Method
False precision
A converted figure with two decimals carries the precision of the original, not of the decimals; quoting it finer than it was measured is a small dishonesty.
Guides on Method: Bone-pressed length, start to finish, Stretched flaccid length, and why the clinics take it, Girth, done the way the studies do it, The measurement glossary
A converted figure is only as precise as the reading it came from, however many decimals it shows. Six inches, converted, is 15.24 cm. That does not mean the measurement is good to a hundredth of a centimetre; it means a calculator can print two decimal places, which is a different fact entirely.
Where the extra digits come from
A conversion multiplies by 2.54, a factor that is exact by definition - the US National Institute of Standards and Technology notes the inch has been "exactly equivalent to 25.4 mm" since 1959. The factor adds no error, but it also adds no precision: the multiplication does not know how the original number was obtained. If the reading was taken to the nearest eighth of an inch, the true precision is about 0.3 cm, and every digit past the first decimal in the converted figure is an artefact of arithmetic rather than a claim about the shaft. Writing 15.24 cm dresses up a coarse reading as a fine one, and anyone comparing it to a properly rounded figure is comparing two different levels of confidence without knowing it.
The direction this distorts things
This is not a rounding error in the arithmetic sense - converting correctly still produces a number with false precision if the input itself was coarse. The distortion runs the same direction every time: more decimals implies more certainty, and a reader has no way to tell a genuinely precise clinical figure from an eighth-of-an-inch estimate wearing two decimal places. Even published decimals need reading with care. Veale et al. (2015), in BJU International, report a pooled erect mean of 13.12 cm, but with a standard deviation of 1.66 cm across 692 men - the second decimal describes an average of many readings, not the precision of any single one.
What to write instead
State the reading to the resolution your instrument and technique actually reached, then convert once and round to match. 6 inches stays 15.2 cm, not 15.24 cm, unless the original reading really was taken to a resolution that supports the second decimal.
This is arithmetic, not judgement, which is worth separating from other numbers that get attached to a measurement. A percentile or a score carries a different kind of uncertainty, the kind a rating service like Rate Cock is built around rather than trying to eliminate, and a subjective opinion from a human reviewer on a site like Rate Penis has no decimal places to misplace in the first place. An AI estimate from a photograph, the sort of figure a tool like AI Penis produces, can display as many digits as the software wants without any of them being measured - the false-precision problem there is worse, not fixed by more decimals. None of that is a reason to relax a length reading's precision to match; if anything it is the argument for keeping the two kinds of number, and the composite scores a site like Penis Rater builds from them, clearly apart.