Why girth is not one number

Length is a single number because a shaft has one base and one tip. Girth is not a single number, because circumference changes along the length of the shaft, and it does not change the same way from one person to the next.

Three shapes, not one

Picture the shaft in profile from base to tip. Some run roughly even the whole way, some taper narrower toward the tip, and some are widest somewhere in the middle rather than at either end.

Even:      ====================
Tapering:  ======---------.....
Mid-bulge: ----========--------

None of these is unusual, and none is "correct." They are just different profiles, and each one means a girth reading taken at the base, the tip, and the middle will disagree with itself by a different amount depending on which profile is in front of you. On an even shaft the three readings sit close together. On a tapering or mid-bulge shaft they can differ by a centimetre or more, which is a large gap next to the roughly ±0.2 cm that careful technique alone contributes.

Why this makes site the whole argument

A length figure is comparable to another length figure because both are measured the same way, bone-pressed, base to tip, along the top. A girth figure is only comparable to another girth figure if both were taken at the same site on the shaft, because "girth" without a site is really three or more different measurements sharing one word. Reporting "16 cm" with no site attached tells a reader almost nothing, because 16 cm at the base and 16 cm at the tip can belong to very differently shaped shafts.

This is also why a girth figure needs the site recorded in the measurement log alongside the instrument and the state, the same way a length figure needs bone-pressed and dorsal recorded. A number without its conditions is not really data, on either side of the ruler.

Why an average number would not help

One tempting fix is to imagine an "average girth" that captures the whole shaft rather than a single site - some blend of base, middle, and tip. This does not actually solve the comparability problem, for two reasons. First, no two people's profiles blend the same way, so an average computed from three sites on a tapering shaft is not comparable to an average from three sites on a mid-bulge shaft, even if the arithmetic mean happens to land on the same number. Second, and more practically, nobody measuring at home is going to take three careful girth readings when one will do, and asking for three multiplies the chances of a bad reading rather than averaging them away. A single, fixed, named site is simpler to take and, because everyone is taking the same one, is actually more comparable across people than an averaged figure would be.

The convention exists for a reason

The published literature settles on one site as the reported convention rather than leaving it open, precisely because taper varies too much between people to average sensibly otherwise. Which site that is, and how to find it on your own shaft, is a separate, more mechanical question from the anatomical point being made here - the point here is only that site has to be fixed at all, not which one gets fixed. The convention also settles a sequencing question worth stating plainly - why length comes before girth in how this site orders its own method pages, and in the studies it draws on.

What this has nothing to do with

None of this is about which shape is larger or better. A taper is a shape, not a score, and this site does not rank shapes any more than it ranks lengths. If the interest is in how a shape reads to another person rather than in its circumference, that is a subjective judgement made by a viewer or a rating tool - Rate Cock does that kind of assessment from a photograph - and it is answering a different question from the one a tape measure answers, which is why the two numbers should never be quoted as if they were the same currency. A photo-rating score works from the same photograph and the same logic, and neither it nor a tape wraps the other's answer. A human reviewer forming an impression of a photograph is doing the same kind of subjective work, and Rate Penis is explicit that a judge's opinion is exactly that, not a substitute for a site-specific circumference. An AI model scoring the same photograph is pattern-matching against a rubric rather than resolving a circumference from pixels, which AI Penis explains in terms of what the model can and cannot see in an image.

The rule in one line

A girth figure without a site is an anecdote. Fix the site, name it next to the number, and the reading finally means something to anyone comparing it to their own, or to a published one.