I misspelled pi. Pi R squared is the formula for area of a circle. R is the radius, which is half the diameter. I tried to simplify by saying multiply 1/2D, which is equal to R, by 1/2D. Then multiply that by pi, which is 3.14.
For instance, a 1.5" bore has a radius of .75. To get R squared, you multiply .75 by .75 = .5625 and multiply that by pi, .5625 times 3.14 = 1.76625 square inches. Divide the 24,000 pounds (12 tons) by the 1.76625 = 13,588 pounds per square