Gear
The bendy ruler
A flexible curve ruler follows an arc and reads its length, which is precisely the number the literature does not use; useful for curiosity, not for comparison.
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A flexible curve ruler reads the length along a bend, not the straight-line base-to-tip length the studies report, so its number is not comparable to the literature. Laid along the top of the shaft, the bendable drafting strip follows any curve present and reports the arc length: the total distance travelled along the bend.
Why that is a different number
The published literature measures length as a straight line, base to tip, regardless of any curve in the shaft - the chord, not the arc is the rule the standard method follows. The reference data are defined narrowly: the 2023 review by Belladelli et al. in the World Journal of Men's Health only pooled studies measuring from the pubo-penile junction to the tip of the glans on the dorsal surface, and some cohorts, such as Habous et al. (2015), excluded men with Peyronie's disease or congenital curvature outright. An arc is always at least as long as the chord between its two endpoints, and strictly longer whenever there is any curvature at all, so a flexible-ruler reading on a curved shaft will read higher than a correctly taken bone-pressed length, by an amount that grows with how much curve is present. On a shaft with no curvature the two numbers converge, which is part of why the difference is easy to miss unless you already know to look for it.
Roughly how much it inflates a reading
The gap between arc length and chord length grows with the amount of curvature and the length of the segment involved, and it is strictly one-directional: an arc is never shorter than its chord, so a flexible-ruler reading is never an underestimate of the straight-line figure, only ever equal to it or larger. On a shaft with a slight, gentle bend the two numbers sit close together; on a pronounced curve the gap becomes large enough to matter, which is exactly the population for whom the base-to-tip straight-line rule was written in the first place, since a curve is precisely the case where "along the top" and "in a straight line" stop being the same instruction.
What it is good for
As a way to see and record the shape of a curve itself - not the length - a flexible ruler is a reasonable tool: bend it to match, lift it off, and trace or photograph the resulting arc. That is a genuinely different question from length, and answering it does not require pretending the arc length is comparable to anything published.
Not a substitute for the standard method
For an actual length figure, bone-pressed, straight-line, base to tip is the method that produces a number comparable to the literature; a flexible ruler answers a related but different question and should be labelled as such if you record it. None of this touches a rating, either - something like Rate Cock produces a score from a photograph, read the way any photo-based rating is, and a human opinion on Rate Penis is a verdict rather than an arc length, whatever instrument sat nearby. Nor does any of it resemble what AI Penis does with a photo - a separate, image-based inference rather than a physical instrument bent along a curve.