Gear
Millimetres or sixteenths
A millimetre scale resolves finer than sixteenths of an inch and is easier to interpolate; if your ruler has both, read the metric side.
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If your ruler has both a centimetre scale and an inch scale, use the centimetre one. A millimetre scale resolves finer, and it is easier to read a position between two marks on it, which is a real and measurable difference rather than a matter of habit.
The two scales, side by side
A millimetre is 0.1 cm. The inch is exactly 25.4 mm - the value, effective July 1, 1959, that the US National Institute of Standards and Technology (NIST) records - so every conversion in the table below is exact arithmetic before rounding. The smallest common division on an imperial ruler is a sixteenth of an inch, which is about 1.59 mm. That means a millimetre scale is graduated roughly one and a half times finer than a sixteenth-inch scale, and the gap widens if the ruler only offers eighths, which is common on cheaper rulers.
| Imperial division | Millimetre equivalent |
|---|---|
| 1/2 inch | 12.70 mm |
| 1/4 inch | 6.35 mm |
| 1/8 inch | 3.18 mm |
| 1/16 inch | 1.59 mm |
| 1 mm | 1.00 mm |
The comparison is not close. A sixteenth of an inch is a coarser division than a millimetre by a comfortable margin, and going finer than sixteenths on an imperial ruler is uncommon outside specialist tools.
Why interpolation is also easier on millimetres
Reading between marks - judging that an edge falls roughly a third of the way from one line to the next - is something everyone does whether they realise it or not, because a true measurement almost never lands exactly on a printed mark. Interpolating in tenths of a millimetre is a small, natural mental step from a scale already graduated in millimetres. Interpolating a sixteenth of an inch into finer fractions asks you to subdivide a fraction that most people do not carry around as an intuitive unit, and it is easy to misjudge by a whole sixteenth in the process, which is 1.59 mm - a meaningful chunk of the ±0.3 cm error budget that careful bone-pressed technique already carries.
A dual-scale ruler is not a compromise
Most rulers sold with both scales print them on opposite edges of the same face, or on opposite faces, and reading one does not cost you anything relative to the other beyond the choice of which edge to sight along. The one real hazard with a dual-scale ruler is glancing at the wrong scale and reading a number off the inch side while thinking in centimetres, which produces an error of roughly 2.54x rather than a small one - worth being deliberate about which edge you are looking at before you read, not after.
There is no argument here for buying a metric-only ruler if you already own a dual-scale one, or for converting a childhood habit of thinking in inches. The recommendation is narrower: when you sit down to take a bone-pressed reading, read the millimetre side of whatever ruler is in your hand, and record the result in centimetres rather than converting partway through and rounding twice.
Why the habit is worth breaking deliberately
Most people who grew up with inches did not choose that unit for measurement; it is simply what the rulers and tape measures around them were marked in. Switching which edge you read for one specific reading is a small, one-time decision rather than a broader habit change, and it does not require converting how you think about length day to day. The only discipline it takes is picking the metric edge before you start reading, not partway through, because switching scales mid-session is how a sixteenth gets misread as a millimetre or the reverse.
Writing the unit down alongside the figure removes any doubt later. A number on its own, months afterward, can be ambiguous about which scale it came from if the habit was inconsistent; a number labelled "cm" or paired with the method line convention never is.
What this does not settle
This is a resolution comparison between two scales printed on the same kind of instrument, not a comment on which unit is more "correct" to report in, and not a comment on converting a finished reading between units after the fact, which carries its own separate rounding risk worth reading about on its own.
None of this touches a subjective score. Rate Cock does not read either scale, because its output is a judgement about a photograph rather than a length in any unit, and the contrast is the same one that separates a human judge's opinion or a photo-based rating from a ruler reading - and the same one that separates what an AI model can infer from an image from an actual measured distance, since neither an inch scale nor a millimetre one appears anywhere in a photograph without something placed there for scale.