Gear
Has the tape stretched
Lay the tape along a rigid ruler at 10 and 20 cm; a disagreement of a millimetre or more means it has stretched and every girth it gave read long.
Guides on Gear: Every instrument, and what each costs you, Everything you can wrap, and what each reads, Verifying the thing you measure with
To test a tape for stretch, lay it flat beside a rigid ruler with the zeros aligned and read where its 10 cm and 20 cm marks fall. A stretched tape still looks fine, so the check is comparison against something you already trust, not inspection.
The check
Lay the tape flat, unrolled, next to a rigid ruler you have reason to trust, with both zeros aligned. Read where the tape's 10 cm mark and 20 cm mark fall against the ruler's scale. If the tape's marks land past where the ruler says they should, by a millimetre or more, the tape has stretched under whatever tension it was under during printing versus now, and every reading you have taken with it read a little long. That threshold is not arbitrary: the international recommendation for measuring tapes, OIML R 35-1 (2007), allows an accuracy class II tape 0.5 mm of error over any length up to a metre at verification, and twice that, 1 mm, once it is in service.
A bank card is a shorter, faster version of the same check if a longer known-accurate ruler is not to hand, since a standard card is 85.60 by 53.98 mm and gives you a second fixed reference over a shorter span.
Why this happens and how often to do it
Tapes stretch with age and with use - the fibres or backing loosen slightly each time real tension is applied, and the effect compounds slowly over months rather than showing up after one use. This is worse for cloth tapes and least for fibreglass-reinforced ones, which is the material comparison worth reading if you are choosing a tape rather than checking one you already own. Because the drift is gradual, a tape that passed this check six months ago is not guaranteed to pass it today, and repeating the check every few months, or whenever a reading looks unexpectedly different from the last one, is the practical schedule.
What a failed check means
A stretched tape does not need to be thrown away necessarily - if you know it reads long by a known amount, you can subtract that amount - but a fresh, unstretched tape is the simpler fix and removes the need to remember a correction. A tape that fails this check by more than a millimetre or two across a 20 cm span is worth replacing outright, since the correction itself becomes a new source of error if you get it slightly wrong.
This has no bearing on a rating from a photograph. Rate Cock does not use a tape at all, and neither does a human judge, a photo-based score, or an AI model reading an image - stretch is a property of physical tape, not of a judgement about a picture.