To get the most accurate measurements for an individual's penis size, it has been recommended that several measurements be taken at different times, preferably with different erections on different days. Then average these figures together. This is to account for what may be natural variability in size due to factors such as arousal level, time of day, room temperature, frequency of sexual activity, and unreliability of the measurement methods.
Length
The length can be measured with the subject standing and the penis held parallel to the floor. The penis is measured along the top, from the base to the tip. Results are inaccurate if the measurement is taken along the underside of the penis, or if the subject is seated or prone.
Circumference
Penis girth is the measurement of the circumference of the completely erect penis. It is variously quoted as an average of three measurements: just below the glans penis, in the middle of the shaft, and at the base.
Problems
A penis must be completely erect in order to obtain an accurate measurement. This may be difficult to achieve in a clinical setting. At least one Brazilian doctor resorted to injecting penises with drugs to induce erection, which gave more consistent results.Some clinicians measure the penis by stretching the flaccid penis as far as comfortably possible. Self-reported measurements tend to be unreliable because men often want to report a larger penis size.