Years ago, before everyone (or anyone) had electronic calculators, I had a pocket slide rule which I used in secondary school until the first TI-30 cane out.

Recently I dug it out. Here's a photo of one end of it.

Slide rule showing mysterious constant on the C and D scales

As you can see, there's a number $C$ marked at about $1.128$ (times some power of $10$; with a slide rule you supply that yourself) on the C and D scales. Reading across to the A scale, its square is about $1.27$. By the C1 scale (which reads reciprocals of the C scale) its reciprocal is about $0.886$ (times some power of $10$).

The only two special numbers marked are $C$ and $\pi$.

I'm not sure whether it's some frequently used constant that's used (eg) in some branch of engineering, or a number which is useful for some trick for using the slide rule.

Unlike $\pi$, which is marked on most of the scales, this mysterious $C$ only appears on the C and D scales, which are the main ones used for multiplication and division.

If you need me to, I can give more explanation of the various scales on the rule and how calculations are done. That might give some clues as to what $C$ is for.

I'm sure the instructions explained what $C$ was, but I last saw those in the 1970s.

Has anyone any idea what $C$ is and why it would be useful on a slide rule?

  • $\begingroup$ Please tag as appropriate—I'm not sure if the right tags and can't yet create a new "slide-rules" tag. $\endgroup$
    – timtfj
    Nov 30, 2018 at 2:59
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    $\begingroup$ Funnest question in forever. $\endgroup$
    – Randall
    Nov 30, 2018 at 3:03
  • $\begingroup$ Did anyone else here buy their kid a slide rule when he/she went off to engineering school. My daughter ended up with two (we are a family with many engineers). She dutifully sat through my instructions on slide rule use (twice), but I don't think she paid any attention. $\endgroup$
    – Flydog57
    Nov 30, 2018 at 17:34
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    $\begingroup$ Shouldn't this go in retrocomputing.SE? ;-) $\endgroup$
    – shoover
    Nov 30, 2018 at 18:34
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    $\begingroup$ Thanks for the question ! I took my slide rule (I did buy it in 1958) and, thanks to you, I nticed that this $C$ was present ! I never noticed. Shame on me ! $\endgroup$ Dec 5, 2018 at 4:31

1 Answer 1


I found the answer by googling "slide rule markings"! It took me straight to the Glossary of the International Slide Rule Museum, which gives C its own entry:

C - Gauge mark found on the C and D scales denoting $\sqrt{4/\pi} = 1.128$ for calculating the area of a circle and the volume of a cylinder. Place the C mark on the C scale over the diameter of a circle on the D scale. The area of the circle is found above the index on the A scale. If this is the base of a cylinder, without moving the slide, move the cursor to the height of the cylinder on the B scale. The volume is read on the A scale. This gauge mark was rendered obsolete with the advent of multi-lined cursors.

And there is, of course, so much more at that site.

  • 2
    $\begingroup$ Amazing. Like a moron I googled 1.128 and didn't get anywhere. $\endgroup$
    – Randall
    Nov 30, 2018 at 3:11
  • $\begingroup$ I've just tried calculating $\sqrt{4/\pi}$ on the slide rule and the result does line up nicely with the $C$ mark, so this is definitely it!. Thanks! $\endgroup$
    – timtfj
    Nov 30, 2018 at 3:23
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    $\begingroup$ The museum is amazing! I feel much younger now (maybe that such a musem can make me feel younger should make me feel old...) $\endgroup$
    – Francesco
    Nov 30, 2018 at 5:32
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    $\begingroup$ @Francesco "It makes me feel like a kid again, and that just reminds me how long it's been." $\endgroup$
    – Nic
    Nov 30, 2018 at 22:01
  • $\begingroup$ I wish I had that mark on my slide rule. I am always calculating the volume of a cylinder at the beer joint. But calculation of the volume of a Starbucks cup is best done with Simpson's rule. $\endgroup$ Dec 5, 2018 at 4:10

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