I was planning on putting a hydroplaning calculation tool here but may not. After doing some research, the formulas used to calculate this differ depending upon tread depth, tire surface area, tread design, vehicle weight, depth of water, and even the consistency of water.

Most people, myself included, would not be able to enter the majority of the variables. So, here's the best I could find to play with.

Most car tires are pressurized to 35 psi, so hydroplaning can occur at 60.76 mph. You can plainly see the importance of keeping your tires properly inflated since hydroplaning speed will decrease with a decrease in pressure. By how much?

A quick way to estimate this is to use 61 mph for 35 psi and subtract 1 mph for every psi lost (1 for 1 exchange). I like easy math.