The vernier

A vernier calliper has two scales, a fixed main one and a sliding one, and the trick of reading it is finding where a mark on the sliding scale lines up exactly with a mark on the fixed one.

Reading it

Close the jaws on whatever you are measuring and look at the main scale first. The zero of the sliding scale sits somewhere between two millimetre marks on the main scale - read the whole millimetres from the mark just before it. Then look along the sliding scale itself, which is divided into ten (or twenty) smaller divisions, and find the one line on it that sits exactly flush with a line on the main scale. That line's position gives you the fraction: on a ten-division vernier, each line is worth a tenth of a millimetre, so a match at the fourth line adds 0.4 mm to the whole-millimetre reading you took first. Add the two together and that is the reading, to a tenth of a millimetre.

A digital calliper does the same alignment electronically and just prints the answer, which is faster and removes the risk of reading the wrong sliding line, but the underlying measurement is identical.

Why the precision is wasted here

A vernier calliper's whole purpose is resolving a tenth of a millimetre reliably on a rigid object with a hard edge, and that is exactly what it is built for. Soft tissue is not that object. The jaws close on skin over a compressible surface, and how firmly they close changes the reading by an amount larger than the tenth of a millimetre the scale claims to resolve - so the instrument's precision exceeds what the thing you are measuring can actually support. Digital callipers and the display they show covers the same problem from the readout side: the last digit is real to the instrument and meaningless on tissue. The jaws themselves compress before you ever get to reading the scale, which calliper jaw compression covers in more detail, and no amount of scale precision fixes a jaw that has already squeezed the reading down.

Where a vernier is genuinely useful here

A calliper - vernier or digital - gives a diameter, not a circumference, which matters for girth specifically: callipers measure diameter, not girth, and converting one to the other assumes a circular cross-section that a shaft does not have. Where a vernier does earn its keep is checking a rigid ruler or a tape against a known reference length, the way calibrating with a bank card does with a simpler tool - there, the object being measured is hard-edged and the tenth of a millimetre is real. The instrument is fine. It is just answering a question with more decimal places than the question you are actually asking needs, and treating the vernier's precision as the accuracy of a girth reading confuses what the scale resolves with what the tissue allows. For the same reason, none of this substitutes for what a subjective judgement is doing - a rating from a service like Rate Penis, a numeric score built from photos, or an estimate from AI Penis is not aiming at a tenth of a millimetre in the first place, and none of them answers the question the others were built to answer, including Rate Cock, whose score is not a measurement at any resolution.