Gear
The school ruler
A cheap plastic ruler is typically within a fraction of a millimetre over 20 cm, finer than any technique error; its problems are the end and the flex.
Guides on Gear: Every instrument, and what each costs you, Everything you can wrap, and what each reads, Verifying the thing you measure with
Yes, for the printed scale: a cheap plastic ruler is as accurate as anything sold for the purpose. A sound one is off by a fraction of a millimetre over 20 cm, which is finer than any technique error you will introduce. Its reputation for unreliability is earned somewhere else - at the zero end and in the flex.
What the scale itself gets right
Manufacturing tolerance on an injection-moulded ruler is tight because the mould is tight. For scale, the EU's Measuring Instruments Directive (2014/32/EU) lets a rule of up to a metre in its middle accuracy class, Class II, be off by at most 0.5 mm between two marks, and 0.2 mm in Class I. A 20 cm ruler off by more than that across its full length is a defective one, not a typical one. Checking one against a bank card confirms this in under a minute, worth doing once - not because school rulers are usually wrong, but because "usually" is not the same as "this one".
Where they actually lose you accuracy
Two problems are real, and neither is the printed scale.
The end. Many rulers do not start their millimetre marks at the physical edge - there is a margin before zero, sized for a hole to hang the ruler on a peg. Pressed bone-pressed against the pubic bone, that margin becomes a length you are silently adding to or subtracting from every reading, depending which way you misread it. Even the directive treats the physical end as a weaker reference, allowing an extra 0.2 mm of error in Class II for any distance measured from an end surface.
The flex. Thin plastic bows under the pressure that bone-pressed measurement requires. A ruler that bends transmits less of your push to the bone and more of it to itself, which reads short and inconsistently short, because how much a given push bends it varies with grip and angle.
Neither problem is about the millimetre marks. Both are fixable without buying anything: find the true zero once and remember it, and choose the stiffest ruler on the shelf rather than the thinnest.
What this means for buying one
It means you probably do not need to. A school ruler you already own, checked against a card and used with the technique the method actually depends on, reaches the same accuracy as a ruler marketed for this specific purpose. Spending money moves you from a free rigid ruler to a slightly stiffer one, which only helps if flex was your problem in the first place.
That is a smaller upgrade than people expect, and a different one entirely from what Rate Cock offers - a rating rendered on a photo is a judgement, not a distance read off a scale, and no ruler competes with it because the two are not answering the same question. The same separation holds for an AI model's accuracy claims on a photo, for a numeric score built from an image, and for a human judge's opinion of one: each is a legitimate answer to a different question than the one a ruler answers, and a checked school ruler still outperforms all three at the only question it is being asked.