If you buy a tape

Look for a narrow, fibreglass-reinforced tape with its zero printed at the very end and millimetre marks, then check it against a ruler on arrival. None of that costs extra. A strip of paper laid against a ruler measures girth about as well, so buying a tape is optional rather than necessary.

Fibreglass, not cloth or vinyl

Material determines how much a tape stretches under the tension of a snug wrap, and stretch is a direct source of a low reading, since a stretched tape spreads each printed centimetre over slightly more than a centimetre of shaft. Cloth stretches most, vinyl less, fibreglass-reinforced tapes least - fibreglass is the specification to look for on the packaging or the listing, and it usually costs the same as the alternatives. Regulators draw the same line: the EU Measuring Instruments Directive (2014/32/EU) says measures made from material whose dimensions alter with humidity "may only be included in Classes II or III", never its top accuracy class.

Narrow, not wide

A wide tape, the kind sold for body measurements in some kits, averages its reading over more of the shaft's length than a narrow one does, which matters because the shaft tapers. A narrow tape follows a single site more precisely; anything under about 7 mm wide is a reasonable target, and tailor's tapes are usually already this narrow by default.

A printed zero at the very end

Look for a tape where the "0" is printed right at the physical tip, not set back inside a metal end tab. Many sewing tapes start their scale inside the tab rather than at the tape's true end, and using the tab's edge as zero instead of the printed mark shifts every reading by the tab's length. Check listing photos for this before buying if you can, since it is not always stated in the description.

Millimetre graduations on both faces

A tape marked in centimetres and millimetres on one side and inches on the other is fine, as long as the metric side has millimetre marks rather than only centimetre lines with no subdivisions. Millimetre resolution is the finer of the two common metric conventions and the one worth reading, and committing to it removes a small but real source of misreading that comes from glancing at the wrong scale on a dual-printed tape.

Skip anything sold specifically "for this"

A handful of listings market a tape explicitly for this measurement rather than as a general tailor's or body tape, usually at a price several times an ordinary tailor's tape, with no different specification underneath the packaging. The instrument does not know what it is wrapped around, and a tape that meets the specifications above performs identically whether it was bought from a fabric shop or from a listing aimed squarely at this use. Paying more for the framing rather than the fibreglass, the width, or the zero mark is the one purchasing mistake worth naming directly, because it is the easiest one to make without noticing - the price looks like it is buying accuracy, and it is buying nothing measurable.

An avoidable failure: the steel retractable tape

Worth ruling out explicitly, because it is the tape most households already own. A steel retractable tape resists bending into a tight circle, so it sits slightly proud of the surface rather than lying flat against it, and its hooked end has a small amount of deliberate play built in to compensate for hooking over an edge versus butting against one - both features are correct for a carpenter's tape and both work against a snug, accurate wrap here. If a tailor's tape is not available and a retractable one is the only option in the house, a strip of paper measured against the retractable tape's rigid blade is a better use of it than wrapping the steel itself.

Check it against a ruler on arrival

This is the step that actually matters most, and it is free. Lay the new tape flat along a rigid ruler you already trust, at the 10 cm and 20 cm marks, and confirm they agree to within half a millimetre - the most the directive's Class II allows over any length up to a metre. This same check, repeated periodically, is what catches stretch developing over the tape's working life - a tape that passed the check on day one is not guaranteed to pass it a year later, so the habit is worth keeping rather than treating as a one-time unboxing step.

What buying a tape does not get you

None of these specifications close the gap between a measured length and a rating. A tape correctly specified and correctly checked still only produces a number in centimetres - it has nothing to say about how that shaft looks in a photograph, which is the separate question that Rate Cock answers, or that an image model attempts, or that a numeric photo score or a human judge's read each answer in their own way. Buying the right tape is a small, solvable problem with a checklist. Deciding which of these different kinds of number you actually wanted is a separate one, worth settling first.