MIDI note numbers: names, frequencies, and why charts disagree

What a MIDI note number is

A MIDI note number is an index from 0 to 127, and that is very nearly all it is. When a key goes down, the message on the wire carries that number and a velocity; it does not carry a note name, and it does not carry a frequency. Everything else — what to call the note, and what pitch to sound — is decided by whatever is reading the message.

The MIDI Association put this plainly when someone asked them about it directly: MIDI notes, they wrote, are simply a note index and do not really define the pitch of a note, the specification only defines middle C as note number 60, and there are two major standards for octave nomenclature. So one number is fixed, and the naming is not.

That is why two pieces of equipment can display the same incoming note as C3 and C4. Neither is malfunctioning and neither is wrong: they have picked different octave-numbering conventions, and the byte they received is identical. This page shows all three conventions at once rather than choosing for you.

How to use it

1. Pick what you are starting from. A note number, a note name, or a frequency in hertz. Names are read using the convention selected below, so C4 means different notes under different settings — which is the whole problem, made visible.
2. Set the convention and the reference pitch. The convention changes every octave number on the page and nothing about the notes themselves. The reference defaults to 440 Hz; 432, 442 and 443 are one click away, and all of them are really used.
3. Read the result and the table. The result panel shows the note under your chosen convention, and beneath it the same note named under all three. The full 128-note table at the bottom re-renders for whatever reference you set.

Why the conventions disagree, and which one has a claim

Three conventions are in circulation, differing only in what they call the octave containing middle C: C3, C4 or C5. The letter is never in dispute — everyone agrees note 61 is a C sharp — so the disagreement is entirely about the number written next to it. All 128 notes are affected, not a handful near the edges.

One of the three can be derived rather than merely asserted. ISO 16 fixes the tuning frequency of the A above middle C at 440 Hz, and that A is universally written A4. In any system where the octave number changes at C, an A4 sitting above middle C makes middle C into C4. So the C4 convention — scientific pitch notation, also called ASPN or IPN — is pinned by the tuning standard itself, which is a stronger reason than popularity.

What this page will not do is tell you which manufacturer uses which. That mapping is repeated confidently all over the internet and the sources contradict each other: one widely-cited summary attributes C3 to Yamaha and C4 to Roland, while the MIDI Association's own post attributes C4 to Yamaha and calls the C3 convention the most common. When two sources disagree and one of them disagrees with itself, the honest answer is to name the conventions by what they call middle C and let you check your own gear.

The practical advice that survives all of this is short. The number is what travels; the name is a display choice. If two tools disagree by exactly one octave, you are looking at a convention mismatch and not a bug, and nothing about the underlying data needs fixing.

What this cannot tell you

The frequencies here assume twelve-tone equal temperament, which is a tuning system rather than a fact about MIDI. Any note is the reference multiplied by two to the power of the semitone distance over twelve — exact, and exactly what most equipment does, but a harpsichord tuned to a historical temperament will not agree, and neither will a synthesiser using the MIDI Tuning Standard. That extension exists precisely because note numbers never carried pitch.

The reference pitch is a field for the same reason. 440 Hz comes from ISO 16, not from the MIDI specification, and orchestras that tune to 442 or 443 are not violating anything. Changing it here scales every frequency by the same factor and leaves every note number and name untouched, which is a useful demonstration of how little the protocol actually pins down.

Two smaller things. The lowest notes have negative octave numbers under two of the three conventions — note 0 is C-1 in scientific pitch notation — and charts that start their numbering at zero have quietly dropped that. And a frequency you type will almost never be a note exactly; the cents figure says how far off it is, and it can never exceed fifty, because past that the next note is nearer.

Why is it free?

This is one exponentiation and a lookup table, and both run in your browser. There is no server in the loop, so there is nothing to bill for and no account to create.

Nothing is uploaded. Reload the page and it has forgotten what you typed.