Ruler material

Wood, plastic and steel rulers all measure a bone-pressed length to essentially the same accuracy, provided each one is stiff enough not to flex. The material itself is not where accuracy is won or lost here.

The one real failure: bowing

A rigid ruler is pressed firmly against the pubic bone, and "firmly" is the whole point of the technique - it is what removes the fat pad as a variable. A thin, cheap plastic ruler can bow slightly under that pressure, curving away from a true straight line between the two points you care about. The bow is usually well under a millimetre, but it works in the same direction as not pressing hard enough, and it stacks with technique error rather than cancelling it. In metrology terms it is a systematic error, one that "in replicate measurements remains constant or varies in a predictable manner", so repeating the reading with the same ruler does not average it out. A ruler that is thick enough, or reinforced along its length, does not do this. Wood and steel essentially never bow at hand pressure. A cheap flexible plastic ruler, the kind that snaps back with a twang when you let go of it, is the one to leave in the drawer for this purpose.

Thermal expansion is not a real concern

Steel, wood and plastic all expand and contract slightly with temperature, and in principle a ruler measured cold reads a hair different from the same ruler warm. At room-temperature swings and a scale of fifteen to twenty centimetres, the effect is a small fraction of a millimetre - far below the ±0.3 cm that careful technique already carries as its error bar. It is a real physical effect and an irrelevant one here.

What actually distinguishes rulers

Stiffness, a square end, and markings you can read at a glance matter more than what the ruler is made of. Choosing a rigid ruler covers the fuller checklist; this post is only about material. Length matters more than material for a straightforward reason - how long a ruler needs to be determines whether a reading near the top of the scale is even usable, regardless of what the ruler is made from. Once stiffness is settled, where the zero mark actually sits is a bigger source of error than any material choice, and worth checking on whichever ruler you end up using.

Not a verdict on which is "best"

There is no material recommendation here beyond "not floppy," because none is warranted. A steel engineer's rule, a wooden metre stick cut down, and a stiff plastic school ruler all reach the same practical accuracy once the bowing problem is ruled out. That is a narrower and more useful claim than a "best ruler" list, and it is the honest one.

This sits well outside anything Rate Cock does - a rating is a judgement about a photograph, not a length taken with an instrument of any material, and the two do not compete for the same question. It is also outside what a human judge, a photo-based score, or an AI model reading an image can speak to, since none of them is holding a ruler against anything.