On 3/19/2025 11:06 PM, Fran?ois via groups.io wrote:
Surface area =? length x width.? 27 1/4"? = 27.25"?
5/16 = .315"
7/8" = .875"?
etc, etc.
?
Ah, ok.
You convert the mixed numbers into decimals and you get "? (square inches).
?
5/16 = .315"
7/8" = .875"
?
Do you have these correspondences in your head like the multiplication tables?
Many of them, I do, yes. Many of us who have done very much "homebrewing" of chassis, panels, etc. through the years have sorta memorized those fractions of inches. At the least, you will almost always find charts posted on our workshops showing those fractions in large print so we can easily read it across the shop (when we can't remember one right off the bat). And yes, we also sometimes more-formally call our system of inches and feet the "imperial" measurement system; although more often, we call it the SAE system, which stands for Society of Automotive Engineers, which in years past, used to "champion" the use of inches and feet especially in American automobiles.
Also, approximating converting mm to inches isn't very difficult: just remember that 1 mm is just a little bit less than 0.04" (0.039371", to be more exact). Therefore, you're almost always "close enough" to just use 0.040" as the length of 1 mm. The easy way to imagine 0.04" is that it's just a little bit wider than the gap of olden time automotive spark plugs used to be set at (which was almost always 0.032", sometimes as wide as 0.038"; but more recently with the use of high performance electronic ignition systems, 0.045" or even 0.060").
Thus, 3.5 mm is 3 times 0.04", plus half of another 0.04" or 0.02"; and so 3.5mm = 0.12 + 0.02 = a total of about 0.14", compared to 0.13779535" if you use all the least significant digits of the exact measurement. You would only be off by a bit more than 0.002" which is less than the thickness of a sheet of tissue paper. As the saying goes, "close enough for government work" 8-)
It's fairly easy to imagine 3mm since that's just a little bit less than 1/8" (which is actually 0.125").
Similarly, 1 inch is 2.54cm; just to get in the ballpark, simply use 2.5. Thus, 20" = 20 x 2.5 or 50cm, then that little bit extra is 20 x 0.04 = 0.8. And so adding all that together, you get 50.8cm.
Converting meters in inches becomes more difficult since you have a much larger portion of a whole to account for, 39.38" = 1 meter; that 0.38 becomes significant and makes it a bit more difficult just to "guess" the total. Typically, we might "guess" and add a third plus a bit more for that 0.38. Or sometimes simply multiply by 39.4 instead of 39.38, then subtract just a tiny bit.
For another but wholly different type of "approximate-guesswork": say you want to know approximately how long a hunk of coax is. First you estimate the diameter of the cable when rolled up; then count the number of turns in the coil of cable, and simply multiply that times 3, then add in a bit more (because circumference = diameter times Pi, or 3.14).
Say you have 30 turns of coax in a coil that's roughly 1 foot in diameter; then it's "at least" 30 times 3 feet long = 90 feet, more probably about 95 feet or so altogether when you add in that portion needed for the 0.14. If you cheat and use your calculator instead, it says your coil of cable is actually 94.2 feet long: once again, "close nuff fer guvmint work" 8-).
Steve K0XP
