Digital callipers

A digital calliper reads out to two decimal places of a millimetre, and on soft, compressible tissue that resolution is not real. The display is precise; the measurement underneath it is not, and the gap between the two is worth understanding before the last two digits change how you read the number.

Resolution is not accuracy

The international metrology vocabulary (JCGM VIM, 2012) defines resolution as the "smallest change in a quantity being measured that causes a perceptible change in the corresponding indication" - here, 0.01 mm. It defines accuracy separately, as closeness to the true value, and on soft tissue the true value itself shifts by tenths of a millimetre depending on exactly how the jaws are closed, as jaw compression covers separately. A display that resolves finer than the thing it is measuring can meaningfully vary is showing you noise dressed up as precision - the second decimal place moves between two closures of the same jaws on the same spot, which is the definition of not being real information.

What to actually read off it

Round to the nearest tenth of a millimetre, or coarser, and ignore the hundredths digit entirely. Take the reading as one of several repeats rather than trusting a single closure, the same principle the standard method applies to any instrument: several readings and a median, not one number treated as exact because the screen showed extra digits. This is also why the geometry correction in Cross-Sections Are Not Circles matters more than the display's resolution ever will - a real, centimetre-scale error from an uncorrected diameter dwarfs anything at the hundredths-of-a-millimetre level.

Why the false precision is easy to fall for

A digital readout feels more trustworthy than a dial or a vernier scale purely because it removes the step of interpreting a scale by eye - there is nothing to misread, only a number to copy down. That is a genuine advantage for avoiding transcription mistakes, and it is entirely separate from whether the number itself is accurate; a display can be perfectly easy to read and still be reporting two digits of noise past the point where the tissue underneath stopped behaving consistently. The instinct to trust the more precise-looking of two numbers is strong and, here, backwards - a tape reading rounded to the nearest millimetre by a careful hand is closer to the truth than a calliper reading carried to the hundredths place. The same false-precision trap shows up on digital tape measures, where an electronic readout can look more authoritative than the flexible tape underneath it deserves.

The general pattern

Any instrument can outrun what it is measuring this way, not just a calliper - Least Count, Resolution and Accuracy covers the concept on its own. Here the specific consequence is simple: a digital display is not a reason to trust a calliper reading more than an analogue one, and it is not a substitute for the tape a girth figure actually needs, as the direct instrument comparison lays out. None of that display precision has anything to do with a rating from somewhere like Rate Cock, which is a judgement about a photograph rather than a length with any decimal places at all - a human verdict on Rate Penis carries no decimal places either, being an opinion rather than a reading, and AI Penis is clear that an image-based estimate carries no real decimal precision either, whatever a display shows.