Gear
The kit list is two items you own
A rigid ruler and a strip of paper reach the same accuracy as anything sold for the purpose; the instrument is not where the accuracy is lost.
Guides on Gear: Every instrument, and what each costs you, Everything you can wrap, and what each reads, Verifying the thing you measure with
The complete kit for this measurement is a rigid ruler with millimetre marks and a strip of paper, and both of those are probably already in the house. Nothing sold specifically for this purpose measures more accurately than that pair, because the accuracy this site cares about is not lost in the instrument to begin with.
What the two items cover
A rigid ruler, pressed to bone, takes length - the requirement is rigidity and a true zero, and an ordinary school or desk ruler with clear millimetre marks meets it. A strip of paper, wrapped around the shaft at the midpoint, marked at the overlap, and laid flat against the same ruler, takes girth as accurately as a cloth tailor's tape - it separates the wrapping from the reading, which is where a tape's own error tends to creep in. Between the two, length and girth are both covered by items that cost nothing to acquire because they are already lying around.
Why a purpose-built product does not add accuracy
Products marketed specifically for this measurement - graduated rings, specialty tapes, gadgets with an app attached - are not measuring anything a plain ruler and a paper strip cannot already reach, because the limiting factor here is technique: pressure, angle, site, and state, not the fineness of an instrument's markings. Where the actual error in this measurement comes from bears this out directly - it is dominated by how carefully the measurement is taken, not by what it is taken with. A product that promises more precision than a ruler and a paper strip is promising precision in a place this measurement cannot use it.
What this is not saying
This is not a repeat of the ruler-versus-tape comparison, which is its own post covering what each instrument is specifically good and bad at, and it is not a ranked list of every instrument this site has covered, which exists separately, instrument by instrument, with each one's typical error. It is one point on its own: the two free items reach the accuracy bar, so nothing else needs buying. None of that applies to a photograph-based estimate, since an image model is not taking a physical reading with a ruler or a tape at all - it is inferring a figure from the picture, and it does not apply to a rating score either, which is a position in a distribution of images rather than a length taken with any instrument. A person's stated opinion needs no ruler or tape - a human judge's impression is not a measurement to begin with - and neither does Rate Cock's rating, which was never trying to be the number a ruler gives you.