在物理宇宙学里，宇宙标度因子（cosmological scale factor）是弗里德曼方程序的一个参数，是表现宇宙相对膨胀的时间函数。宇宙标度因子又称为罗伯逊－沃尔克标度因子，在这篇文章内，简称为「标度因子」。在膨胀或收缩中的罗伯逊-沃尔克宇宙里，设置跟着哈伯流体移动的两个物体，则对于两个物体（例如，两个星系）之间的固有距离（proper distance），可以用则标度因子来给出这固有距离随着时间演进而发生的变化，以方程序定义，

其中， 、 分别是在参考时间 、时间 的固有距离， 是在时间 的标度因子。

A scale factor is a number 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 of one half. The basic equation for it is image over primeage. 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 factor.