Which tape

Fibreglass-reinforced tape is the one to use: cloth stretches the most, vinyl less, fibreglass the least. A stretched tape spreads its printed marks apart, so it reads a girth slightly short, and the error grows as a cloth or vinyl tape is pulled and stored a few dozen times.

Why the backing material matters

A tape does its job by wrapping snugly around a circumference and holding still long enough to read. If the material itself stretches under the tension you apply while wrapping, the printed scale is no longer telling the truth about the length of tape you have laid down - it stretched between the point where it was printed and the point where you are reading it, and the printed number now understates how much tape is actually wrapped, which reads as a girth number that is slightly low.

Cloth tapes, the traditional sewing kind, stretch the most and continue to stretch further with repeated use and age, so an old cloth tape reads differently from a new one even before anyone has done anything wrong. Vinyl tapes, plastic-coated cloth or plastic film, resist stretch considerably better and hold their calibration longer, which is why they largely replaced cloth in sewing kits. Fibreglass-reinforced tapes, with fine glass fibre woven into the material, barely stretch under normal hand tension and are the most dimensionally stable of the three, which is why they are the standard recommendation for a tape used repeatedly over months.

How much this actually moves a reading

The effect is small on any single reading - a few tenths of a millimetre on a tape in good condition - but it accumulates two ways. It gets worse as a cloth or vinyl tape ages and stretches further, so a reading taken with a six-month-old cloth tape is not directly comparable to one taken with the same tape when it was new. And it sits on top of, rather than instead of, every other source of error in a girth reading, including how snugly the tape is pulled and the fact that a wide tape can lie differently on a tapering shaft than a narrow one does. None of these effects is large individually. Together they are a meaningful share of the ±0.2 cm that a careful girth reading carries as its error bar.

Which one to actually buy

A fibreglass-reinforced sewing tape, sold cheaply for dressmaking, is the practical recommendation. It costs about the same as a cloth or vinyl tape, holds its length far better over repeated use, and needs no special care beyond not leaving it folded under something heavy for months. Stiffness is not a downside here the way it is with a rigid ruler - a girth tape needs to bend into a circle, so a slightly stiffer tape that still wraps cleanly is fine, and the reinforcement that resists stretch does not meaningfully resist the bend. Beyond the backing material, what else to check before buying a tape covers the rest of the checklist, from end-tab wear to how legibly the scale is printed.

If you already own a cloth or vinyl tape and are not measuring for a comparison across months, the difference is small enough not to be worth replacing it over. It becomes worth caring about once you are tracking a girth figure over time, where testing the tape for stretch periodically is the more direct check than guessing at the material's age.

Storage matters more than the label suggests

How a tape is kept between uses affects it almost as much as what it is made of. A tape left rolled tightly under something heavy for months, or hung in a way that keeps constant tension on part of its length, stretches faster than the same tape stored loosely. Humidity affects cloth more than vinyl or fibreglass, since natural fibres absorb moisture and swell or shrink slightly with it, which is one more reason cloth is the least stable of the three over a season rather than a single measurement. Legal metrology treats this as disqualifying for top-grade instruments: the EU Measuring Instruments Directive (2014/32/EU) says measures made from material whose dimensions alter with relative humidity "may only be included in Classes II or III". None of this is a reason to fuss over storage - a tape kept in a drawer, loosely coiled, ages slowly regardless of material - but it explains why two tapes bought on the same day from the same reel can drift apart in accuracy if one lives somewhere warm and damp and the other does not.

Not a claim about size, and not a rating

None of this is a route to a more flattering number - a fibreglass tape reads more consistently, not larger or smaller in any particular direction, and a stretched tape reads short, which is not what anyone hoping for a bigger figure would want anyway. It is also unrelated to what a photograph-based score is doing: Rate Cock produces a rating from an image, not a circumference from a tape, and the same distance from tape material sits between a girth reading and a photo-based score, a human judge's opinion, or what an AI model reads from a picture - none of them touch fabric at all.