Reference or measure the amount of force your body weight exerts. Weigh yourself to determine the pounds of force exerted onto the ground. For our example, we will use a person who weighs 160 pounds.
Calculate the area the force is being applied through. When wearing shoes, your feet are roughly rectangular, so you would measure the width and length of your foot and multiply the measurements. As an example, if you had a 4-by-12 inch shoe, then the area of one foot would be 48 square inches. Since you have two feet to disperse the weight, your total area would be 96 square inches.
Divide the force by the area to calculate pressure. In the example, you would divide 160 pounds by 96 inches to produce a pressure of 1.667 pounds per square inch.
As a comparison, say you strapped on a pair of rectangular, 12-by-24 inch snow shoes. Your weight doesn't change, but the total area of exerted force is now spread over a total of 576 square inches, by calculating 12 times 24 and then multiplying that by two. This produces only 0.278 pounds per square inch.