We have all been duped to measure the penis size incorrectly. Here's a quick calculation that will assist you in measuring your penis properly. This goes without saying that all sizes are perfect!
Reference:
Schultz W W, van Andel P, Sabelis I, Mooyaart E. Magnetic resonance imaging of male and female genitals during coitus and female sexual arousal BMJ 1999; 319 :1596 doi:10.1136/bmj.319.7225.1596