Well, the opposite. I was designing a CD shelf the other day; decided upon 12 CD's per compartment. Whipping out the trusty tape-measure, a jewel-box looked to be 3/8 inch. Quick math, 4-1/2 inches per dozen. Easy. Prototyped it. Hmmm, only fits 11. When all else fails, have a look at the official specs: 10 mm. Okay, that's a little more than 3/8 inch, in fact a dozen winds up being something like 4.72 inches.... Back to the drawing board, wondering why I didn't just measure a dozen in the first place. What, and take the fun out of this??!
In searching for the specs, I did find a link to an inventive but highly-impractical origami-like CD-case. I expect a certain newly-minted 20-year-old to teach me how to do this when she is home for Thanksgiving break.