The two popular measures for quantitatively determining if a process is capable are

The process capability ratio and the process capability index

Explanation:

The process capability ratio is calculated by dividing the difference of the upper and lower specification limits by six standard deviations. It is a ratio of specification to the process variation. It determines whether a process meets design specifications or not.

Whereas the process capability index measures the proportion of variation between centre of processes and the nearest specification limits.

These two are the statistical measure of a process capability which determine quantitatively that a process is capable or not.