Let me state at the outset that there is no direct relationship between flaccid and erect penis size in men. As per NHS (National Health Service), UK and several researches carried out in different parts of the world, it has been proven that the average human male penis size ranges from 5.5 inches to 6.3 inches in erect state. The average penile girth in erect state varies from 4.7 inches to 5.1 inches. A penis can be considered unusually small only if it is shorter than 3 inches in length when erect.
These inferences have been drawn based on 50 different studies conducted on penis sizes since 1942, covering as many as 11,531 penises.
NHS also has directions on measuring the penis size correctly, should anyone be curious. A penis is measured best when it is in erect state. The measurement must be taken from the base of penile shaft (from the side towards the tummy) and till the tip of penile head.
Regardless of the clinical significance of such data, it can never be assumed that someone with a big flaccid penis will have a huge erection. Similarly, someone whose penis may look very tiny in flaccid state could swell up like anything when erect.
An analysis carried out by Alfred Kinsey revealed that men with shorter flaccid penises gain almost twice the length when erect, than men with longer flaccid penises.