When you are in the field and need to come up with a quick approximation of the surface area of a cylinder’s piston, rod or annular area, try this quick method.There is a formula for area which is D. x D. x .7854. Instead of using .7854, use .75, or the fraction 3/4. What this means is that if you have a square, and draw the largest circle inside of it that you can, the area encompassed by the circle is about 3/4 of the area of the square. If you have a 4” bore, to find the area, multiply 4 x 4 and then take 3/4 of it. Three fourths of 16 is equal to 12, and that is a good approximation. The complete math comes out to 12.57. The shortcut got us within 5%.
By: Dan Turner, Turner Hydraulics