The shaku (尺) or “Japanese foot” is a unit of length that is now standardized as 30.3 centimeters or 11.9 inches. For example, a 1.8 shakuhachi is 1 shaku and 8 sun, which works out to be 54.54 cm (1.8 x 30.3). The number 8 in Japanese is hachi (八); therefore, shakuhachi literally means “1 foot, 8 inches”. This is the standard size which is why the word shakuhachi can refer to any length. 1.8’s are almost always in the key of D above middle C; middle C being the key of the standard silver flute.
All is not always clear-cut when it comes to the relationship between the length and the pitch. Various factors can cause minor or major discrepancies between the shaku-length and the expected pitch for that length, such as the width of the bamboo, the size of the bottom opening, and whether or not the shaku-length is rounded up or down. For example, a shakuhachi with a wider bore or a smaller opening at the bottom will play flatter than a more standard instrument; therefore, it is often necessary to provide the exact length in centimeters in addition to the precise pitch.