Find out using the Running In The Rain Calculator.
A summation of some basic considerations to determine if it is worth running in the rain. Enter the various parameters about yourself and the rainfall and see how wet you would get while running to a shelter.
It seems that as soon as rain starts to fall, people speed up their pace. There could be many reasons for this but one is the expectation that by running - or at least walking faster - they will arrive drier at their destination.
If you add the rate the droplets hit your top and front areas, you now have droplets/second - so multiply this by the amount of time you will be in the rain - d/v. A few comparisons might lead you to worry less about how fast you run, but there are a lot of cases to consider: wind in your face, wind from behind, wind from the side, drizzle vs downpour, and the various possible running speeds.
As an object falls through air, it starts to experience drag forces. The higher its speed, the more resisting drag. It will eventually reach a "terminal velocity" where the drag force equals its weight, hence there is zero net force and it stops accelerating. The value of the terminal velocity depends on the size, shape and mass of the object.
For raindrops at sea level, the range of terminal velocities is 2 m/s (0.5 mm drop - drizzle) up to 9 metres per second (5 mm drop - downpour).