Most rulers are approximately one foot long. But not exactly. I’m wondering why not. Usually there’s a bit extra at each end, such that you can’t just look at the 5 inch point (for example) and that will be five inches from the end of the ruler. It would be a lot more convenient if such were the case. So why the extra bit on each end?
Wear and tear on the ends.
What inspired this thread is a metal ruler I have which is similarly larger than one foot long. Something like this.
The same also goes for plastic rulers that I’ve seen.
All good rulers are flush at one end, so that you can measure from the end. I suppose it must be more expensive to make them like that.
But all good rulers are also made of something tough enough so that the don’t easily change shape and have the ends worn off. I remember that all of my cheap school rulers got the ends worn off in a year or two.
Depends on the purpose of the ruler. If used for measuring then they should run to the end of the edge.
If used for drafting purposes then they are inset. It’s much easier to set a lead tip or ink tip on the -0- mark than trying to set it on the edge of the ruler.
It’s more expensive to make them that way. You can buy machinists rulers with explicit tolerances if you’re willing to pay for it.
I was taught, when using a ruler with a flush end, that you should always start your measurement at the 1 unit mark, and then subtract 1, because rulers made that way always had some wear and tear (or more to the point, you can’t know whether they have any wear and tear).
I suspect this is related to the phenomenon that low-sodium foods cost more than the regular kind.
I splurged $10 or $15 on a nice plastic ruler that starts from 0 (not 0.something) and I love it!
That’s exactly what my industrial arts teacher said as well.
Ah, those glorious old days in my youth of the one foot long ruler.
Believe they are prohibited imports over here now. [sigh]
Mind you the 30cm one which I use now has precisely the same character albeit with slightly different marks. 
I’ve actually wondered about that: What is the length of a standard school-supplies ruler, in sane countries? Here, most rulers now have both scales on them, with 12 inches on one side and 30 cm on the other, but the length is dictated by the 12 inches and 30 cm is just (approximately) what that corresponds to. But I could imagine purely-metric rulers settling in on 30, or 33 1/3, or 25, or whatever.
The Starrett 12" Spring-Tempered Steel Rule with Inch Graduation features a satin chrome finish. Graduations at 8ths, 16ths, quick-reading 32nds, 64ths. 12", 4R Grad, Certification.
Only $44.50.
Rulers here in Canada are all metric, but still a foot long. I think it’s an ergomatic thing more than anything else: a foot is a nice handy length for a desk ruler.
And in contrast, good metal tape measures are a bit short at the 0" end – typically by 1/16 or half that. The metal hook at the end of the tape is always a bit loose, sliding back and forth by an amount equal to the thickness of the hook. This enables you to get an accurate inside or outside measure.
ETA: See:
How to Use a Tape Measure the Right Way
Starrett FTW!
Every machinist (in the imperial units world) has a 6-inch scale in their pocket, used for everything from prying stuff to scraping gunk to … making accurate measurements. The Starrett version is as close to 6 inches long as you need, and has deep engraved graduations to 1/64". The scale is 3/4" wide and has a small scale in 1/32 increments on each end. Being hardened steel, they last forever.
You can safely measure directly from the end.
As opposed to the rulers we had in grade school with the blunt ends and that thin copper-colored strip embedded in the edge. Those definitely were in need of the “start measuring from 1” guideline.
$44.50 FTL!
BTW, in case this wasn’t clear, my point is Post #3 was to cast doubt as to whether the rationale for the extra length was to allow for wear, since this wouldn’t apply to metal rulers (or plastic, for the most part).
That’s probably the cost of the 12 inch version.
The ubiquitous 6 inch one costs $27 on Amazon right now.
Mine has lasted me 30 years, so there’s that.
As far as your doubts on plastic and metal rulers, many of them are silkscreened, and it’s probably not very easy to accurately silkscreen to the end. By having the whole scale in the silkscreen, there is a little bit of wiggle room for printing accuracy.
I’ve got one of these printed circuit board reference rulers and, aside from all sorts of measurements and example pads, it appears to be exactly 12 inches.
I think it’s because of the way we measure things. Needing to measure within an object like and inseam or room, you use a tape measure usually
Either 30 or 15 cm, depending on if it’s supposed to fit in your pencil case or not.