Flaccid and erect percentiles

A man's flaccid and erect figures rarely land on the same percentile, and the erect one is the more informative. That is not a mistake in either measurement: flaccid length predicts erect length only weakly, it wanders from day to day, and the two reference distributions are shaped differently.

Two distributions, not one distribution twice

Veale et al. (2015), in BJU International, report flaccid and erect length as separate distributions from clinician measurements: flaccid 9.16 cm (SD 1.57 cm, 10,704 men) and erect 13.12 cm (SD 1.66 cm, 692 men). They are not the same curve shifted along an axis. In absolute terms the two spreads are close, so one centimetre above average is a similar step on either chart. Relative to its mean, though, the erect distribution is tighter, so a man who sits the same proportion above average in both states lands further out on the erect chart. A figure that sits comfortably mid-pack flaccid can sit further out erect, or the reverse, without anything unusual being true about the man.

Why the flaccid figure carries more noise

Flaccid length responds to temperature, recent activity, arousal state and time of day in ways that erect length mostly does not. Two readings of the same man twenty minutes apart, both taken carefully, can differ meaningfully in the flaccid state and barely at all in the erect one. A percentile built on a noisier input is itself less stable - the same man measured on two different afternoons can land at two different flaccid percentiles for reasons that have nothing to do with a change in his body, only in when the tape came out.

The correlation between the two is weak

Flaccid length predicts erect length only weakly at the individual level, and that weak correlation is the mechanical reason the percentiles diverge rather than move together. If the two states were tightly linked, a high flaccid percentile would reliably forecast a high erect one. They are not, so it does not. This is a separate fact from the percentile-shape argument above - both push in the same direction, but neither one is redundant with the other.

Which percentile to trust

For most practical purposes, the erect figure is the informative one. It is more repeatable, its distribution is better characterised in the pooled data, and it is the state the clinical literature leans on when discussing size at all. A flaccid percentile is worth noting as a curiosity, not as a number to anchor an opinion of yourself to - given how much it moves on its own, treating it as a fixed fact about your body is treating noise as signal.

A concrete way to see the shape difference

Imagine a man whose flaccid and erect figures both sit 10% above their respective means. Ten percent of the flaccid mean is less than a centimetre, while ten percent of the erect mean is more than one, and the erect standard deviation is only slightly larger than the flaccid one. So the two "10% above average" statements sound identical and describe two different percentile jumps, the erect one larger, purely because of how tightly each distribution is packed relative to its own mean. This is illustrative arithmetic, not a claim about any specific man's numbers, but it is the mechanism behind the mismatch, stated plainly rather than left as an assertion.

A practical takeaway for anyone comparing the two

If your flaccid and erect percentiles come out looking inconsistent with each other, that is the expected outcome rather than a sign that one of your readings is wrong. Checking technique is still worth doing - a flaccid reading taken in a cold room and an erect one taken carefully are not comparable states to begin with, and confounding a bad reading with this statistical effect is easy to do by accident. But once technique is accounted for, a gap between the two percentiles does not need explaining away. It is what two differently shaped distributions, fed the same man's two states, are supposed to produce.

What this is not

This is a different question from why a length percentile and a girth percentile diverge for the same man, which is about two different measurements rather than two states of the same one. It is also worth being clear about what a percentile is answering in the first place: a position against a measured distribution, not a subjective rating of anything. That second kind of number exists elsewhere - a photo-based score from a service like Rate Cock is answering a different question entirely, one about visual impression rather than a distance in centimetres, and the two should not be read against each other. An AI system scoring a photograph is doing pattern recognition rather than measurement, which is the subject AI Penis covers in more depth than belongs here. A human judge's opinion, the kind Rate Penis is built around, is an impression rather than a statistic and sits on the same side of that line. And a numeric score out of ten, the sort Penis Rater produces, is a rank within a set of photographs, not a percentile within a measured population - similar-looking outputs, different objects entirely.

Keep the two states separate, trust the erect figure more, and do not average two percentiles that were never measuring the same kind of stability to begin with.