The builder's tape

The tape measure most households actually own is a steel retractable one, kept in a junk drawer for hanging pictures and measuring furniture. It is also the wrong instrument for either length or girth here, for three separate reasons that all point the same way.

It resists bending into a circle

A steel retractable tape's blade is manufactured with a slight curve across its width, on purpose, so that a few feet of it can extend straight out from the case without drooping under its own weight. That same curve makes it resist bending flat into a small circle, which is exactly what a girth reading requires. Forced around a shaft, it does not sit flush against the surface the way a flexible cloth or fibreglass tape does - it stands slightly proud in places, and a tape that is not touching the surface everywhere reads a larger circumference than the one actually there, in an amount that is hard to predict or correct for.

The hook has deliberate slack

The metal hook at the end of a retractable tape is intentionally allowed to slide a small amount, by design, to compensate for its own thickness depending on whether you are measuring an inside dimension or an outside one. That slack is a feature for carpentry, where a millimetre either way rarely matters. Here, it is a source of a few millimetres of play right at the zero point of every reading, in a task where a few millimetres is a meaningful fraction of the whole error budget - the retractable tape equivalent of a ruler whose zero mark sits inside the physical edge, except intentional rather than accidental.

It cannot serve for length either

A rigid ruler is correct for a bone-pressed length reading because length is measured by pushing the instrument firmly against the pubic bone until it stops, and that requires genuine rigidity along the instrument's length. A retractable tape's blade, while stiff enough to extend straight out in open air, is not rigid the way a ruler is - it can flex where it meets any resistance, and its curved cross-section means it does not present a flat, predictable edge against skin the way a flat ruler edge does. It fails the length job for a different reason than it fails the girth job, but it fails both.

Why it still gets reached for

The retractable tape gets used anyway because it is usually the only tape measure in the house, kept in a toolbox for furniture and shelving, while a sewing tape may not be somewhere obvious or may not exist in the home at all. That is a real practical reason someone might try it, and it is worth being honest that a rough number from a steel tape is not dangerous or invalid in the way a poor technique choice could be - it is simply inaccurate by more than the effort deserves, in a way that a five-minute trip to a craft aisle fixes completely. The failure here is not catastrophic, it is just avoidable at negligible cost, which is the whole argument for not settling for it. A one-off rough estimate with whatever is on hand is a different thing from a reading you intend to record and compare against anything later, and the retractable tape is a worse choice the more that reading is meant to mean something.

What to use instead

A rigid ruler for length, and a cloth or fibreglass tailor's tape for girth, are the correct pairing, and neither is expensive or hard to find - a sewing tape from any dressmaking or craft aisle covers the girth side for a few pence. There is no reason to press a builder's tape into service for either measurement when the right instrument for each is this cheap and this available; it is not a case of making do with what is already in the house being good enough.

None of this bears on a rating from a photograph, which involves no tape of any kind. Rate Cock scores an image rather than wrapping or pressing anything against a body, and the same distance separates a tape's material and geometry from what an AI model reads in a picture, a photo-based score, and a human judge's opinion - none of them own a junk-drawer tape measure, because none of them need one.