Compressive strength involves testing and calculating how well a given specimen, product or material can survive compressive stress. Unlike tension, which expands or pulls, compression means a specimen, product or material is shortened or pressed down. Compressive strength of a material is the point at which the material fails. Calculating compressive strength involves testing to find this failure point and using the data from the test to feed the calculation. The final compressive strength number is in poundsforce per square inch, or psi.
Things You'll Need
 Compressive strength testing apparatus
 Calculator

Set up a test to determine the maximum load of the specimen or material for which you are looking to calculate the compressive strength. This test has to be unique to your specimen. Determine the maximum amount of compressive stress the specimen can handle in units of poundsforce, or lbf. Place your specimen on a solid surface and apply a force to your specimen with the testing apparatus until the specimen fails or crushes. Upon failure, record the failure force in units of lbf. The force has to be adequate for the specimen to fail within 15 minutes of applying it.

Calculate the crosssection area, or A, of the specimen. If your specimen is circular or cylindrical, use the formula A = 3.1415r^2, where r is the radius of the specimen. If the specimen is foursided, use the formula A = LW, where L is the length and W is the width. The area will be in units of in^2 or square inches. For example, if we assume you have a circular specimen with a radius of 2 inches, your area will be 12.56 in^2:
A = 3.1415r^2 = (3.1415)(2^2) = (3.1415)(4) = 12.56 in^2

Calculate the compressive strength using the formula S = P/A, where S is the compressive strength, P is the maximum load applied to the specimen and A is the area. Your final compressive strength value will be in units of lbf/in^2, which is the same as pressure or psi. For example, if your maximum load is 20,000 lbf and your A is 12.56in^2, your compressive strength is 1,592 lbf/in^2 or 1,592 psi:
S = P/A = 20,000 lbf/12.56 in^2 = 1,592 lbf/in^2 = 1,592 psi
Tips & Warnings
 Although psi is an English unit in common use in the US, the preferred scientific equivalent unit of pressure is the pascal. Because pascals indicate relatively small amounts of pressure, measurements for compressive strength typically use the MPa, or megapascal for metals, minerals and other common materials.
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