Inches, measured straight
Length is measured straight, from the base to the tip, before any wave. Wavy and curly hair reads two to four inches shorter than the number on the label — that is the wave, not a mistake. Ten inches is about 25 cm, sixteen inches about 41 cm.
Find your own reference
Stand in front of a mirror and mark where each length would fall on you: chin, collarbone, shoulder, below the shoulder. Photograph yourself at that angle. It is far more reliable than imagining the number.
What density actually looks like
Density is the amount of hair on the base, not the thickness of each strand. 110% reads as natural everyday hair on most people; higher densities photograph beautifully but feel warmer and take longer to dry.
Longer means heavier
Length and density multiply. If you are new to wearing, start at the shorter end of what you like and at 110% — you can always go longer next time, and the first piece is the one you learn on.