Because I've never measured mine before. erplexed From crown to tips? Or from the base of your neck to tips since that's where the actual length begins or is most obvious. Pardon the noobish question.
Like in your siggy, i see where the ends of the hair stop. I try to use the same shirt. I don't use measuring tape, that would become an obsessive habit that i don't want to start, lol.
I'd do root to tip. So whichever section of hair you get, be it crown, above the ear, nape, or middle of the forehead. I'd measure root to tip in a couple sections to get a good gauge.
I use a little black ruler which seem to have disappeared....i think someone who is about 3 ft tall, runs really fast, and has a habbit of being a little sticky fingers misplaced it, so I'm going to start using a measuring tape. But when I do measure, I measure my front, both sides, crown, and nape. For me, usually my front half grows alot faster than my nape, but maybe it's because I give it more attention than I do my nape.