When sai is used as a unit for weighing lumber (especially valuable lumber), 1 sai is deemed to have the volume of 1 sun square multiplied by 1 ken length or by 2 ken length in some regions of Japan (1 sun is about 3.03 centimeters, and 1 ken is about 1.8182 meters).
木材、特に貴重材の体積（材積）については、一寸角で長さ1間（地方によっては2間）の体積を指す。