PSI is short for pounds per square inch. It is a unit of pressure that is used to describe the strength of a person’s grip. It is calculated by dividing the weight of the object by its surface area. For example, if a person holds a hammer with a 10 pound weight and a 2 square foot surface area, then the PSI value would be 20. Scientists use PSI values to compare different types of dogs. When comparing two dogs of the same breed, they can determine which one has a higher PSI value.