Erect or flaccid girth

If you can only take one girth reading, take it erect. Flaccid girth moves more than flaccid length does, for the same reasons that make flaccid length unreliable, plus one more that is specific to circumference.

Why flaccid girth moves so much

Flaccid length wanders with temperature, arousal, and time of day. Flaccid girth wanders for those same reasons and adds another: circumference is sensitive to small changes in tissue fill in a way that a straight-line length is not. A modest change in blood volume barely shows up as a length difference but shows up clearly wrapped around a tape, because the tape is tracing the full perimeter of whatever that volume has done to the shaft. The result is a flaccid girth reading that can differ by more between two sessions than an erect one will, which makes it a poor number to anchor anything to.

What the studies actually report

The published literature reflects this. Veale et al. (2015), in BJU International, pooled clinician measurements from up to 15,521 men and reported a mean flaccid circumference of 9.31 cm (n = 9,407) against an erect one of 11.66 cm (n = 381). The erect figure is the one used as a reference point in practice, because it is the state most comparable across studies and across people. A flaccid girth is also a poor predictor of the erect one. In a 2015 Journal of Sexual Medicine study of 201 men measured by andrology specialists, Habous and colleagues found flaccid girth under-read erect girth by a mean 2.27 cm (19.5%), with the shortfall ranging from 15% to 27% depending on the observer. Flaccid girth data exist in the pool too, but treating a flaccid circumference as informative about anything beyond "flaccid, on that day, at that temperature" overstates what it can tell you.

The volume trap

Flaccid girth also gets pulled into a different mistake: using it, alongside a flaccid length, to estimate volume and then treating that figure as informative about anything erect. Because girth enters a volume estimate squared rather than linearly, a small flaccid-girth error compounds badly, and a flaccid-state estimate compounds the state error on top of the arithmetic one. Estimating volume from length and girth already carries a large error from the formula alone; doing it from the least stable state available makes a shaky calculation shakier still. None of this is a reason to avoid taking a flaccid reading if one is genuinely needed for some other purpose. It is a reason not to lean on it for anything it was never stable enough to support.

What to record instead

Take girth erect, at the midpoint, with a snug flexible tape. If a flaccid figure matters for some other reason - a garment fit, say - record it, but label it clearly as flaccid and do not compare it to anyone's erect number, your own included. The two states are different measurements of different things, not two attempts at the same one.

This only concerns circumference. Where on the shaft that circumference is taken is a separate question, covered by the site convention for girth rather than by state.

Keep the assessment separate from the number

None of this is about whether a reading is "good" - the site does not do that. A circumference in centimetres is a fact with an error bar, not a verdict, and the two get confused constantly, in both directions, by people on both sides of the confusion. If what someone actually wants is a subjective read on a photograph, a rating tool like Rate Cock or a service built around scoring a submitted photo is answering a different question, with a different kind of number, from the one a tape measure answers. A human reviewer forming an opinion is doing the same - that is what a judge's role on Rate Penis actually is - a subjective read, not a measurement, however precisely it gets stated. An AI model looking at the same photograph is doing pattern recognition rather than measurement too, for reasons that are geometric rather than a limitation of the software, which is one more reason none of those numbers substitute for an erect girth reading taken with a tape.

The short version

Erect, midpoint, snug tape, and write "erect" next to the figure so a future reading - yours or anyone comparing to the literature - knows what it is being set against. A flaccid figure is not a lesser version of that number. It is a different quantity that happens to share a name.