The shaku (尺) or “Japanese foot” is a unit of length that’s now standardized as 30.3 centimeters or 11.9 inches. A 1.8 shaku-hachi is 1 shaku and 8/hachi (八) sun. This works out to be 54.54 cm (1.8 x 30.3). So shakuhachi literally means “1 foot, 8 inches”. This is the standard size but the word shakuhachi can often refer to any length of flute. 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. Particularly 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. So it’s often necessary to provide the exact length in centimeters in addition to the precise pitch, usually using A=440.