Piano octaves: what they are and how they are numbered

An octave on the piano is the distance from one key to the next key with the same letter name, such as C4 to C5 or A3 to A4. It spans 12 half steps, or 13 keys counting both ends, and the upper note vibrates exactly twice as fast as the lower one. That 2:1 ratio makes the two notes sound so alike that music gives them the same letter. A full 88-key piano covers seven octaves and a minor third, from A0 to C8, and its octaves are numbered 0 to 8.

The lit keys on the keyboard above are three As, each an octave from the next: A3 at 220 Hz, A4 at 440 Hz and A5 at 880 Hz. Play them in turn and each one sounds like a higher copy of the one before.

Why it is called an octave

Octave comes from the Latin octavus, "eighth". Counted on the white keys, an octave from C runs C, D, E, F, G, A, B, C: the eighth note is the same letter as the first. Counted in half steps, the same span is 12 steps, one from each key to the next, white or black. That is why the piano's pattern of seven white and five black keys starts over at every octave.

The doubling works in both directions. A4 is 440 Hz, so A5 is 880 Hz and A3 is 220 Hz, and the same holds for every note: the frequency of any key is twice that of the key an octave below it.

The diagram shows the two octaves between the lit keys, A3 to A4 and A4 to A5. Each holds the same seven white and five black keys in the same order, so an octave can start on any note, not only on C. The octave number, though, changes at C: the diagram's Cs read C4 and C5.

How many octaves an 88-key piano has

From A0 to C8 is 87 half steps: seven octaves of 12 (84 half steps) plus three more. Three half steps make a minor third, so the range is described as seven octaves plus a minor third, or seven and a quarter octaves. Counted as complete C-to-C octaves, the keyboard holds seven, from C1 up to C8, with three keys left over below C1: A0, A#0/Bb0 and B0.

Octave numbers

Octaves are numbered in scientific pitch notation. Each numbered octave starts on C and ends on B, so the number changes between B and C, not between G and A: B3 is followed by C4, and A0 belongs to octave 0 because it comes before the first C. The older names in the table come from traditional notation, in which middle C's octave is the one-line octave.

Each C in the last column has twice the frequency of the C above it in the table, allowing for rounding. Some keyboard manuals and music programs number every octave one lower, so that middle C reads C3 and the top key C7; the middle C guide explains that convention. The piano notes chart gives the frequency of every key.

Octaves on smaller keyboards

Every 12 keys add one octave. A keyboard that starts and ends on C has one extra key, the top C that closes its last octave: 61 keys are five octaves, 49 are four, 37 are three and 25 are two.

Ranges differ by model; these are typical. For how many white and black keys each size has, see how many keys on a piano.

Octave buttons and octave shift

Keyboards with fewer than 88 keys can usually shift their whole range up or down an octave at a time, which is how a small keyboard reaches notes it does not have:

• MIDI controllers usually shift with a pair of buttons, marked Octave − and Octave + or Oct − and Oct +. On one 25-key controller, for example, each press moves the keys one octave, up to four octaves in either direction.
• Portable keyboards often keep the setting in a function menu instead. One 61-key model offers an octave setting for its main sound from two octaves down to two up.
• Digital pianos sometimes let each sound be shifted by octaves when two sounds are layered or split.

A shifted keyboard plays different notes from the same keys. After one press of Octave +, the key that normally plays middle C (MIDI note 60) plays C5 (note 72). When everything sounds an octave too high or too low, check the octave setting first.

Octave signs in sheet music

Printed music uses octave signs to avoid stacks of ledger lines:

A dashed line or bracket after the sign shows how far it lasts. 8va stands for the Italian all'ottava, "at the octave".

Playing octaves with one hand

In piano music, an octave played with one hand means both notes at once, most often with the thumb and little finger, fingers 1 and 5. On a standard keyboard an octave measures about 6.5 inches (16.5 cm) across its seven white keys, and a hand playing one stretches across eight white keys, counting both ends. For players with small hands, narrower keyboards exist with octaves of about 6.0 inches (the 15/16 size) and 5.5 inches (the 7/8 size), though few pianos are built with them. To hear octaves across the whole range, from A0 to C8, try the full online piano on our companion site.