depending on the units of measurement, if you have, for example;
5.5 miles = x km
the unit conversion for mile -> km is x 1.609
so, 5.5 m x 1.609 = 8.8495 km
if you were to go from km -> miles, the unit of conversion is 0.6214 (they are not the same!!)
8.8495 km x 0.6214 = 5.5 m
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If the two measurements are of the same type, such as length, we could use that ratio to convert from one measurements to another. An example is the ratio of 1.609 Km to 1 mile. Here, we can multiply some number of miles by 1.609 and find the kilometer distance. If the two measurements are of different types, that is often used as a definition of another quantity such as speed. Speed is defined as the ratio of the distance traveled to the amount of time it takes. These two examples are the most common uses when taking the ratio of two measurements, yet there may be a more generalized term or theorem here, but I've not heard of it yet.
It can be a conversion factor - though not necessarily. For example, 68 deg Fahrenheit = 20 deg Celsius. But there is no conversion factor for F-to-C: instead there is a linear equation.
Usually such ratios have a specific name. For example, the ratio of a mass measurement (g) over a volume measurement (mL) is called density. The result is a number with the unit g/mL. Most cases that have different units will have a specific name if they're commonly used. Other examples include molarity, molality, joules, newtons, and ppm.
scale factor