Mine does vary given the circumstances. An average, run of the mill "I'm just looking at some porn to jerk off" erection will be hard enough for me to get off, but if I'm actually really turned on or it's been awhile since I had an orgasm my erection will be fuller and harder. The difference is usually most visible to me by looking at whether it stays all the way up or if it fades into a semi quickly. I don't know if the difference is 1/2 an inch, but I can definitely tell a "good" day from an average day.