Around 5% for me, though I don't have a reliable test. It's a genetic thing I suppose, I'm right on the line for underweight (I usually keep my coat and shoes on when they ask me to step on the scale at the Dr.'s office so the nurse doesn't put me down as underweight.) I've tried everything--donuts, whole milk, eating right before I go to bed, working a sedentary job, etc.--and nothing except exercise gets my weight up, and even then regular weight lifting gets it up only by a few pounds.
Oddly enough, that's partially what drove me to a greater interest in clothes--off-the-shelf clothes for the "American male" are a dreadful fit on my frame, so I went far and wide in search of better-fitting clothes and, as it turns out, ended up with higher-quality and better-looking clothes in the process.