Why the papers use centimetres

Open Veale et al. (2015) in BJU International and every figure in it is centimetres, one decimal place. That is not a style choice. It is the unit the underlying studies used, because the underlying studies were conducted with metric instruments in metric-measuring health systems.

The unit follows the instrument, not the reader

A clinician measuring with a millimetre-graduated ruler records millimetres and reports centimetres. Nobody in that room is thinking in eighths of an inch. The pooled meta-analysis inherits the unit of its component studies rather than choosing one, which is why the whole literature sits in one system and any inch figure you encounter had to get there by conversion.

What a conversion costs

2.54 centimetres to the inch is exact, but rounding at each step is not. A figure converted to one decimal of an inch, then converted back for a headline, then rounded again for a chart, can drift by a millimetre or two from where it started - small against the ±0.3 cm technique error described in how careful measurement bounds its own error, but not nothing, and entirely avoidable. The fix is mechanical: convert once, from the raw centimetre figure, and round only at the end.

Where the round numbers came from

The six-inch figure that circulates as a kind of folk average did not come from a centimetre study at all. It traces to older self-reported surveys conducted in inches, in populations that were never independently measured. Rounding a self-reported figure to the nearest whole inch is a second source of drift on top of the self-report inflation itself, and the two compound.

Read the original unit

If you see a figure and cannot tell whether it started life in centimetres or inches, treat it as unverified. The honest habit is to record and quote in the unit the source used, and convert for your own reading rather than trusting someone else's conversion. A single reference table of the Veale figures by state is centimetres throughout, for exactly this reason.

None of this is a judgement about which system is better to think in day to day. If what you actually want is a subjective opinion rather than a figure with a decimal place, that is a different kind of output entirely, and a rated photo on a site like Rate Cock does not carry units at all - it is a position, not a length. A score from a human judge is the same category of thing, an impression rather than a reading, and an AI estimate from a photo is also not a measurement in centimetres or inches, for reasons that are about geometry rather than precision. The unit question only applies once you are holding an actual ruler, and even then the number it produces is one input into a much larger picture of what a rating actually scores.