys.
| Note | MIDI number | Frequency (Hz) |
|---|---|---|
| A0 | 21 | 27.50 |
| A1 | 33 | 55.00 |
| A2 | 45 | 110.00 |
| A3 | 57 | 220.00 |
| A4 | 69 | 440.00 |
| A5 | 81 | 880.00 |
| A6 | 93 | 1760.00 |
| A7 | 105 | 3520.00 |
How the frequency of A is calculated
Modern instruments use twelve-tone equal temperament, where an octave is split into twelve equal semitones. With A4 fixed at 440 Hz, any note’s frequency is:
f = 440 × 2(m − 69) / 12
where m is the MIDI note number. For A4 that is
m = 69, giving 440.00 Hz.
A and its neighbours
Each semitone changes the frequency by a ratio of 21/12 ≈ 1.0595 — about 5.95%. Going up one octave doubles it. So A4 at 440.00 Hz has a neighbour 415.30 Hz a semitone below and 466.16 Hz a semitone above, and A5 at 880.00 Hz an octave above.
Frequently asked questions
- What is the frequency of A4?
- A4 is 440.00 Hz in equal temperament with A4 = 440 Hz. It is MIDI note 69.
- How many A notes are on an 88-key piano?
- An 88-key piano runs from A0 to C8 and contains 8 A notes, from A0 (27.50 Hz) to A7 (3520.00 Hz).
- What MIDI number is A4?
- A4 is MIDI note 69. MIDI note numbers run from 0 (C−1) to 127 (G9), with middle C (C4) at 60 and A4 at 69.
- Why does the frequency double every octave?
- An octave is a 2:1 frequency ratio. In twelve-tone equal temperament the octave is divided into twelve equal steps, so each semitone multiplies the frequency by the twelfth root of two (about 1.0595). Twelve of those steps multiply out to exactly 2.
Last updated: 2026-09-01