A flexible measuring tape is extended from a predetermined location on a face a predetermined distance to a location on a scalp. A 2 cm by 2 cm square bundle of hair about that location is isolated. The bundle of isolated hair is captured in a slit in a two-part cartridge, one part being slidable in the other part about a changeable length slit 1 mm wide. The cartridge is placed in an "L" shaped end of a plunger extending from a first end of a body of a measuring device. The plunger is drawn into the body of the device to compress the hair in the changeable length slit as the two parts of the cartridge are pushed together between an end of the "L" and the first end of the body. A spring is provided on the plunger between a second end of the body of the device and an inner end of a knob at an outer end of the plunger. The spring applies the same load regardless of the bundle's size and urges the "L" shaped end toward the body to compress the bundle of hair and the amount of compressed hair is read off of a gauge as DDI, Density-Diameter Index, which represents the cross-sectional area of the compressed bundle of hair. After the measurement is made, the smaller part is pulled part way out of the larger part of the cartridge so the cartridge easily can be pulled off of the bundle of hair and discarded.