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A **scale factor** is usually a decimal which scales, or multiplies, some quantity. In the equation *y* = *Cx*, *C* is the scale factor for *x*. *C* is also the coefficient of *x*, and may be called the constant of proportionality of *y* to *x*. For example, doubling distances corresponds to a scale factor of two for distance, while cutting a cake in half results in pieces with a scale factor for volume of one half. The basic equation for it is image over preimage.

In the field of measurements, the scale factor of an instrument is sometimes referred to as sensitivity. The ratio of any two corresponding lengths in two similar geometric figures is also called a scale.