Gear
Two scales on one tape
Dual scales exist for the sewing market, not for accuracy; they cost nothing if you commit to one edge, and a factor of 2.54 if you do not.
Guides on Gear: Every instrument, and what each costs you, Everything you can wrap, and what each reads, Verifying the thing you measure with
Most cloth tapes sold for body measurement print centimetres along one edge and inches along the other. Neither scale is more accurate than the other - they are the same physical tape, marked twice.
Where the dual scale comes from
Tailoring and dressmaking are the actual market these tapes are made for, and patterns in that trade come from both metric and imperial traditions depending on where they originate. A single tape that serves both saves the manufacturer from printing two products, so nearly every cloth tape sold for body measurement carries both scales, one down each long edge. Nothing about that design decision has anything to do with measuring girth here - it is a manufacturing convenience from an unrelated industry that happens to be the source of the tool.
What it costs you if you are careless
The scale itself is not the problem. The problem is reading the wrong one without noticing, which happens most easily when the tape has wrapped around and the edge you started reading from is not the edge you finish reading from - dual-scale tapes and the wrong-side misread is the specific way this goes wrong. A number read half in inches and interpreted as centimetres, or vice versa, is not off by a rounding error. It is off by a factor of 2.54, which is large enough that most people would notice the resulting figure looks absurd - but not always, if the true figure happens to fall in a range where the wrong-unit version still looks plausible.
Living with one, safely
Pick one edge before you start and read only that one for the whole session. The literature reports in centimetres, so centimetres is the more useful default if you intend to compare your figure to anything published, including the reference this site leans on throughout: Veale et al. (2015), in BJU International. Write the unit down next to the number in your log, every time, even though it feels redundant in the moment - it is the cheapest possible insurance against a mix-up weeks later when you are comparing readings and cannot remember which edge you used. If you ever convert between the two, do it once from the raw reading rather than at every step, because repeated rounding drifts the figure further than a single conversion does.
None of this is a concern for a subjective score rather than a physical reading - a rating from Rate Cock or a judge's opinion on Rate Penis carries no unit at all, and neither does an estimate from AI Penis or a comparison drawn on Penis Rater - the dual-scale problem is specific to instruments that actually put a number in centimetres or inches in your hand.