I have a model with a suspect declared scale. How do I establish the scale of a model when I can measure it and compare it to the size of the real one? 
Measure a distance of the model. Divide the same measurement of the real one by the measurement of the model. Make sure both are in the same unitsā¦inches, centimeters, whatever.
Exampleā¦Real wingspan 252 centimeters. Model wingspan is 3.5 centimeters. 252 / 3.5 = 72. Model is 1/72 scale
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Thanks!! 
Iāll be honest, I donāt know if that is exactly correct, but that is what I was led to believe.
Did you hear about the constipated mathematician? He worked it out with a pencil. 
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I just did that with that Tarhe I ordered and when I kept getting 32ā I kept thinking I was doing something wrong!
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It is correct.
The scale factor is the quota between the large and the small.
That is why it is usually written as 1/number (1/72, 1/48 et.c).
To get the size of the small divide the measurement of the large by the
scale ānumberā, 252 divided by 72 is equal to 3.5.
Dividing by 72 can also be written as a multiplication with a number smaller
than one, 1/72 is 0.01388888ā¦ so we can take the 252 cm wingspan and multiply
it by 0.013888888ā¦ and round off the result to 3.5 (depends on how many 8ās we use
on the calculator, with only 4 8ās the result is 3.4999776 which is close enough to 3.5
on my vernier calipers, less than the thickness of a layer of paint).
Why oh why do we use these awkward scales? In a metric world it would be easier to use
1/50, 1/10, 1/100, 1/20 in a pinch.
Well, it so happens that the inches and feet caused this.
There is twelve inches to a foot so if we let each foot on the real thing become one inch
on the model we get 1/12 scale, half inch ā 1/24, quarter inch ā 1/48, eighth inch ā 1/96,
sixteenth inch 1/192. Just grab any inch based ruler and you have a scale ruler, all the inc fractions are already there, the thirtyseconds markings on the ruler equals 1/384.
There are some odd scales though, 1/72 is one sixth of an inch per foot. This is usually not shown on standard rulers, one third of an inch would be 1/36 (rounded off to 1/35???).
No idea about the origins of 1/32.
Railway model scales or gauges:
https://www.world-of-railways.co.uk/techniques/model-railway-scales-and-gauges-explained/
(a brief history of Making Things Difficult ā¦)
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I use this app for scaling and for much more. It is an excellent app.
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I donāt understand how anyone who already has enough wherewithal to build models wouldnāt know that is correct, or at least be able to prove it to themselves in mere seconds.
Nothing uses 1/384 scale.
The older way of expressing scale was inches per inch or inches per foot. One inch = 6 feet is 1/72. Your other example, 1/32, is a common fraction, but is was usually expressed as 3/8 inch = 1 foot, a common architectural drawing scale.
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With regard to the discourse on division, apropos to me is I think only in the Americanized imperial system, canāt help it. Work aside where Metric or SI prevail for precision I go nowhere without a conversion capability. So at days end for modeling, translating between measurement systems is essential. I have rulers for different scales in different units. The challenge though is to obtain an actual measurement of something say on a real tank, say the road wheel diameter, you can then on the tank scale anything up or down in your system of choice and with proper rounding achieve proportional precision.
Not quite as rare as hens teeth but not very common as a scale.
Revell did use some rather odd scales (adapted to fit the boxes?).
Seems to me I learned about how this worked in elementary school. 5th grade maybe.
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Well, it seems to me that it is harder to understand in the imperial system, then in the metrical (lucky us metricals) as 1:72 simply means 1 cm in model is 72 cm (replace cm with metres if you in to the larger size kits) in the real thing and we donāt work with x/8th of a inch or that kind of logic(?)ā¦
Indeed,
but the big difference is that our rulers have cm and mm engraved on them
while imperial rulers have markings for 1/2, 1/4, 1/8, 1/16, 1/32 (shorter than 1 mm)
and sometimes even 1/64 inch.
Measure on the drawing or real object and use the same number of the corresponding fractions.
It gets more complicated with 1/72, see post by @KurtLaughlin.
We in the metric world have to measure (same), divide by the scale, measure again (same) so
we get an extra division operation (measure 2 feet, measure 2/8ths, done).
The imperial guys are in trouble when the 1:1 scale measurement isnāt a whole number of feet ā¦
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There are two ways of writing a modelās scale: one is as a fraction, like youāre doing above, the other is as a ratio such as 1:72, 1:35, 1:10, etc. The numbers are exactly the same, the notation is slightly different (/ vs. :) and the mathematical meaning is also slightly different, but the end result is the same.
It sometimes puzzles me too that many modellers have a very poor grasp of what āscaleā means. But Iāve seen plenty of posts like the one that started this thread, so it looks like a good number of modellers never actually learned this. Of course, if youāre not trying to correct models, measuring in drawings and photos, etc. then it probably doesnāt really matter in practice.
Usually affects scratch builders.
Once upon a time there was a model builder, maybe in the old rec.models.scale who
couldnāt believe that his rocket model was a 1/96 (or 1:96, by the way, a fraction is a division, some write division as / and some write : but it all comes down to divide the number to the left of the symbol by the number to the right of the symbol, a horizontal line is awkward in typed text ā¦) scale model of the real thing. He was comparing visual size, basically volume, and that sure as didnāt equal 1/96th. We explaine that volume scale is the cube of the length scale (96 x 96 x 96 = 884736). My brother explained this to me over 50 years ago when I said that my Wiking (1/90 scale, close to H0) automobile was way to light since 1.5 tons divided by 90 was 16.66 kg and not the few grams that the model car weighed ā¦
No issues . Screw the 1/8th , 1/16th crap ā¦ I use 1/100ths. 1.75 inchs X 72 (or 48, or 32ā¦) If you want it in feet multiply by 12. (same as mm X 100= cm. ) A 2 inch 1/35 figure (2X35 =70 inches , divided by 12 ) would be 5.83 feet or 5ā10". And keep in mind numbers over 23 I run out of things to count on. Must admit that pints, cup, quart is a cluster. (when it comes to Guinness though, a pint is a pint! 
Letās get back to the good old days when everyone knew an inch was the length of the kingās thumb from knuckle to tip! So what if that unit changed with each passing king?.. 
With people getting larger over time (even kings), then the size of an inch should getting bigger? Maybe now about 3 cm? And whats with nations that donĀ“t have a king?
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They have no choice but to go metric!
Unless the Presidentās thumb can serve?..
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