How are rates calculated?
Mathematically speaking, any rate will respect the formula:
In a more advanced notation, using calculus:
So, to calculate a rate, you must have two values changing, you fix a time, or any equivalent measure, and calculate their changes, then divide them. Your rate will tell you how the numerator variable changes with the denominator variable.
Examples are: velocity - how space changes with time; acceleration - how the velocity changes with time. See that most rates, not necessary mandatory, will be something with respect time.
See for more: Rate (mathematics)