To calculate the pounds per square inch (psi) from the compression ratio, one needs the compression ratio and the atmospheric pressure measurement. Then, the first number in the compression ratio is multiplied by the atmospheric pressure, then divided by the second number in the ratio.

For example, if the atmospheric pressure is 14.7 psi and the compression ratio is 11:1, the equation to solve for the psi is (14.7*11)/1. Therefore, the answer is 161.7 psi. People use this method of calculation to determine the pounds per square a piston produces.

The atmospheric pressure can vary, but the standard atmosphere produces 14.696 psi at sea level. For more exact calculations, the altitude should be considered.