An octave number (in Spanish and French, the índice acústico) is the digit added to a note name to show which octave the note belongs to. The letter name C repeats in every octave, so the number tells you which C you mean: middle C, the C an octave above it, and so on. The catch is that two numbering systems are in use, the scientific (international) one and the Franco-Belgian one, so the same note can get two different numbers.
What is an octave number?
Every note has a letter name (C, D, E…) and a specific pitch. But the name repeats octave after octave: there are many Cs on a piano. Octave numbers remove the ambiguity by numbering the octaves: each note gets the number of the octave it is in (C3, A4, F5…). So C3 and C4 are two different notes, one octave apart.
The number changes every time you reach a C: all the notes from one C up to (but not including) the next C share the same number. For example, in the scientific system, starting on middle C: C4, D4, E4, F4, G4, A4, B4, and then the next C starts the series 5.
Scientific pitch notation (the international system)
Scientific pitch notation is the standard in most of the world. It starts counting from the lowest octave: 0 is given to the lowest C on a large pipe organ (C0, about 16.35 Hz). In this system:
- Middle C on the piano (261.63 Hz) is C4.
- Concert A, the tuning pitch (440 Hz), is A4.
It is the system used by notation software, synthesizers, MIDI documentation and most international books. The lowest key on a standard 88-key piano is A0 and the highest is C8.
The Franco-Belgian system
The Franco-Belgian system is the one traditionally used in Spain, France, Belgium and Italy, and in conservatories that follow those traditions. Its numbering starts one step later, so the same middle C gets a number one lower:
- Middle C (261.63 Hz) is C3.
- Concert A (440 Hz) is A3.
For low notes this system uses negative numbers (C₋₁, C₋₂…). The lowest organ C, which is C0 in scientific notation, is C₋₁ here.
These countries also teach fixed-do note names (Do, Re, Mi, Fa, Sol, La, Si) instead of letters, so in a Spanish or French book you will see “La3” where an American book writes A4.
Conversion chart
The rule is simple: the Franco-Belgian number is always one less than the scientific number. The note sounds exactly the same; only the digit changes. The chart shows the Cs of each octave:
| Scientific (international) | Franco-Belgian | Reference |
|---|---|---|
| C0 | C₋₁ | Lowest C of a large organ (16.35 Hz) |
| C1 | C0 | — |
| C2 | C1 | C on the 2nd ledger line below the staff in bass clef |
| C3 | C2 | — |
| C4 | C3 | Middle C (261.63 Hz) |
| C5 | C4 | — |
| C6 | C5 | — |
| C7 | C6 | — |
| C8 | C7 | Highest key on a standard piano (4186 Hz) |
And the most quoted case, concert A: it is A4 in scientific notation and A3 in the Franco-Belgian system, both at 440 Hz.
Which one should I use?
It depends on the context. If you read books or study with material from a Spanish, French or Belgian conservatory tradition, middle C will be C3. If you work with music software, synthesizers or English-language books, middle C is C4. Some software and keyboard makers use yet another offset (for instance, labeling middle C as C3 or C5), so always check the manual. The important thing is to state which system you use, because the same number points to different notes in different systems. To convert, add 1 (Franco-Belgian to scientific) or subtract 1 (scientific to Franco-Belgian).
This page builds on the staff, the clefs and the names of the notes: middle C, for example, is the reference note that links the treble clef and the bass clef.