Manufacturers typically rate jackets using two numbers. The first, in millimeters, describes how water resistant it is. If you were to stand a 1 in. by 1 in. tube over the jacket’s fabric, and then fill it with water, doing so would measure how high the water would have to be before it started leaking through.
In the case of a 10K jacket (or 10,000mm), you could stack a 1 in. by 1 in. column of water 10,000mm high before it even started to seep through the fabric’s other side—that’s almost 33 feet!
A rating of up to 10K is enough to handle light to average rain for a short amount of time. Ratings between 10K and 15K can handle a moderate amount of rain for much longer, and jackets rated between 15K and 20K or higher are serious shells for heavy, intense rain over a prolonged period.