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Can someone provide a rational reason why are fractions used for measuring, instead of decimals or percentages?

To me they don't seem intuitive and I fail to see how can be intuitive to anyone because you are forced to memorize them in order to be able to imagine the size in your head.

There are some exceptions like 1/2 and 3/4, they are easy to imagine because they are so common. But what happens when you deal with uncommon fractions 5/6, 7/8, 6/10, 18/14 etc. ?

On the other hand 0.5, 0.75, 0.45, 0.30, 0.95, 0.anything, 50%, 75%, anything% are very easy to imagine.

katie
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    It depends in which standard your mind is preconditioned... – soosai steven Oct 20 '16 at 08:54
  • Are you telling me that someone with the fraction-preconditioned mind can imagine a 32/27 size in his mind instantly? Sorry, but I find it hard to believe that. – katie Oct 20 '16 at 10:33
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    The fractions used in English Standard measurements are more regular than you think. Only powers of 2 appear in the denominator (e.g. 2nds, 4ths, 8ths, 16ths, 32nds, 64ths) in common building or carpentry measurements using English Standard measurements. 32nds are the smallest increment of an inch on common English Standard tape measures that I've seen. While it is still not as straightforward as metric measurements, it is not impossibly complex to work with either because you really only need to divide/multiply a measurement by 2 a few times in order to compare it to another. – statueuphemism Oct 20 '16 at 10:57
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    Tempted to give +1 just for " ... rational reason... fractions..." :-) . BTW, would you mind providing your age and educational background? A question like this is reasonable for a beginner. – Carl Witthoft Oct 20 '16 at 15:32
  • @katie , 32/27 is hard to evaluate but 27/32 is not that hard because divide a length by 32 is simply the 5th stage of continuous division by 2. You only need to consider the 30th multiplication of the 32th division and subtract 3 portions from it. – soosai steven Oct 20 '16 at 23:50

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The simple answer is: tradition and convention. You could as well ask why people are using inches, not millimeters.

Someone used to the metric system will certainly measure with decimals. It is absolutely normal to do that. 1.5 centimeters, what else would you say... surely not 3/2 or something different.

However, to someone used to imperial measurements, neither the "odd" units nor fractions are unnatural. A 3/8 inch pipe has an immediately recognizable thickness (half that of a 3/4 as it happens). Something the size of 1/16 inch (about 1.5mm) is perfectly imaginable, and makes perfect sense.

I just depends on what you are used to, and also what your tools are designed for. For example, you may have a router bit or a rabbet plane that does a 1/2'' rabbet.
Why would you want to try expressing this in any other way than "half inch", if that is exactly what it is?

Damon
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    1/2 and 3/4 are immediately recognizable to me too, but what about the uncommon fractions? because uncommon decimals and percentages are definitely immediately recognizable. when you say 1.36 mm (or any unit) you can imagine that size in your mind, but is this the same for 15/11? – katie Oct 20 '16 at 10:49
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    Well, you don't use something like 15/11, but e.g. 15/16 (which is 1 - 1/16, easy to imagine). Also note that "real craftsmen" do not measure all that much anyway. You often make a guide or jig, whatever exact size it is, and then mark all pieces with the same jig. Or, you transmit a lenght rather than measuring and re-measuring on the other piece. Or, you divide by half a distance. Or you use a gauge block (the same one) on all pieces. This is, in almost all cases, the much better approach, and it matters little what actual exact size it has -- what matters is the pieces fit. – Damon Oct 20 '16 at 11:26