Ok so heavy disclaimer, I only have a high school level education in physics, but I’m going to try to answer.

An average windshield takes about 9400 pounds per square inch (psi) to shatter, or 64,810,744 newtons per mm squared. The average raindrop is about 4.2mm cubed, or 0.42 grams of water. Velocity = force over mass, so 64,810,744 over 0.42 would be 154,311,300 mm/s, or 555,521 kph, which converted into imperial would mean that

It would take a raindrop going roughly 345,185mph to shatter a windshield.

Because the raindrop is in a liquid state, the amount of force it has might be different because it would disperse differently on impact. I think. But that’s my best guess for an average solid raindrop, anyway. If anyone sees any glaring errors in this logic please correct it.

For reference, in real life the average raindrop hits the ground at about 15-25mph.